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Microwave/vacuum and osmotic drying of cranberries

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MICROWAVE/VACUUM AND OSMOTIC DRYING OF
CRANBERRIES
by
Predrag S. Sunjka
Department of Agricultural and Biosystems Engineering
McGill University, Montreal
March 2003
A thesis submitted to McGill University in partial fulfillment
of the requirements for the degree of Master of Science
®Predrag S. Sunjka 2003
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ABSTRACT
PREDRAG S. SUNJKA
M.Sc.
Agricultural and Biosystems Engineering
MICROWAVE/VACUUM AND OSMOTIC DRYING OF CRANBERRIES
Modem food industry dictates strict conditions on energy use and application,
preventing unnecessary energy dissipation. Energy demanding processes such as
distillation and drying have to be optimised to the highest extent, while retaining or
improving the final product quality. Pretreatments to drying can be used in order to
optimize drying, and some of cranberry pretreatments such as chemical, mechanical and
osmotic dehydration were optimized. Chemical pretreatment consisted of dipping
cranberries into solution of ethyl oleate and sodium hydroxide at different temperatures,
and process times. Mechanical pretreatment was cutting of berries into halves or quarters.
Tested parameters for osmotic dehydration were the duration of process, osmotic agent
type and its concentration.
Once the appropriate pretreatment was selected, cranberries were subjected to
hybrid drying under subatmospheric pressure and using microwaves as an energy source.
Evaluated process parameters were microwave power level, microwave power mode, and
the operating pressure of process. This drying method showed good potential, but in order
to verify the results obtained, it was compared to microwave/convective drying. Slight
advantages of the microwave/vacuum process over the microwave/convective process
were apparent in almost all product quality parameters, as well as in process efficiency.
ii
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RESUME
PREDRAG S. SUNJKA
M.Sc.
Genie Agricole et des Biosystemes
SECHAGE DE LA CANNEBERGE PAR OSMOSE ET A L’AIDE D’UN SYSTEME
MICRO-ONDE/SOUS-VIDE
Le sechage des denrees alimentaires requiert une grande quantite d’energie.
II est done essentiel de chercher a optimiser ce procede tout en retenant ou ameliorant la
qualite du produit fini. Cette etude a porte sur 1’evaluation et la comparaison de differents
pretraitements de la canneberge permettant d’en optimiser le sechage. Les trois types de
pretraitements utilises etaient: chimique, mecanique et osmotique. Les pretraitements
chimiques ont consiste a tremper des canneberges dans une solution d'oleate d'ethyle et
d’hydroxyde du sodium a temperatures differentes, et durees du processus. Le
pretraitement mecanique coupait les baies en demi ou quart. Les parametres testes pour la
deshydratation osmotique etaient la duree de processus, le type de l'agent osmotique et sa
concentration.
Une fois traitees, les canneberges ont ete sechees a l’aide d’un systeme hybride
operant sous-vide et utilisant les micro-ondes comme source d'energie. Les canneberges
seches ainsi obtenues ont ete comparees a celles sechees a Taide d’un systeme hybride
micro-ondes/air chaud. Cette etude a permis de demontrer que le systeme hybride microonde/sous-vide etait legerement superieur puisque l’efficacite du procede et presque tous
les parametres de qualite des canneberges sechees a l’aide de ce systeme etaient superieurs
a ceux obtenus a Taide du sechoir micro-onde/air chaud.
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ACKNOWLEDGEMENTS
I would like to express my endless gratitude towards my supervisor Dr. G.S. Vijaya
Raghavan, James McGill Professor and the Chair of Department of Agricultural and
Biosystems Engineering, for his guidance, advice and assistance in this research. His
instructions were always constructive and positive, helping me to finish my thesis with
thorough understanding of the problem.
Successful finishing of this thesis would be impossible without numerous
professional and supporting people from Macdonald Campus, in the first place Timothy
John Rennie who helped me with my cul-de-sacs and dealt with my disorganized scientific
papers. Special appreciation is extended to Valerie Orsat and Yvan Gariepy, merci mes
amies pour toutes vos instructions et recommandations, ce serait impossible sans vous. I
would like to mention a few other colleagues that helped me, among them were Claudia
Beaudry, Jianming Dai, Venkatesh Sosle, and Samson Sotocinal. Thank you all my
friends, it was a pleasure studying and working with you.
My parents and brother showed me with their example the importance of academic
education, and without their support all this would be difficult, and I owe them eternal
gratitude.
At the end, the most important word of appreciation goes towards my love, my
wife Slavica. XBajia th Hane 3a CBe ihto ch mh npyacnna.
iv
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FORMAT OF THESIS
The format of this thesis is manuscript-based format, suitable for journal
publication with minor modifications. The Faculty of Graduate Studies and Research,
McGill University approved this thesis format, as outlined in the “Thesis Preparation and
Submission Guidelines”, Section I: Thesis preparation, Part C: Manuscript-based thesis:
“C. Manuscript-based thesis
As an alternative to the traditional thesis format, the dissertation can
consist of a collection of papers of which the student is an author or co­
author. These papers must have a cohesive, unitary character making
them a report of a single program of research. The structure for the
manuscript-based thesis must conform to the following:
1. Candidates have the option of including, as part of the thesis, the text
of one or more papers submitted, or to be submitted, for publication, or
the clearly-duplicated text (not the reprints) of one or more published
papers. These texts must conform to the "Guidelines for Thesis
Preparation" with respect to font size, line spacing and margin sizes and
must be bound together as an integral part of the thesis. (Reprints of
published papers can be included in the appendices at the end of the
thesis.)
2. The thesis must be more than a collection of manuscripts. All
components must be integrated into a cohesive unit with a logical
progression from one chapter to the next. In order to ensure that the
thesis has continuity, connecting texts that provide logical bridges
preceding and following each manuscript are mandatory.
3. The thesis must conform to all other requirements of the "Guidelines
for Thesis Preparation" in addition to the manuscripts.
The thesis must include the following:
1.
a table of contents;
2.
a brief abstract in both English and French;
3.
an introduction which clearly states the rational and objectives of
the research;
4.
a comprehensive review of the literature (in addition to that
covered in the introduction to each paper);
5.
a final conclusion and summary;
6.
a thorough bibliography;
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7.
Appendix containing an ethics certificate in the case of research
involving human or animal subjects, microorganisms, living cells, other
biohazards and/or radioactive material.
4. As manuscripts for publication are frequently very concise
documents, where appropriate, additional material must be provided
(e.g., in appendices) in sufficient detail to allow a clear and precise
judgement to be made of the importance and originality of the research
reported in the thesis.
5. In general, when co-authored papers are included in a thesis the
candidate must have made a substantial contribution to all papers
included in the thesis. In addition, the candidate is required to make an
explicit statement in the thesis as to who contributed to such work and
to what extent. This statement should appear in a single section entitled
"Contributions of Authors" as a preface to the thesis. The supervisor
must attest to the accuracy of this statement at the doctoral oral defence.
Since the task of the examiners is made more difficult in these cases, it
is in the candidate's interest to clearly specify the responsibilities of all
the authors of the co-authored papers....”
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CONTRIBUTIONS OF AUTHORS
The work presented here was performed by the candidate and supervised by Dr.
G.S.V. Raghavan of the Department of Agricultural and Biosystems Engineering,
Macdonald Campus of McGill University, Montreal. The research project was conducted
in the Department of Agricultural and Biosystems Engineering, Macdonald Campus,
McGill University, Montreal.
The authorships for the papers are as follows:
1st paper (Chapter III): P.S. Sunjka and G.S.V. Raghavan
2nd paper (Chapter IV): P.S. Sunjka and G.S.V. Raghavan
3rd paper (Chapter V): P.S. Sunjka. T.J. Rennie, C. Beaudry, and G.S.V. Raghavan
vii
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TABLE OF CONTENTS
ABSTRACT..............................................................................
II
R ESU M E......................................................................................................................................Ill
ACKNOW LEDGEMENTS.......................................................................................................IV
FORMAT OF T H E SIS................................................................................................................V
CONTRIBUTIONS OF A U TH O R S...................................................................................... VII
TABLE OF CONTENTS........................................................................................................ VIII
LIST OF TABLES...................................................................................................................... XI
LIST OF FIG U R ES................................................................................................................. XIII
NO M EN CLATU RE................................................................................................................. XV
I. GENERAL INTRODUCTION..............................................................................................1
1.1 Introduction ................................................................................................................. 1
1.2 H ypothesis ..................................................................................................................... 2
1.3 O bjectives ...................................................................................................................... 3
1.4 Sc o p e ............................................................................................................................... 3
II. REVIEW OF LITERATURE................................................................................................4
2.1 General Introduction on Cranberries ................................................................4
2.1.1 Cultivation characteristics............................................................................4
2.1.2 Harvesting and handling...............................................................................5
2.1.3 Storage conditions........................................................................................6
2.1.4 Chemical composition, nutritive and medicinal value o f cranberries.......... 7
2.2 P retreatment of fruits ............................................................................................. 8
2.2.1 Methods o f pretreatment...............................................................................8
2.2.2 Osmotic dehydration.....................................................................................9
2.3 D rying as a D ehydration M e t h o d .......................................................................12
2.3.1 Drying o f fruits............................................................................................13
2.3.2 Drying methods...........................................................................................16
2.4 Quality A ssessment ................................................................................................. 24
2.4.1 Sensory evaluation......................................................................................24
2.4.2 Skin colour determination...........................................................................25
2.4.3 Texture characteristics................................................................................26
2.4.4 Determination o f rehydration capacity offruits.........................................29
2.4.5 Water activity..............................................................................................30
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III.
ASSESMENT OF DIFFERENT METHODS FOR SKIN PRETREATMENT AND
OSMOTIC DEHYDRATION OF CRANBERRIES...................................................... 32
3.1 A bstract ...................................................................................................................... 32
3.2 Introduction ...............................................................................................................32
3.2.1 Skin pretreatment........................................................................................33
3.2.2 Osmotic dehydration...................................................................................34
3.3 O bjectives .................................................................................................................... 35
3.4 M aterials and M e t h o d s .........................................................................................35
3.4.1 Chemical and mechanical pretreatment.....................................................35
3.4.2 Osmotic dehydration...................................................................................36
3.4.3 Quality evaluation.......................................................................................37
3.4.4 Experimental design....................................................................................38
3.5 R esults and D iscussion ........................................................................................... 38
3.5.1 Chemical and mechanical pretreatment.....................................................38
3.5.2 Osmotic dehydration...................................................................................42
3.6 C onclusions ................................................................................................................46
3.7 A cknowledgements ................................................................................................. 47
3.8 R eferen ces ..................................................................................................................47
CONNECTING T E X T ..............................................................................................................50
IV. MICROWAVE/VACUUM DRYING TECHNIQUE ON CRANBERRIES............51
4.1 A bstract ...................................................................................................................... 51
4.2 Introduction ...............................................................................................................51
4.2.1 Microwave/vacuum drying o f food materials.............................................52
4.3 O bjectives .................................................................................................................... 55
4.4 M aterials and M e t h o d s .........................................................................................55
4.4.1 Cranberry pretreatment..............................................................................55
4.4.2 Microwave/vacuum drying..........................................................................55
4.4.3 Quality evaluation.......................................................................................57
4.4.4 Experimental design....................................................................................59
4.5 Results and D iscussions .........................................................................................59
4.5.1 Energy efficiency.........................................................................................59
4.5.2 Colour parameters......................................................................................60
4.5.3 Textural properties......................................................................................61
4.5.4 Rehydration ratio........................................................................................64
4.6 C onclusions ................................................................................................................65
4.7 A cknowledgements ................................................................................................. 66
4.8 Referen ces ..................................................................................................................67
CONNECTING T E X T ............................................................................................................. 70
V.
MICROWAVE/CONVECTIVE AND MICROWAVE/VACUUM DRYING OF
CRANBERRIES: A COMPARATIVE STUDY............................................................ 71
5.1 A bstract .................................................................................................................... 71
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5 .2 In t r o d u c t io n ..................................................................................................................................72
5.3 D r y in g M e t h o d s ..........................................................................................................................72
5.3.1 Microwave/convective drying..................................................................... 73
5.3.2 Microwave/vacuum drying.......................................................................... 73
5.3.3 Drying efficiency......................................................................................... 74
5 .4 M a t e r ia l s
and
M e t h o d s ........................................................................................................76
5.4.1 Microwave/convective drying..................................................................... 76
5.4.2 Microwave/vacuum drying.......................................................................... 76
5.4.3 Quality evaluation....................................................................................... 77
5.5 R e s u l t s
and
D is c u s s io n ...........................................................................................................78
5.5.1 Colour and textural properties................................................................... 78
5.5.2 Temperature during drying......................................................................... 79
5.5.3 Energy aspects............................................................................................80
5.5.4 Sensory evaluation......................................................................................83
5 .6 C o n c l u s io n s ...................................................................................................................................84
5 .7 A c k n o w l e d g e m e n t s .................................................................................................................. 84
5.8 R e f e r e n c e s ..................................................................................................................................... 85
VI. GENERAL DISCUSSIONS AND CONCLUSIONS...............................................88
REFERENCES..................................................................................................................91
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LIST OF TABLES
Table 2.1:
Respiration rates of cranberries at different storage temperatures..................7
Table 2.2:
Nutritional information of fresh cranberries per 48 g serving.........................7
Table 2.3:
Food types according to moisture content........................................................ 31
Table 3.1:
Average change in sugar and moisture content for chemically treated and
osmotically dehydrated cranberries under the same conditions (HFCS 24
h, mass ratio 2:1, room temperature)................................................................. 39
Table 3.2:
Change in mass gain, solids gain and moisture loss of cranberries treated
with different chemical method and osmotically dehydrated under similar
conditions........................................................................................................... 40
Table 3.3: Average change in sugar content and moisture content for mechanically
treated and osmotically dehydrated cranberries under same conditions
Table 3.4:
41
Change in mass loss, solids gain and moisture loss of cranberries treated
with different mechanical methods and osmotically dehydrated under
same conditions.................................................................................................. 41
Table 3.5: Average change in sugar content and moisture content for cranberries at
different times of osmotic dehydration (HFCS, 2:1)......................................... 43
Table 3.6:
Average change in sugar and moisture content of cranberries with
different osmotic agent and its concentration during treatment of 24 hours
Table 3.7:
44
Average values of mass gain, solids gain and moisture loss for
osmotically dehydrated cranberries using different osmotic agent type and
concentration...................................................................................................... 45
Table 4.1:
Average values of total drying time, power-on drying time and drying
efficiency for cranberries dried at 3.4 kPa of absolute pressure........................ 59
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Table 4.2:
Average values of total drying time, power-on drying time and drying
efficiency of cranberries dried with 125 W power level and MW mode of
30 s on/30 s off, under different absolute pressures........................................... 60
Table 4.3:
Average colour values for cranberries dried under different MW power
levels and MW modes, at 3.4 kPa...................................................................... 60
Table 4.4:
Average colour values of cranberries dried with 125 W power level and
MW mode of 30 s on/30 s off under different absolute pressures..................... 61
Table 4.5:
Average values of Young’s modulus and toughness of cranberries dried
with 125 W power level and MW mode of 30 s on/30 s off under different
absolute pressures............................................................................................... 63
Table 4.6:
Average values of rehydration ratios for cranberries dried with 125 W
power level and MW mode of 30 s on/30 s off under different absolute
pressures............................................................................................................. 64
Table 5.1:
Comparison of colour values of MW/convective and MW/vacuum dried
cranberries.......................................................................................................... 78
Table 5.2:
Toughness and Young’s modulus for MW/convective and MW/vacuum
dried cranberries................................................................................................. 79
Table 5.3:
MW power-on times and drying efficiencies for cranberries dried with
MW/convective and MW/vacuum method........................................................ 82
Table 5.4:
Organoleptic analysis of three cranberry samples dried using three
different drying methods.................................................................................... 83
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LIST OF FIGURES
Figure 2.1:
Wet harvesting of cranberries..........................................................................5
Figure 2.2:
Cranberry harvester.........................................................................................6
Figure 2.3:
The effect of cranberry juice on infection of urinary tract (Avom et al.,
1994)................................................................................................................. 8
Figure 2.4:
Schematic demonstration of osmotic dehydration process..............................10
Figure 2.5:
Isotherm for water vapour sorption isotherm.................................................. 14
Figure 2.6:
Typical drying curve........................................................................................ 15
Figure 2.7:
L*a*b* color three-dimensional system (HunterLab, Reston, VA).................. 25
Figure 2.8:
Kramer shear press.......................................................................................... 28
Figure 2.9:
Typical stress-strain diagram obtained from INSTRON machine.................. 29
Figure 3.1:
Average change in mass gain, solids gain and moisture loss in time for
osmotic dehydration during dehydration with HFCS, 2:1 mass ratio HFCS
to fruit................................................................................................................ 43
Figure 4.1:
Schematic representation of MW/vacuum drying equipment..........................56
Figure 4.2:
Instron Universal Testing Machine................................................................. 58
Figure 4.3:
Average values of Young’s modulus for cranberries dried under different
MW powers and MW modes at 3.4kPa of absolutepressure............................ 62
Figure 4.4: Average values of Toughness for cranberries dried under different MW
powers and MW modes at 3.4 kPa of absolutepressure.................................... 63
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Figure 4.5:
Average values of rehydration ratios for cranberries dried under different
MW powers and MW modes at 3.4 kPa of absolute pressure........................... 64
Figure 5.1:
Typical temperature profile during MW/convective, MW/vacuum and
convective dried cranberries.............................................................................. 80
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NOMENCLATURE
a
*
Chromacity coordinate (redness or greenness)
a*st
Chromacity coordinate a* of fresh cranberries
aw
Water activity
b*
Chromacity coordinate of fresh cranberries
b*st
Chromacity coordinate b* of fresh cranberries
C
Constant (nondimensional)
c
♦
Chromacity index (saturation index)
CaS04
Calcium sulphate
Csaml
Specific heats of the sample at the initial stage of drying (kJ/kg-°C)
Csam2
Specific heats of the sample at the final stage of drying (kJ/kg-°C)
DEa
Drying efficiency (MJ/kgw)
DEb
Drying efficiency (kgw/MJ)
E lo ss
Energy losses during process (kJ)
EO
Ethyl oleate
HFCS
High fructose com syrup
h°
Hue angle in chromacity
ip
Specific enthalpy of water vapour (kJ/kg)
K
Dehydration constant ( h 1)
fa , i, 2
Empirical drying coefficients (nondimensional)
b-M
Drying constant (h_1)
L*
Chromacity coefficient (lightness)
L * st
Chromacity coefficient of fresh cranberries
M
Average moisture content (ratio, dry basis)
M
Moisture content (%, wet basis; or ratio, wet basis)
M0
Initial moisture content (ratio, wet basis)
m, n
Empirical coefficients in determination of specific heat
mi
Mass before drying (kg)
m2
Mass after drying (kg)
mdh
Mass of dehydrated sample (g)
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Mdh
Moisture content of the dry sample (%, wet basis)
Me
Equilibrium moisture content (ratio, dry basis)
mevw
Mass of evaporated water (kg)
Mf
Final moisture content (ratio, dry basis)
Mi
Initial moisture content (ratio, dry basis)
nii
Initial sample mass (kg)
Min
Initial moisture content (%, wet basis)
mrh
Mass of rehydrated sample (g)
mSami
Initial mass of the sample (kg)
msam 2
Final mass of the sample (kg)
MW
Microwaves
NaOH
Sodium hydroxide
OD
Osmotic dehydration
P
Microwave power level (W)
Po
Reference microwave power (W)
Pa
Water vapour fugacity over the material (Pa)
PAsat
Water vapour fugacity over the pure solvent (Pa)
Pbi
Power input from the air blower (kW)
Phe
Power input from the air heater (kW)
P mw
MW power input (kW)
PR
Microwave pulse ratio
Pvp
Power of the vacuum pump (kW)
Q
Absolute pressure (Pa)
Qo
Reference absolute pressure (Pa)
RH
Relative humidity
RR
Rehydration ratio of dried cranberries
t
Time (s, min, hour)
tand
Loss tangent
Tfm
Final temperature of the sample (°C)
Tin
Initial temperature of the sample (°C)
tMWon
Total microwave power-on time (s)
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ton
Total microwave power-on time (s)
hot
Total drying time (s)
Greek
^
Cumulative energy efficiency (ratio)
6
Penetration depth of microwaves (m)
Aa
Difference in chromacity coordinate a* between fresh and dried
cranberries
Ab*
Difference in chromacity coordinate b* between fresh and dried
cranberries
AE
AL
Total colour difference between fresh and dried cranberries
$
Difference in chromacity coefficient between fresh and dried
cranberries
Electric permittivity complex
e
e
’
Dielectric constant
e ’’
Loss factor
£in
Instantaneous energy efficiency (ratio)
Xo
Wavelength in free space (m)
xvii
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I. GENERAL INTRODUCTION
1.1 Introduction
Food needs of contemporary world have never been greater. Reaching its peak of
more than 6 billion people in 2002, human population is faced with lack of nutritive
commodities, apparent as never before. Therefore, new methods for preserving perishable
foodstuff have to be developed, as well as old approved techniques must be improved to
the greatest extent.
Fresh food of plant origin, or to be more specific - fruits and vegetables are
especially sensitive to spoilage by internal or external factors. In the last century, science
has offered a proof for a long known truth: how valuable and exceptional are fruits and
vegetables for human organism. Their nutritive values and position in human diet can not
be magnified enough.
Fruits contribute to overall vitamin, mineral, and cellulose intake, and hence are
often called "protective" foods. Because of their varied shapes and sizes, fruits add
pleasure and enjoyment to eating. However, fruit is very perishable commodity and even
the most sustainable fruit has to be preserved in some way. Fruit has a seasonal character,
but improved transportation, refrigeration, storage, conservation and packaging
procedures have made available a constant supply of fruits from the many areas of the
world to the most remote countries.
Cranberries (Vacciunium macrocarpon) are bright red berries used for sauces,
jellies, juices, and as dried product in bakery industry for a wide group of products:
muffins, breakfast cereals, snack bars, and others. Canada with its continental climate
represents a suitable environment for the growing of cranberry fruit. Canada ranks second
after US in total world production of cranberries. In 1998 with more than 32,906 metric
tonnes and an "estimated" farm value of $55.4 million, the industry reached all-time highs
(The Government of Canada, 1999). Different drying methods have been tested for
cranberries, methods such as convective air-drying, freeze-drying, vacuum drying, and
many others (Beaudry et al., 2003c; Grabowski et al., 2002; Yongsawatdigul and
1
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Gunasekaran, 1996a,b). Every method has its advantages and disadvantages, and every
suggested process must be tested on aforementioned fruit in order to establish the
soundest method.
Because of cranberry's impenetrable epidermis, the migration of moisture is
impeded. Thick waxy layer represents a barrier that has to be surmounted. Many
pretreatments are suggested and tested for tackling of this problem. They can be divided
in two groups: chemical and mechanical pretreatments. It is well known that it is
recommendable for high moisture food (e.g. cranberry) to be partially dried in some way.
Purpose of this operation is the primary diminishing of water content, and as a
consequence, the facilitation of the principal drying. This can be achieved by placing a
product in an environment with high osmotic pressure. Several types of sugar
surroundings are commonly used for fruits as an agent for osmotic dehydration.
After determining of the most appropriate pretreatment, the main drying method
was performed - combination of microwaves as an energy source use and application of
low pressures in order to obtain a high quality product - dried cranberries.
1.2 Hypothesis
This work is focused on the evaluation of pretreatment methods for cranberries, as
well as the comparison of different conditions during microwave/vacuum combined
drying. The hypothesis is that with proper combination of appropriate pretreatment
techniques and drying conditions a high quality product can be obtained, while
maintaining high process efficiency. The final product must preserve initial features of
fresh berries to the highest extent. Original characteristics like skin colour, skin surface,
aroma and flavour must be retained as much as possible, simultaneously keeping water
content and water activity at minimum.
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1.3 Objectives
Aforementioned hypothesis can be summarized into three main objectives:
1. Optimization of chemical and mechanical pretreatment of cranberry skin
comparing several different effects such as time and concentration.
2. Optimization of osmotic dehydration using two sugar agents and changing their
treatment parameters: time and sugar to fruit ratio.
3. Optimization of microwave/vacuum conditions for cranberry drying with the
purpose of acquiring of a product with a quality equivalent or better, compared to
current drying methods. These conditions include microwave power density,
microwave power cycling period, and vacuum applied.
1.4 Scope
This study will be based on cranberries. Applied methods and treatments can be
applicable to other fruits, but proper research must be done with the intention of
determining appropriate conditions of moisture removal from fruits. Every fruit has its
own characteristics, and thus must be subjected to each suggested method.
The scope of this thesis is somewhat similar to that presented in works of Beaudry
et al. (2003a,b, and c), but there are significant differences such as mechanical
pretreatment of fruit, and consequently, totally different results. In the drying part of this
work, a novel method of drying was applied: microwave/vacuum drying, and this method
wasn’t discussed in the works of Beaudry et al. (2003a,b, and c).
3
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II. REVIEW OF LITERATURE
2.1 General Introduction on Cranberries
Cranberries (Vaccinium Macrocarpon) are classified as small fruits and berries
from temperate climate zone (Kader and Barret, 1996). One of several fruits indigenous
to North America, this evergreen vine is cultivated on sandy soils or peat bogs. Cranberry
was well known among North American Indians for its characteristics and was used as a
food, medicine, dye, and as a symbol of peace and friendship eaten on feasts. The large or
American cranberry has long been prized for its sour red berries, which are high in
vitamin C content.
2.1.1 Cultivation characteristics
First to start cranberry cultivation of cranberry was Henry Hall of Dennis,
Massachusetts, on Cape Cod in about 1816 (Eck, 1990). Consumption of cranberries was
once limited to Thanksgiving and Christmas meals in the form of jellies and sauces.
Starting in the early 1960’s new products began to appear, such as cranberry juice or
juices with combined cranberry and another fruit (grape, orange, apple...). The leading
countries in cranberry production at this moment are United States, Canada, Latvia and
Poland, although there are some recent experimental trials in Chile, Austria, Germany and
Russia.
The plant is a native of swamp areas and stands submergence under water for long
periods without injury. The cranberry thrives best where the summers are relatively cool
(Seelig, 1974). The vines grow on drained land near an abundant water supply. The rich
peat soil is levelled and covered with a three-inch layer of sand. Cuttings from cranberry
vines are then planted deep enough to take root in a peat soil beneath. After five years,
bog is ready for its full harvest. If properly kept in good condition, vines can produce
berries indefinitely (Eck, 1990).
Proper site selection, site preparation and cultivar selection are very important
factors of cranberry cultivation (Eck, 1990). Water management should be as efficient as
possible, because it is used to protect the vines from winter injury. Cultivar selection has
4
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to be based on demonstrated success with the cultivar near proposed commercial site and
under anticipated management system.
2.1.2 Harvesting and handling
Cranberry harvest begins in early September and is generally completed by
November. Harvesting methods can be "dry" or "wet". "Dry" harvesting is by mechanical
pickers, similar to lawnmower in appearance. Its rotating teeth slip under the berries and
hold them gently as the vines are rolled away by the forward motion of machine. The
berries drop on a conveyer belt, which carries them to a box on the guide bars. Full boxes
are collected, placed on waiting trucks, and carried to receiving stations and processing
plants where they are prepared for market. Dry harvesting is used for marketing as a
whole fresh fruit (Seelig, 1974).
For processing purposes, cranberries are wet harvested: bog is flooded to a depth
about 10-30 cm. Mechanical water reels (Figure 2.2) are then used to stir up the water
with sufficient force to remove the ripe berries. The berries float to the surface, forming
an "island" of red berries (Figure 2.1). They are gathered using floating wooden booms,
raked onto elevators which raise them into loading trucks (Seelig, 1974).
Figure 2.1: Wet harvesting of cranberries
5
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Figure 2.2: Cranberry harvester
2.1.3 Storage conditions
Part of cranberry crop is stored in field boxes for marketing as fresh fruit during
peak Thanksgiving and Christmas holiday seasons. For a longer storage, it is necessary to
maintain certain conditions: 36-40°F (2.2-4.4°C) with 90-95 % relative humidity (RH).
Under these conditions cranberries can be stored no longer than two months (Seelig,
1974).
More recent research is showing similar data: 60-120 days at 2-4°C, 90-95 % RH,
at very low ventilation rate. It must be noted that the freezing point of cranberries is
-0.9°C. Respiration rates of cranberries can be found in Table 2.1, adopted from Uddin,
1997.
6
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Table 2.1: Respiration rates of cranberries at different storage temperatures
Temperature (°C)
Respiration rate (ml C02/kg h)
0
1.34-1.67
5
1.67-2.00
10
2.34-3.67
15
3.34-5.68
20
5.18-6.68
Cranberry is prone to many diseases, such as physiological rot, freezing and
chilling injury, smothering injury, old age (senescence), fungus rots and many others
(Seelig, 1974).
2.1.4 Chemical composition, nutritive and medicinal value of cranberries
Increase in cranberry production has various reasons, but one of them is its
nutritive and therapeutic value. Chemically, the cranberry consists of water, plant fibre,
sugar, acids, pectin, waxy materials and the various ash constituents such as calcium,
magnesium, potassium and phosphorus, plus various vitamins.
Cranberry chemical composition shows an outstanding health benefits (The
Government of Canada, 1999):
Table 2.2: Nutritional information of fresh cranberries per 48 g serving
Ingredient
Quantity
Energy
23 cal (100 kJ)
Protein
0.2 g
Fat
0.1 g
Carbohydrate
6.0 g
Dietary fibre
0.6 g
Sodium
1 mg
Potassium
71 mg
Percentage o f recommended daily intake per 48 g serving
Vitamin C
11 %
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A number of studies have been made in reducing the ammoniacal odour of urine,
in combating urinary infection, and dissolving urinary calculi (Seelig, 1974). Research
conducted by Harvard Medical School showed that cranberry juice is very beneficial in
the prevention of urinary tract infections. Data in Figure 2.3 confirms this fact (Avom et
al., 1994) showing correlation between the consumption of cranberry juice and the
presence of urinary infections.
Bacteriuiia/Pyuria and cnmbuiiy Juice
•S
30 -
I
25 -
20
-
15 -
lO -
O
1
2
3
5
6
Month
□--------
Placebo
»------- Cntnbmiiy
Figure 2.3: The effect of cranberry juice on infection of urinary tract (Avom et al.,
1994)
2.2 Pretreatment of fruits
Fruit has to be pretreated in some way before the main drying treatment. Purpose
of this unit operation is to facilitate and accelerate the main treatment. Common
pretreatments in food industry are washing, peeling and slicing, dipping into alkaline or
acid solutions, sulphuring or sulphating, blanching, different dehydration processes and
many others (Salunkhe, 1991).
2.2.1 Methods of pretreatment
Pretreatments prior to drying have as an objective reducing of initial moisture
content and increasing the drying rates, hence the final product will have higher quality.
8
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The main idea is to change the skin of fruit and modify its features, because it is the main
obstacle in moisture removal. Skin pretreatment can be chemical or mechanical.
Chemical pretreatment involves immersion of the product in alkaline or acid
solutions of oleate esters prior to drying. Alkaline dipping facilitates drying by forming
fine cracks on the skin (Salunkhe et al., 1991), and dipping in oleate esters causes the wax
platelets
on the
fruit
skin to
dissociate,
and help the
moisture
removal
(Venkatalachapathy, 1998). It is determined by Ponting and McBean (1970) that for fruits
with waxy layer is the most effective treatment with ethyl esters of fatty acids, especially
oleic acid. Saravacos et al. (1988) and Tulasidas et al. (1994) used ethyl oleate as a
pretreatment, and found out that it can improve drying rate and therefore increase drying
speed, but it has little effect on product quality. Venkatalachapathy (1998) used alkaline
solution of 2 % ethyl oleate and 0.5 % NaOH as a pretreatment of strawberries and
blueberries. Beaudry et al. (2003b) tested different concentrations and times of dipping
for cranberries, and concluded that it has no significant influence.
Mechanical pretreatments can replace or complement chemical, mainly because
the consumers are hesitant to buy chemically treated fruits. Mechanical pretreatment
comprises peeling, surface abrasion, cutting in various shapes (halves, cylinders, cubes).
Shi and Maupoey (1993) peeled, cored and cut in cubes apricot and pineapple prior to
vacuum osmotic dehydration; Kiranoudis et al. (1997) cut apple, kiwi and pear into
spherical particles before microwave vacuum drying, Jia et al. (1993) sliced carrots prior
to the heat pump/microwave drying. Geyer et al. (2003a) tested two mechanical drying
pretreatment of guavas: cutting in slices and in spheres.
Mechanical pretreatment of cranberries is recorded in Yongsawatdigul and
Gunasekaran (1996a), where they cut cranberries in halves. Beaudry et al. (2003b)
examined different skin pretreatment on cranberries - cutting in halves, surface abrasion
of skin and puncturing of skin by a needle, and realized that cutting in half is the best
possible method.
2.2.2 Osmotic dehydration
The quality and processing cost are the two most important factors when choosing
a food preservation method. The application of the osmosis principle has been primarily
motivated by economical factors and the quality improvement. High moisture fruit as
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cranberries is difficult to dry in one step; it is time and energy consuming. Osmotic
dehydration is a preservation method that makes the achieving of high quality product
possible by means of water removal without phase change. Osmotically dehydrated fruits
have good retention of flavour, aroma, and nutritional content because osmotic
dehydration has small impact on the mineral content and vitamin losses; it preserves
organoleptic properties (Barbosa-Canovas and Vega-Mercado, 1996).
During the osmotic dehydration, a two-way counter-flow of mass exchange takes
place (Lenart, 1996):
•
The most important is water diffusion from a sample to surrounding hypertonic
solution.
•
Opposite stream is stream of osmotic substance (sugar, salt...) which is incoming
into fruit.
It is important to mention that water takes out soluble substances such as
saccharides, organic acids, and vitamins. However, this loss is not noteworthy, except for
minor deficit in nutritive value and organoleptic properties (Lenart, 1996). These two
flows can be seen in Figure 2.4:
PRODUCT
cell membrane
OSMOTIC
SUBSTANCE
WATER
SoluMe substances
I
(organic acids,
<£• —
saccharides. salts,
minerals,,..)
Figure 2.4: Schematic demonstration of osmotic dehydration process
10
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The application of osmotic dehydration to fruits, and in smaller quantity
vegetables, has received attention in recent years as a technique for production of
intermediate moisture foods and shelf-stable foods, or as a pretreatment prior to drying in
order to reduce energy consumption and/or heat damage (Jayaraman and Das Gupta,
1992).
Two most important factors that have influence on osmotic dehydration are
concentration of substrate and temperature. Other important factors are the type of sugar
(sucrose, maltose, fructose), external factors (pressure), and pH (Barbosa-Canovas and
Vega-Mercado, 1996).
Many scientists have done research on osmotic dehydration. Yongsawatdigul and
Gunasekaran (1996a) used high fructose com syrup (HFCS) as an osmotic agent for
dehydration of cranberries. Karathanos and Kostaropoulos (1995) treated apple slices
with two sugar solutions - glucose and sucrose. Palou et al. (1994) assessed twoparameter non exponential model by Peleg (1988) for osmotic dehydration of papaya.
Papaya was also used in work of Argaiz et al. (1994), and was dehydrated with
com syrup solids. They suggested the application of com syrup solids and maltodextrins
in production of food with reduced water activity aw since they are less sweet than other
common sugars and have relatively high molecular weight, providing higher moisture
loss and smaller sugar gain for fruits that are already sufficiently sweet. Shi et al. (1996)
concluded that osmotic dehydration of strawberries is excellent for jam processing, since
strawberries keep their original aroma, flavour, nutritive components and natural colour.
Spent syrup was also used as an ingredient to prepare pectin solution needed for jam
processing.
Lenart (1996) deduced four main advantages of using osmotic dehydration:
1. Reducing of heat applications, hence negative changes of colour and aromatic
substances are diminished in subsequent drying.
2. The cell membranes are not absolutely resistant to osmotic substance, assuring a
small flow of sugar into the cell, causing sweeter taste of dehydrated food.
3. Osmotic dehydration as a pretreatment provides shorter drying time and increases
the dryer's potential.
4. Energy consumption is smaller at a rate of 20-30 % when compared to the
convection drying.
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One of interesting attempts is the one from Shi and Maupoey (1993). They
performed osmotic dehydration of apricots and pineapples at two levels of vacuum.
Conclusion was that low pressures were beneficial in speeding up the water diffusion, but
had no influence on sugar increase. It is important to mention their remark that vacuum
osmotic dehydration is more effective for fruits with higher porosity.
Beaudry et al. (2003b) tested different sugar solutions and times for cranberry
dehydration. Advantage of this process is twofold for cranberries: the removal of water
and addition of sugars. Because of their sour taste, cranberries are seldom used in fresh
state, and for this reason sugar increase from osmotic agent is very favourable. The best
treatment was with HFCS at 1:1 fruit to sugar ratio, for 24 hours. Similar conditions were
used by Yongsawatdigul and Gunasekaran (1996) - HFCS (at 30 and 60°Brix) for 24
hours.
2.3 Drying as a Dehydration Method
The technique of dehydration is probably the oldest method of food preservation
applied by mankind. The difference between dehydration and drying - drying is removal
of moisture from the material with phase change, whereas in dehydration (e.g. osmotic
dehydration) water mostly stays in liquid form.
Drying is one of the most energy demanding unit operation, and consequently
huge amount of research must be done in order to achieve the highest possible energy
consumption. It is wise to use several operations in successive manner instead of only one
extensive, wide-ranging and excessive energy required process. Especially in food drying,
the process itself must be conducted slowly because of product nature, and energy
demands will therefore be excessive (Ratti and Mujumdar, 1996). The most energy
consuming drying method is freeze drying, but it offers product with the highest
obtainable quality, with high preservation rate of initial characteristics.
Two processes take place at the same time (Menon and Mujumdar, 1987), transfer
of energy and transfer of mass. Energy transfer can be conductive, convective, radiative,
or any combination of these three. Mass transfer includes the removal of moisture that
12
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moves from the interior of dried material towards object surface. All these processes must
be understood and scrutinised prior to any industrial or pilot-scale laboratory application.
The elimination of water from a material is accomplished usually by use of dry air
(apart from some drying methods and osmotic dehydration) which removes moisture
from the surface and carries it away (Barbosa-Canovas and Vega-Mercado, 1996). For
that reason, the properties of dry air and its mixture with water vapour must be identified.
Water is moving in dried product under capillary forces, liquid diffusion due to
concentration gradients, surface diffusion, water vapour diffusion in pores filled with air,
flow due to pressure gradient as driving force, and flow owed to a vaporisationcondensation system (Barbosa-Canovas and Vega-Mercado, 1996).
Heat supplied from external source is transferred by some of three transfer
methods to the material and the vapour pressure of material moisture is increased, causing
the diffusion of vapour to the surface. On the surface, vapour is taken away either with
flowing air (convective drying) or by itself (vacuum drying, freeze drying)
(Venkatachapalathy, 1998).
2.3.1 Drying of fruits
Artificial drying of fruits is an important method of preservation and production of
a wide variety of products, and the major aim is to prolong the fruit’s storage life.
Unfortunately, changes in the physical and biochemical structure are inevitable because
the fruit is treated with thermal, chemical and/or other treatments (Ratti and Mujumdar,
1996).
The advantages of dried fruits are following (Somogyi and Luh, 1986):
•
Almost unlimited shelf life, because inhibition of microbial and enzymatic
reactions is high.
•
Substantially lower costs of handling, transportation and storage.
•
Mineral content is practically unaffected, as well as calorie-providing constituents.
•
Providing of consistent product - seasonal variations are diminished.
•
They can be modified in size, shape and form, and price is constant throughout the
year.
•
Dried fruits are packed in recyclable packages; this is not always done with fresh
fruits.
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•
They can be used in snack products and other processed food.
The extent of final product quality damaging depends on the time-temperature
history during drying. Obviously, the lower the temperature and the shorter the time of
drying process: the higher the quality of dried product. The knowledge of sorption
properties of dried food is essential in studying of drying kinetics. The equilibrium
material-moisture is given in terms of the so-called sorption/desorption isotherms. These
are graphs that relate moisture contents in food and ambient air under constant
temperature and equilibrium status. The most appropriate option for illustration of dried
bio-material (i.e. food) moisture content is its water activity (aw). Water activity informs
us not only about the material moisture content, but incorporates other important factors,
such as material structure, type of bonds between water and material, biological activity,
water availability for microorganisms, and many others (Mousa and Farid, 2002).
Figure 2.5 represents typical sorption isotherm, and can be divided into three
sections (Kaminski and Kudra, 2000): the first section is section of strong desorptive and
chemi-sorptive bonds between water and material, the second section is characterised by
polimolecular adsorption, and in the third section water fills the pores in material
structure, hence this type of connection is not strong, based only on the mechanical
attachment (Van den Berg, 1986).
0.6
0.5
0.4
0.2
H
Isection
0.1
I section
Figure 2.5: Isotherm for water vapour sorption isotherm
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The drying process depends on heat applied to the product, which increases the
water activity within the product, but also helps in the removal of the water causing
reduced moisture, the main cause of entire process. The speed of operation is very
important, since high moisture food is perishable, it is essential to lower the moisture
content as fast as possible before any significant spoilage can occur. Therefore, drying
kinetics must be determined before every drying process. Figure 2.6 shows typical drying
curve: drying pace vs. drying time, adopted from Somogyi and Luh (1986). During the
constant rate period, water is on the surface of the product, and drying rate is dependent
on temperature, relative humidity of surrounding air, and air velocity. After this rate,
surface moisture has disappeared, and remaining moisture has to diffuse from inside
towards the surface, that is mainly influenced by material porosity (Somogyi and Luh,
1986).
Constant rate
Falling rate
s
Internal diffusion
Figure 2.6: Typical drying curve
The rate of drying during the constant rate period may be determined using both
heat and mass transfer equations. To estimate average drying time during the falling rate
period Fick's law of diffusion can be used (Jayaraman and Das Gupta, 1992).
Drying kinetics can be described with a straight line equation 2.1, which is a
simplified form of Fick's law (Jayaraman and Das Gupta, 1992):
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M -M .
„ _K,
= C-e~“
Mj - M e
(2.1)
Where:
C = constant (non-dimensional)
K = dehydration constant (h'1)
M = average moisture content (kgw/kgdm)
Mj = initial moisture (kgw/kgdm)
Me = equilibrium moisture (kgw/kgdm)
t = time (h).
2.3.2 Drying methods
Several types of dryers and drying methods have to be developed and adapted for
each specific situation, and to be commercially applicable. There are several types of
drying methods that can be divided according to many factors, such as pressure
(atmospheric,
subatmospheric),
type
of
unit
operation
(continuous,
batch,
semicontinuous), temperature (freeze drying, hot air convective) and many others.
As indicated by Jayaraman and Das Gupta (1992) and Somogyi & Luh (1986),
there are three basic types of drying processes:
1. Sun drying and solar drying.
2. Atmospheric dehydration including:
a) Stationary or batch processes (kiln, tower and cabinet dryers).
b) Continuous processes (tunnel, continuous belt, belt-through, fluidised bed,
explosion puffing, foam-mat, spray, drum and microwave-heated dryers).
3. Subatmospheric dehydration (vacuum shelf, vacuum belt, vacuum drum and
freeze dryers).
In industry, it is imperative to know what we dry and what we want to attain. This
is a prerequisite for the appropriate choice of dryer and drying conditions. Numerous
experiments have to be performed, for example establishing the data required for
planning, investigation of the efficiency and capacity of existing drying equipment,
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analysis of the effect of operational conditions on the shape and quality of the product,
and considerable study of the drying mechanism for particular product. It is unacceptable
to use solar dryer in northern hemisphere where number of sunny days is not sufficient or
to dry liquids like fruit juices in the cabinet drier.
Microwave applications in drying
Conventional drying of foods is a very slow process, reaching in some cases more
than one day. There are many suggested improvements based on upgrading of drying
operation, but few of them are in reality applied in industry (Ratti and Mujumdar, 1996).
According to Ratti and Mujumdar (1996) there are three main advantages of microwave
heating:
1. Microwave heating has profound penetrating value that is of indisputable quality,
leading to uniform heating of water all over the material depth.
2. Selective adsorption of energy by water, without dangerous heating of material.
3. Rapid response of water to heating, subsequently the control of process itself is
easier.
Microwaves are the portion of electromagnetic spectrum between far infrared and
the conventional radio frequency region. Radiation with frequencies between 300 MHz
and 300 GHz (with wavelengths ranging from 1=1 mm to 1 m) are microwaves, and the
heat that they cause microwave heating (Sanga et al., 2000).
Microwaves are convenient method to combine with other methods of drying,
such as air drying, heat pump (Jia et al., 1993) or vacuum application, but factors such as
dielectric coefficient, shape of the material and its moisture have to be considered while
drying a food by microwave method (Barbosa-Canovas and Vega-Mercado, 1996).
The
propertiesrelated to microwaves'implementations
for heating are the
dielectric properties - dielectric constant s' and loss factor s",that are interrelated with
electric permittivity s complex and loss tangent tanS:
£ = £ '-] ■ £ "
( 2 .2)
tanS = ~~
(2.3)
£'
17
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The meaning of these values is: dielectric constant s' controls the electromagnetic
field distribution within the material and represents an assessment of energy storability in
material, and loss factor s" illustrates the loss interactions and dissipation of energy into
the material. The loss factor must be determined accurately by experiment (Jia et al.,
1993). The loss tangent tanS (or dissipation factor) is the ratio of dielectric loss to the
dielectric constant. The materials with high loss tangent are heated rapidly by microwaves
(Sanga et al., 2000).
The penetration depth 5 can be calculated from following equation (Lian et al.,
1997):
2
X0
2 n
s ’ ■
2
1
(
1 +
-
I
V
-
1
s'
Where:
d - penetration depth (m)
Xu = wavelength in free space (m).
As stated by Sanga et al. (2000), there are six advantages and four disadvantages
of microwave drying compared to other methods:
1. Fast and volumetric heating - the heat is generated within the commodity.
2. Higher drying rate - the volumetric heat generation leads to higher internal
temperatures causing the increase in internal vapour pressure, which helps to push
water in liquid form to the surface.
3. Shorter drying time - this is a consequence of higher drying rate.
4. Quality of the product - the exposure to high temperature is shorter, thus the heat
sensitive components like vitamins or proteins are less damaged.
5. Reduced energy consumption - microwaves are absorbed only by dielectric
materials (water) and therefore energy is not longer used to heat the material itself,
moreover, the drying rate is higher, drying time is shorter, uniformity and
selective heating is significant.
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6. Cost savings - economy is possible in energy savings, cutback of the drying time,
increased capacity, reduced handling time and maintenance costs.
The disadvantages are the following:
1. High initial cost of purchase and installation of the drying equipment, as well as
shortage of documented energy savings.
2. There is some evidence of the aroma loss and developing of Maillard reactions
responsible for generation of colour and specific aromas.
3. Some physical damages caused by extensive local heating.
4. Specific sample size and shape are usually required, because it is difficult to dry
big size food with microwaves.
Heat is generated within a dried product. The dimensions of microwave chamber
are greater than the wavelengths of microwaves and waves are reflected of the walls,
entering the product and conveying energy to the internal moisture (Salunkhe et al.,
1991). Microwave dryers are more likely to be used for finish-drying than for complete
drying process, and can be used to equilibrate moisture of the food pieces that have
gradient of moisture (Somogyi and Luh, 1986).
Vacuum applications in dryins
The main purpose of vacuum drying is to enable the removal of moisture at lower
temperature than the boiling point under ambient condition. Water is boiling on 1 bar at
100 °C, but if the pressure is lowered to 40 mbar, boiling temperature will be 28.96 °C
(Moran and Shapiro, 1996). The important feature of vacuum drying is virtual absence of
air during dehydration, which makes this process attractive for drying of material that
may deteriorate and/or be chemically modified as a result to air or high temperature
exposure. Compared to direct dryers, in whom the product is in direct contact with the
drying medium, the vacuum dryer has a lower maximum drying temperature (BarbosaCanovas and Vega-Mercado, 1996).
All systems that have vacuum application consist of four main parts: vacuum
chamber, heat supply, vacuum producing unit (pump) and device to collect water vapour
(desiccator or condenser). Because of the high installation and operating cost, vacuum
dryers are used only for high-value materials and to dry materials with low final moisture
(Somogyi and Luh, 1986). Vacuum treatment is also useful in combination with some
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other processes, such as microwaves, osmotic dehydration (Argaiz et al., 1994), or as a
finishing drying method.
Combination o f microwave and vacuum methods in dryins o f foods
Obvious advantages of these two methods have guided many scientists to perform
food drying experiments with their combination. Clear benefit is that product temperature
will be lower comparing to other drying methods, drying time will be significantly
reduced, and as a result, the final quality of the product will be higher. Application of
microwave energy in vacuum causes an increase in product temperature, but only to the
boiling temperature of water at a given pressure.
Microwave/vacuum drying is very useful in drying of pastas, powders or porous
materials (Mousa and Farid, 2002). Kiranoudis et al. (1997) suggested following
empirical model for describing moisture transfer in microwave-vacuum drying of fruits
(linear form):
dM = k u M
dt
(2.5)
Where:
M= material moisture content (ratio)
kM- drying constant (h'1).
Minus sign indicates (-) the decrease of moisture in time.
The equilibrium moisture content of the product is considered to be zero, due to
the vacuum application. When drying conditions remain constant, this model has
analytical solution (exponential from):
M = M 0-e~ICM'1
(2.6)
Where Mo is the initial material moisture content (ratio). To determine drying
constant kM, equation 2.7 can be used - it includes two factors that are of importance microwave power level P (W) and the system absolute pressure Q (Pa):
20
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K=K-Pl'-Qk'
Where ko, kj and
(2.7)
are empirical coefficients that can be estimated by fitting the model
employed to the experimental drying curves.
Yongsawatdigul and Gunasekaran (1996a) suggested following equation to obtain
drying efficiency DE:
DEa
ton-P -(l-M ,)-1 0 “6
----f—-----mi -{Mi - M f )
(2.8)
Where:
DEa = drying efficiency (M J/kgwater)
ton = total power-on time (s)
P = microwave power input (W)
mt = initial mass (kg)
Mi = initial moisture content (ratio, wet basis)
M/= final moisture content (ratio, wet basis)
Similar approach can be found in the work of Drouzas et al. (1999), where drying
rate can be expressed by the equation:
(M0- M e)
( 2 . 10)
Where M is moisture content (ratio) after time t (s), Mo is initial moisture content (ratio),
Me is moisture content at equilibrium (ratio) and k is drying constant. For vacuum drying
it is assumed that Me=0.
Many scientists have contributed to modelling of microwave/vacuum drying;
among them are Kim and Bhowmik (1995) who studied modelling of moisture diffusivity
21
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in yogurt during MW/vacuum drying, Lian et al. (1997) observed coupled heat and
moisture transfer in MW/vacuum drying of the concentrated food paste.
Fruits and vegetables are commonly subjected to MW/vacuum drying. Clary and
Ostrom (1995) dried Thomson seedless grapes as an alternative to sulphating. They
determined the relationship among MW power, vacuum, temperature and final moisture
content of grapes and defined levels of required specific energy for attaining of grapes
with high quality. The optimum total specific energy was 0.84 to 0.88 W-h/g for 70-75
min in temperature range of 70 to 80°C, and within this range 95 % of the grapes
exhibited the integrity, puffed character, and colour of the fresh grapes.
Kim et al. (1997) modelled the survival of lactic acid bacteria - Streptococcus
salivarius and Lacto bacillus delbrueckii, a very important feature of yogurt, during
MW/vacuum process. The MW/vacuum drying showed promising results in higher
retention of lactic bacteria, besides its additional advantages - shorter drying time and
lower cost than freeze drying.
Mousa and Farid (2002) dried banana slices with MW under vacuum. They
examined the thermal and drying efficiencies during the whole period of drying.
Conclusion was that these two values are almost 100 % at the beginning of process, but
rapid drop can be observed as moisture content decreases. The effect of low pressure is
particularly important at low moisture values. Parboiled rice was dried with MW/vacuum
method by Wadsworth et al. (1989). Parboiled rice has increased moisture of over 35 %,
and this drying method showed good approach to reduce moisture to below 14 % which is
safe storage level of moisture for rice.
Chen and Chiu (1999) investigated the kinetics of volatile compound retention in
onions. They reported that microwave vacuum drying of onion simultaneously overcame
heat and mass transfer resistance with high drying rate and high volatile retention. Pere et
al. (2001) studied the amount of total absorbed power by dried material (glass beds and
pharmaceutical granules). They proposed analytical formulation that is strongly and
mostly dependent on drying mechanism, for a fixed sample geometry and the drying
mechanism.
Drouzas et al. (1999) dried pectin gels (concentrated orange juice) and declared
that combination of MW heating and vacuum drying resulted in the acceleration of drying
rate for model fruit gels. The experimental pectin gel with 38.4 % of moisture content
22
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was dried to less than 3 % of moisture in less than four minutes. However, due to the
uneven distribution of the MW energy in the MW oven, the location of material in the
cavity should be specified. Color of the dried gel was also better compared to air drying.
The same author and Schubert (1996) dried banana slices, and a high quality product was
obtained, with preserved aroma, smell and good rehydration test. They used pulsed mode
of microwaves - 10 seconds on and 20 seconds off. Compared to freeze drying, no
significant difference was found in colour, shape, aroma or taste. The water activity was
lowered to less than 0.7 that corresponds to 14 % moisture content.
Kiranoudis et al. (1997) examined the drying kinetics of apples, kiwi fruits and
pears by introducing a one-parameter empirical mass transfer model; the model was
tested with data produced in a MW oven equipped with MW apparatus. The best fit of
comparison was obtained with kiwi and worst with apples. It was found that the drying
constant was affected with MW power and vacuum pressure, in highly positive and
slightly negative manner, respectively.
Yongsawatdigul and Gunasekaran (1996a,b) dried cranberries with MW and
vacuum applications. The pretreatment comprised of testing two concentrations of sugar
syrup, and treated berries were dried using a small-scale laboratory drier. The drying
modes were continuous or pulsed. In the continuous mode two power levels were tested,
as well as one level of absolute pressure. In the pulsed mode, only one power level and
two pressure levels were tested, as well as two levels of power-on time and three levels of
power-off time. It was found that pulsed mode was more efficient than continuous mode,
lower pressure assured higher moisture evaporation, and shorter power-on and longer
power-off time proved to be the most appropriate mode. The following conclusions were
drawn:
1. MW/vacuum drying is a viable alternative for cranberry dehydration.
2. In the continuous mode, pressure of 5.33 kPa and power of 500 W provided more
favourable drying time, energy input, drying efficiency of 3.59-5.02 MJ/kg of
water, and drying rate of 4.52-12.63 kg/h-kg dry matter.
3. Pulsed MW mode is more energy efficient than the continuous. The most suitable
power-on and power-off time at 250 W were 30 s and 150 s, respectively. Drying
efficiency and drying rate of pulsed mode were 2.51-4.49 MJ/kg of water and
1.49-6.19 kg/h-kg dry matter, respectively.
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2.4 Quality Assessment
Food quality is the sum of all desirable characteristics which make a food
acceptable to eat. Quality characteristics of a product may be divided into three major
categories: sensory, hidden and quantitative (Salunkhe et al., 1991). The sensory
characteristics are color, gloss, size, shape, defects, odour and taste. Hidden are nutritive
value, presence of dangerous contaminants and poisonous materials. Quantity parameters
are those that as well contribute to overall fruit quality, such as yield of a dried product.
In order to determine the relevance of a drying method, several parameters have to
be examined through a quality evaluation. These parameters are sensory evaluation, skin
colour determination, and texture properties. Cranberry products have a general appeal
that is due to their attractive colour, fruity flavour and acidic astringent taste (Francis,
1985).
2.4.1 Sensory evaluation
Sensory appearance is of high importance, because it is the first characteristic
observed by consumer. Appearance quality includes visual sensations that can be
perceived by senses. These include (but are not limited to) size dimensions, shape, color,
uniformity and intensity, stiffness under fingers, smell (odour), taste (sweetness,
sourness...) and others. Since this is purely subjective factor, it is not consistent
throughout different experiments.
Beaudry et al. (2003c) used numerical sensory evaluation scale of 1-7, from “like
extremely” to “dislike extremely”, respectively. Venkatachalapathy (1998) worked with a
panel of ten or more untrained judges for his sensory analysis of dried strawberries and
blueberries, whereas Rennie (1999) used visual quality assessment scale ranging from 9
to 1, representing range of excellent quality to extremely poor, respectively. Tulasidas et
al. (1995b) used scoring panel of 10 judges to determine the quality attributes of
MW/convective dried grapes, and the ratings assigned were given to a scale of 0 to 5
points, where 0 is the highest quality and 5 points the poorest.
It is important to mention that judges used in test panels have to be untrained or
semi-trained, and it is recommendable to use for every test panel the same judges in order
24
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to maintain homogeneity of results. That judges should be the representative sample of
overall consumer population.
2.4.2 Skin colour determination
Colour is a human impression of light waves reflected from the surface of
material. Colour is one of the first noticed characteristics of fruit, and formerly it was
evaluated only subjectively or with use of colour comparing charts.
The most common technique to assess the colour is by colorimeter. There are
several colour scales in which the surface colour can be represented. The 3-dimensional
scale I*, a* and h* is used in Minolta chromameter. The L* is lightness coefficient,
ranging from 0 (black) to 100 (white) on a vertical axis. The a is purple-red (positive a
value) and blue-green (negative a* value) horizontal axis. Second horizontal axis is b*,
that represents yellow (positive b* value) or blue (negative b* value) colour (McGuire,
1992). This 3D colour system can be seen in Figure 2.5. The values of L*, a and b* can
be converted to hue angle (h°) and Chroma (C*) value, analogous to color saturation or
intensity (McGuire, 1992). Color difference AE can be calculated if one wants to find
difference between the sample and a previously chosen standard (McGuire, 1992).
Figure 2.7: L*a b* color three-dimensional system (HunterLab, Reston, VA)
h*
^ °= arc tan -T
a
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(2.11)
C* = j { a * f + ( b * Y
(2 . 12)
AE = V(AL * f + (Aa *)2+ (Ab * f
(2.13)
Where AL* is the difference between L* of the sample and standard, Aa* is the difference
between a of sample and standard, and Ab* is the difference between b* of sample and
standard (McGuire, 1992).
Beaudry et al. (2003c) used this scale to evaluate the colour of microwave/hot air,
conventional hot air, freeze and vacuum dried cranberries, and concluded that there is no
significant difference among different drying treatments, in spite of expected darker
colour of berries subdued to higher temperatures. Venkatachalapathy (1998) used similar
technique, but with one new value - ratio of a* and b*, where higher a*lb* ratio indicates a
darker product. He determined a significant difference between freeze dried and
MW/hot air dried strawberries, where microwave-hot air dried had lower a*lb* ratio.
Yongasawatdigul and Gunasekaran (1996b) compared MW/vacuum dried cranberries
with those conventionally dried in the same system as described above (L*, a*, b*, h°, C*,
AE), and concluded that MW/vacuum dried cranberries resulted in a better quality product
than hot-air drying method. Also, the redness (a*) decreased with storage time.
2.4.3 Texture characteristics
Texture is a way that food feels to the tongue when eaten (Medved, 1981). From a
physicochemical point of view, Jongen (2000) defines the texture in terms of cell wall
composition and structure. Texture and consistency are evident chemo-structural features
of fruits and vegetables, and are the attributes of a primary importance. Textural
characteristics can be measured by any standard procedure.
Instrumental texture analysis is an analytical procedure which subjects a sample to
known conditions (stress or strain) in a controlled manner from which mechanical
characteristics can be interpreted.
Texture analysis is primarily concerned with the evaluation of mechanical
characteristics where food is subjected to a controlled force from which a deformation
curve of its response is generated. These mechanical characteristics can be further sub-
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divided into primary and secondary sensory characteristics (Szozesniak, 1963). Primary
sensory characteristics are:
1. Hardness (soft, firm, hard)
2. Cohesiveness (crumbly, crunchy, brittle)
3. Elasticity (plastic, elastic)
4. Adhesiveness (sticky, tacky, gooey)
5. Viscosity (thin, viscous)
And secondary are:
1. Brittleness (crumbly, crunchy, brittle)
2. Chewiness (tender, chewy, tough)
3. Gumminess (short, mealy, pasty, gummy)
Instrumental procedures are generally more sensitive and reproducible than their
subjective sensory equivalents where variation in results is generally attributed to
variation in sample heterogeneity rather than instrumental precision.
According to Bruckner and Auerswald (2000), texture together with appearance
and colour is one of the most important and assessed properties. Same authors describe
the fruit firmness using a deformation test. Displacement at the force of 10 N was
measured and the pressure calculated from the puncture force applied. They used a probe
with 3.2 mm in diameter and a crosshead speed of 50 mm/min.
Many types of the instruments have been used to define texture of different types
of fruit and vegetables. All these instruments must have five vital parts: the driving
mechanism, a probe element that is in contact with sample, a sensory system for forcedirection, a sensor for type and rate of application and a system for read-out
(Venkatachalapathy, 1998).
The type and direction of force applied can be different, depending on method
applied, i.e. the probe element that is in contact with food. The most appropriate method
for a sample with many smaller particles like small fruits is Kramer shear press (Beaudry
et al., 2003a), seen in Figure 2.8. The force necessary to shear through a sample results in
combination of stresses, displayed on the read-out system.
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Figure 2.8: Kramer shear press
Typical stress-strain diagram can be seen in Figure 2.9. The straight line can be
used to determine so called Young’s modulus. Tomas Young in 1807 suggested that the
firmness of material can be determined from this straight part of a diagram by using
stress-strain ratio, i.e. Young’s modulus. This modulus is the slope of the straight line
(Riley and Zachary, 1989). This diagram is obtained on Instron Universal Testing
Machine with Kramer Shear Press as a probe element. On the display can be seen several
important parameters, such as modulus, energy required to break, and toughness.
Beaudry et al. (2003a,b) used this method and this machine for texture analysis of
osmotically dehydrated cranberries. Venkatachalapathy (1998) used aforementioned
Instron machine and puncture test for measuring the toughness of the fruit berries.
Yongsawatdigul and Gunasekaran (1996b) used a V-shaped cutter probe to test the
texture of microwave vacuum dried cranberries, previously treated with osmotic agent.
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%
e
•J
Displacement mm
Figure 2.9: Typical stress-strain diagram obtained from INSTRON machine
2.4.4 Determination of rehydration capacity of fruits
The first aim of drying is removing of water because dried food has numerous
advantages. But, before consumption of this food, it has to be rehydrated by adding of
water. Factors that affect rehydration are time, temperature, air displacement, pH and
ionic strength. Rehydration rates can be hasted by ultrasonic treatment of the rehydrated
product placed in water (Salunkhe et al., 1991).
The rehydration characteristics of the dried product are influenced by processing
conditions, sample constitution, preparation of the sample prior to rehydration and extent
of the structural and chemical changes induced by drying (Krokida and Maroulis, 2000).
Drying processes that change product composition in lesser extent are supposed to offer
better rehydration ratio of finished product. An example of these processes is freeze
drying, which offers the smallest changes in structure and therefore the best rehydration
capacity.
This phenomenon can be explained by physical shrinkage and changes in
physicochemical composition during drying at colloidal level (Potter and Hotchkiss,
1995). Proteins are denaturised and they can not completely reabsorb water.
Venkatachalapaty (1998) used equation 2.14 for calculating the coefficient of rehydration
for grapes and strawberries.
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RR = 10- m^ 'j 10Q M ‘»\
mdA-(l00 - M dh)
(2.14)
RR = coefficient of rehydration (non-dimensional)
mrh - mass of rehydrated sample (g)
mdh = mass of dehydrated sample (g)
Min = initial moisture content of the sample before drying (% wet basis)
Mdh ~ moisture content of the dry sample (% wet basis)
2.4.5 Water activity
Water activity (aw) is defined as a ratio of vapour fugacity of water over the
material (pj) to vapour fugacity over the pure solvent ifAsat) at the same temperature
(Kaminski and Kudra, 2000):
a w = -------
(2.15)
P A sat
The water activity is a function of moisture content in food and temperature (Ratti and
Mujumdar, 1996). Water connections in food can be defined by water activity (BarbosaCanovas and Vega-Mercado, 1996):
•
Tightly bound water aw<0.3
•
Moderately bound water 0.3< aw<0.7
•
Loosely bound water aw>0.7
•
Free water aw~ 1.0.
Table 2.3 is adapted from Salunkhe et al. (1991).
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Table 2.3: Food types according to moisture content
aw
Moisture content
Food
Characteristics
>0.9
> 30 %
High moisture
Soft, must be heated to prevent microbial
growth
Semi-moist, firm, prone to Maillard
0.85
20 - 30 %
Intermediate
reactions, less susceptible to fat oxidation
than low moisture food
Hard, firm, resistant to microbial growth
<0.7
< 20 %
Low moisture
and less prone to Maillard reactions, prone
to fat oxidation
Water activity is the main factor of numerous important processes, such as
microbial growth, toxin formation, and enzymatic and nonenzymatic (chemical) reactions
(Leung, 1986). Water activity values lower than 0.7 offer reliable storage of food,
because microbial growth is prevented.
Measurement of water activity implies cognition of many factors: vapour
pressure, osmotic pressure, freezing point depression, boiling point elevation,
psychrometric assessments (dew point and wet bulb depression), suction potential and
many others (Leung, 1986). It is important to mention that water activity must be
measured at a constant temperature, because the results will be compared afterwards.
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III. ASSESMENT OF DIFFERENT METHODS FOR SKIN
PRETREATMENT AND OSMOTIC DEHYDRATION OF
CRANBERRIES
3.1 Abstract
In this research different drying pretreatment methods were tested on cranberry
fruit (Vaccinium macrocarpon). Pretreatments such as mechanical and chemical were
examined, as well as osmotic dehydration. Two types of assessed mechanical
pretreatment were cutting in halves and quarters. Chemical pretreatment consisted of
testing different temperatures (23, 45, and 65°C) of chemical solution (sodium hydroxide
and ethyl oleate) and dipping times (60 and 180 seconds) for cranberries. Osmotic
dehydration involved evaluation of different osmotic agents (crystal sucrose and High
Fructose Com Syrup - HFCS), their concentrations (mass ratios of 1:1, 2:1, 3:1, and 4:1
agent to fruit), and different times of osmotic dehydration (12, 24, 36, and 48 hours).
Mechanical pretreatment that showed the best was cutting in quarters, but chemical
pretreatment showed no significant difference. The best conditions of osmotic
dehydration were HFCS as an osmotic agent, 24 hours of reaction, and 2:1 mass ratio of
HFCS to cranberries.
3.2 Introduction
Drying of fruits and vegetables is one of the most time and energy consuming
processes in the modem food industry. To reduce the process length, thus facilitating and
accelerating dehydration process, a number of suggestions were made. The main obstacle
in this food preservation method is the outer layer of a particular commodity, the skin.
The skin impedes water transport from the interior of a food product to its surface,
slowing the drying process.
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There are two main methods to reduce skin resistance to water transport chemical and mechanical. Chemical pretreatment involves dipping of a product into a
chemical solution (normally alkaline or acid solution of oleate esters) of a specific
concentration for a specific amount of time. Mechanical pretreatment consists of skin
abrasion, puncturing, or cutting the product into smaller pieces.
The most important pretreatment to drying is, without doubt, osmotic dehydration.
Osmotic dehydration is the incomplete removal of water from a food product by means of
an osmotic agent. An osmotic agent is a solution that has a very high osmotic pressure of
the hypertonic substance; most of the time it is either sugar or salt. The main advantage of
this process is its influence on the principal drying method; shortening of the drying
process, resulting in lower energy requirements. Considering that heat is not applied in
this stage, osmotic dehydration offers higher retention of initial food characteristics, such
as colour, aroma, nutritional constituents and flavour compounds.
3.2.1 Skin pretreatment
Chemical pretreatment involves immersion of the product in alkaline or acid
solutions of oleate esters prior to drying. Alkaline dipping facilitates drying by forming
fine cracks on fruit surface (Salunkhe et al., 1991), and dipping in oleate esters causes
wax platelets on the fruit skin to dissociate, helping the removal of moisture
(Venkatachalapathy, 1998).
It was determined by Ponting and McBean (1970) that, for fruits with a waxy
surface layer, the most effective treatment is with ethyl esters of fatty acids, especially
oleic acid. Saravacos et al. (1988) and Tulasidas et al. (1994) used ethyl oleate as a
pretreatment, and found that it can improve the drying rate with only a minor effect on
product quality. Venkatachalapathy (1998) used alkaline solution of 2 % EO and 0.5 %
NaOH as a pretreatment for strawberries and blueberries. Beaudry et al. (2003b) tested
different concentrations and time periods of dipping for cranberries and concluded that
these have no significant influence on subsequent osmotic dehydration.
Mechanical pretreatment might replace or complement chemical pretreatment,
mainly because consumers hesitate to buy chemically treated fruits. Mechanical
pretreatment consists of peeling, surface abrasion, and cutting in various shapes, such as
halves, cylinders, and cubes. Shi and Maupoey (1993) peeled, cored and cut apricot and
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pineapple into cubes prior to vacuum-osmotic dehydration. Kiranoudis et al. (1997) cut
apple, kiwi and pear into spherical particles before microwave-vacuum drying; Jia et al.
(1993) sliced carrots prior to heat pump-microwave drying.
Mechanical pretreatment of cranberries is also recorded in Yongsawatdigul and
Gunasekaran (1996a), where they cut cranberries in halves. Beaudry et al. (2003b)
examined different skin pretreatment techniques on cranberries: cutting into halves,
abrading surface of skin and puncturing of skin by needle, and demonstrated that cutting
in half is the best possible method.
3.2.2 Osmotic dehydration
Quality and processing costs are the two most important factors when choosing a
food preservation method. Economical factors and quality improvement have primarily
motivated application of the osmosis principle. Drying of high moisture fruit, such as
cranberries, is time and energy consuming because such fruit is difficult to dry in one
step. Osmotic dehydration is a preservation method that offers a high quality product by
means of water removal without phase change. Osmotically dehydrated fruits have a good
retention of flavour, aroma, and high nutritional content because osmotic dehydration has
low influence on mineral content and vitamin loss; it preserves organoleptic properties
(Barbosa-Canovas and Vega-Mercado, 1996).
During osmotic dehydration, a two-way counter-flow of mass exchange takes
place (Lenart, 1996):
-
More important of these is water diffusion from sample to a surrounding
hypertonic solution;
-
Less important is opposite stream, the stream of osmotic substance (sugar, salt...)
which incomes into fruit.
It is important to mention that water removes water-soluble substances, such as
saccharides, organic acids, vitamins... However, this loss is not significant, except for a
minor deficit in nutritive value and a small change in organoleptic properties (Lenart,
1996).
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3.3 Objectives
The objectives of the present study were to estimate optimal conditions for the
chemical and mechanical skin pretreatment of cranberries, as well as the different
conditions of osmotic dehydration prior to drying. The hypothesis is that with a sound
combination of chemical and mechanical pretreatments, and with a good choice of factors
influencing osmotic dehydration, cranberry drying can be performed with significant
improvements compared to cranberry dried without such pretreatment.
3.4 Materials and Methods
Each test performed here was performed on thawed cranberries (Vaccinium
macrocarpon) of the Stevens cultivar. These cranberries were cultivated and harvested
from sandy and organic soils. Prior to each experiment, the cranberries were thawed by
immersion in water at room temperature (23±1°C) for one hour before being used for
tests.
3.4.1 Chemical and mechanical pretreatment
Chemical pretreatment
Being one of the most important pretreatments, chemical pretreatment was tested
for multiple variables, such as time of immersion and the temperature of the particular
chemical applied. Tests were performed using a solution of 2 % EO and 0.5 % NaOH
(% mass basis) in distilled water. This concentration of EO and NaOH was recommended
by Beaudry et al. (2003b). Liquid EO was previously kept in a freezer at -20°C, and
granular NaOH at ambient temperature.
After thawing, cranberries were wiped with soft tissues and immersed in prepared
alkaline EO solution for a specific time and at a specific temperature. Two levels of
dipping time were tested: 60 and 180 seconds, and three temperatures of solution:
ambient (23), 45, and 65°C. All experiments were performed four times. Afterwards,
cranberries were rinsed with warm tap water (approximately 40°C) and wiped again with
tissues. Then, for all chemical treatment method combinations, the same mechanical
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pretreatment (cut in quarters) and the same osmotic dehydration method (24 hours, room
temperature, high fructose com syrup (HFCS) with 1:1 and 2:1 concentrations) were
followed.
After 24 hours, the cranberries were rinsed of HFCS syrup, wiped with tissues and
air-dried (placed on a table) for 15-20 minutes to remove surface moisture. At that
moment mass was recorded and the final moisture content was determined by placing
cranberries in an oven set at 70°C, until sample mass became constant (Boland, 1984).
Mechanical vretreatment
Mechanical pretreatment has a profound effect on the later drying process. There
are several methods that can be applied (Beaudry et al., 2003b), for example, puncturing
the skin by needle, cutting the berry in halves or quarters, or abrading skin surface. All
these mechanical pretreatments are used to increase the “active” skin surface where water
can penetrate.
Upon evaluation of optimal chemical pretreatment (pretreatment that offers the
highest moisture loss in subsequent osmotic dehydration), two mechanical pretreatments
were compared to the standard (no mechanical pretreatment). The pretreatments were
cutting the berries in halves and in quarters with a stainless steel knife. All other
parameters were the same: no chemical pretreatment, osmotic dehydration as described
above (24 hours, room temperature, HFCS with 1:1 and 2:1 concentrations). All
experiments were performed four times to assure sound data analysis.
3.4.2 Osmotic dehydration
Once optimal chemical and mechanical pretreatments were determined, different
factors of osmotic dehydration were tested. These factors and their levels are:
• Type of sugar agent (crystal sucrose, HFCS)
• Concentration of sugar agent (mass ratio of fruit to sugar 1:1,2:1,3:1, and 4:1)
• Time of osmotic dehydration (12, 24, 36, and 48 h)
Crystal sucrose was commercially available special fine granulated sugar, and
HFCS was high fructose com syrup - Invertose 2655, at 77±l°Brix. All tests were
performed four times at ambient temperature (23±1°C).
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After defined time of dehydration, cranberries were removed from sugar solution
and rinsed under warm tap water (app. 40°C), gently wiped with a soft tissue and left for
15-20 minutes in ambient air in order to remove surface moisture. Mass at that moment
was recorded, the cranberries were placed in an oven at 70°C until their mass became
constant, and then the mass was recorded again, as recommended by AOAC standards
(Boland, 1984).
After osmotic dehydration, all pretreatment parameters were identified and it
became possible to establish the appropriate pretreatment methods prior to drying.
3.4.3 Quality evaluation
Numerous quality parameters were monitored during this experiment:
•
Initial and final moisture contents
•
Initial and final sugar content in °Brix for sample and HFCS
•
Mass loss in %
•
Solids gain in %
•
Moisture loss in %
Initial and final moisture contents of the sample were determined as described
above, using the following equation:
M = ^ L I^ l.1 0 0
m1
(3.1)
where:
M= moisture content (%, wet basis),
mj = mass before drying (g), and
m2 = mass after drying (g).
Initial and final sugar content in °Brix of samples and HFCS solutions were
determined with a hand-held refractometer (Fisherbrand by Fisher Scientific, Nepean,
Ontario). The fruits were pressed in order to obtain one drop of juice, used for measuring.
Mass loss, solids gain and moisture loss were calculated using the following
equations (Beaudry et al., 2003b):
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.,
. total mass after osmosis - initial mass before osmosis . . .
Mass gain = ---------------------------------------------------------------- 100
initial mass before osmosis
(3.2)
0 ..,
. mass solids after osmosis - mass solids before osmosis , „.
Solids gain = ------------------------------------------------------------------ 100
initial mass before osmosis
(3.3)
,, . x ,
initial moisture content - final moisture content . . .
Moisture loss = --------------------------------------------------------- 100
initial moisture content
(3.4)
These three equations are quantitative description of component transfer under osmotic
dehydration.
3.4.4 Experimental design
All experiments were performed in four replicates in order to assure better
analysis of the statistical data. The data were subjected to the analysis of variance
(ANOVA) and to Duncan’s multiple range tests for pairwise comparison of each variable.
Differences were determined as significant or non-significant at a significance level of
0.05 in all cases.
3.5 Results and Discussion
Each parameter in each method will be observed separately, in order to facilitate
understanding of its significance.
3.5.1 Chemical and mechanical pretreatment
Chemical vretreatment
Chemical pretreatment method of skin showed significant influence for the
majority of observed parameters. But, considering that observed parameters for standard
(no chemical pretreatment) were nearly always between parameters for other methods, it
can be concluded that chemical pretreatment has no significant influence on improving
water transfer during osmotic dehydration of cranberries. Two levels of time (60 and 180
seconds) and three temperatures of NaOH+EO solution (room, 45±1 and 65±1°C) were
tested under constant conditions (cut in quarters, HFCS with 2:1 ratio sugar to fruit, for
24 hours of osmotic dehydration).
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In Table 3.1 average soluble sugar contents in °Brix of HFCS and cranberries at
the start and at the end of experiment, as well as the initial and final moisture contents of
cranberries for each method are presented.
Table 3.1:
Average change in sugar and moisture content for chemically treated and
osmotically dehydrated cranberries under the same conditions (HFCS 24 h,
mass ratio 2:1, room temperature)
Chemical pretreatment
Temperature
Sugar (°Brix)
Cranberries
Moisture (%)
HFCS
Initial
Final
69.4b
88
46.59b
77
63.5e
88
45.91
39c
77
73.5a
88
50.68a
6
54.2a
77
67.5 ,c
88
48.87a
60
6
52.1a,b
77
64.3d’e
88
45.22b,c
180
6
54.5a
77
66.6
88
44.7
6
52.3a,b
77
66.4d,c
88
43.45°
Time (sec)
Initial
Final
Initial
Final
60
6
47.5b
77
180
6
48.3
60
6
180
CQ
23
45
65
Standard - no chemical
pretreatment
Duncan groupings: Means with the same letters are not significantly different
It can be observed from Table 3.1, that it is possible to obtain with appropriate
osmotic dehydration treatment almost 50 % of moisture loss - from 88 % of initial
moisture to 44.7 % of final moisture in the last method.
But, the most important parameters for determination of effectiveness for the
aforementioned methods are the following three parameters: mass gain, solids gain and
moisture loss. Their change can be seen in Table 3.2:
39
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Table 3.2:
Change in mass gain, solids gain and moisture loss of cranberries treated
with different chemical method and osmotically dehydrated under similar
conditions
Method o f chemical pretreatment
Parameters observed
Temperature
CC)
Time (sec)
Mass gain (%>)
Solids gain (%)
Moisture loss (%)
60
6.99a
32.86°
47.07b,c
180
5.62a
45.143,
47.83a’
60
8.96a
41.72a,b
42.41d
180
17.47
48.103’
44.47
60
4.53a
45,29s’b
48.61a,b
180
14.48
51.33a
46.7 l b’°
5.18a
47.8a,b
50.623
23
45
65
Standard - no chemical
pretreatment
Duncan groupings: Means with the same letters are not significantly different
Higher moisture loss for standard (50.62 %) than for any of the chemically
treatments is not statistically significant, and this study confirmed the results from similar
experiment from Beaudry et al. (2003b) - chemical pretreatment has no significant
influence on quantitative parameters that describe water exchange between osmotic agent
and fruit. The majority of parameters had similar results as standard sample (no chemical
pretreatment). Therefore, in further experiments chemical treatment will not be used.
Mechanical pretreatment
The purpose of this method is to increase available surface for water to depart
from a produce. There are several methods of mechanical pretreatment, but only two were
tested here - cutting into halves and cutting into quarters with a stainless steel knife.
These two methods were compared with the standard - whole berries, with intact skin.
The reason why only two methods were tested here is because a similar study was
performed by Beaudry et al. (2003b), and it was determined that cutting into halves has
significant difference compared to skin surface abrasion and puncturing the skin with
needle. Therefore, cutting of the berry into halves was tested against cutting into quarters,
for the same observed parameters. All other conditions were absolutely identical: no
40
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chemical pretreatment, osmotic dehydration in HFCS (sugar to fruit mass ratio 2:1) for 24
hours at room temperature (23°C).
The initial and final Brix degree of cranberries and HFCS, as well as moisture
content of cranberries can be seen in Table 3.3.
Table 3.3:
Average change in sugar content and moisture content for mechanically
treated and osmotically dehydrated cranberries under same conditions
Sugar (°Brix)
Mechanical pretreatment
Cranberries
Moisture (%)
HFCS
Initial
Final
58c
88
59.75'
77
66.4b
88
43.45'
77
IT
88
88a
Initial
Final
Initial
Final
Cutting into halves
6
32.5b
77
Cutting into quarters
6
52.3a
Standard - whole berries
6
6C
Duncan groupings: Means with the same letters are not significantly different
Table 3.4: Change in mass loss, solids gain and moisture loss of cranberries treated with
different mechanical methods and osmotically dehydrated under same
conditions
Parameters observed
Mass gain (%)
Solids gain (%)
Moisture loss (%)
Cutting into halves
-4.163
27.37b
31.76b
Cutting into quarters
5.18 b
47.8a
50.62a
Standard - whole berries
0a
0C
0C
Duncan groupings: Means with the same letters are not significantly different
Table 3.4 confirms that mechanical treatment has significant influence on water
transfer, and that cranberries cut in quarters have maximum “active” surface for water and
sugar exchange. This method offers the highest mass gain, the highest solids (sugar) gain,
and most importantly, the highest moisture loss - over 50 % of the initial moisture was
removed in osmotic dehydration, reducing initial moisture content from 88 % to a final
43.45 %.
41
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The parameter “solids gain” in % is especially important for fruits such as
cranberries, because fresh cranberries are very sour, and their tart taste is an obstacle for
fresh consumption - they have to be processed, or, as in this case, sweetened.
Mechanical pretreatment results confirmed previous studies from Beaudry et al.
(2003b) and Venkatachalapathy (1998) that this method is a very important and
influential pre-drying fruit preparation. Two tested levels, cutting into halves and into
quarters, demonstrated significant difference and thus mechanical pretreatment of cutting
into quarters will be used in subsequent experiments.
3.5.2 Osmotic dehydration
The main pretreatment method is osmotic dehydration, or placing of berries into
some hypertonic agent. This osmotic agent is usually some hypertonic solution with high
osmotic pressure. In this research, the osmotic agents were HFCS and granular sugar
(sucrose).
A number of treatment parameters were tested for osmotic dehydration - osmotic
dehydration time, osmotic agent type, and osmotic agent concentration. During testing of
a particular parameter, all other parameters were maintained constant, in order to ensure
commensurability of the obtained results. Each of these three factors will be examined
separately.
Osmotic dehydration time
Four time periods of osmotic dehydration were applied here: 12, 24, 36 and 48
hours. All other conditions were constant: no chemical pretreatment, cut in quarters,
osmotic dehydration with HFCS agent (syrup to fruit ratio 2:1) for 24 hours at ambient
temperature (23 °C).
Time of osmotic dehydration had a significant effect on observed parameters,
which can be seen in Table 3.5 and Figure 3.1:
42
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Average change in sugar content and moisture content for cranberries at
different times of osmotic dehydration (HFCS, 2:1)
Table 3.5:
Sugar (°Brix)
Time of osmotic dehydration
HFCS
Cranberries
(hours)
Moisture (%)
Initial
Final
61.55°
88
53.57a
77
66.4a
88
43.45°
50.2a
77
63. lb
88
45.75b
54.7a
77
64. lb
88
41.44d
Initial
Final
Initial
Final
12
6
42.6b
77
24
6
52.3a
36
6
48
6
Duncan groupings: Means with the same letters are not significantly different
60
i
|
Mass gain
' Solids gain
Moisture loss
o ----------------------------------------------------1--------------------------------------------------1--------------------------------------------------1--------------------------------------------------1--------------------------------------------------1--------------------------------------: ---------- 1
0
10
20
30
40
50
60
T im e (h o u rs)
Figure 3.1:
Average change in mass gain, solids gain and moisture loss in time for
osmotic dehydration during dehydration with HFCS, 2:1 mass ratio HFCS
to fruit
Duncan groupings: Means with the same letters are not significantly different
43
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Time has a significant effect on solids gain (sugar uptake) and moisture loss;
while for the mass gain time has no effect. However, considering that % of moisture loss
is the most significant parameter of these three, and that osmotic dehydration for 24 hours
had the highest percentage of moisture loss, this time of 24 hours is recommended for
subsequent experiments.
There is also another very important factor to consider: with respect to quality,
samples subjected to longer duration (>24 hours) became darker and emitted an
unpleasant odour, but the samples used for 24 hours did not show these changes.
Osmotic aeent type and its concentration
Two osmotic agents were tested: granular sucrose and HFCS. Four levels of
sucrose concentration and two levels of HFCS concentration were applied. All other
parameters remained constant during experiment: no chemical pretreatment, cranberries
cut in quarters, and experiment duration of 24 hours.
Table 3.6 denotes initial and final average values of Brix content for cranberries
and HFCS (where applicable), as well as moisture content.
Table 3.6:
Average change in sugar and moisture content of cranberries with different
osmotic agent and its concentration during treatment of 24 hours
Osmotic agent
Type
Sucrose
Sugar (°Brix)
Mass ratio
Moisture (%)
HFCS
Cranberries
Initial
Final
NA
88
51.91°
NA
NA
88
52.45°
23.5d
NA
NA
88
58.87b
6
23.2d
NA
NA
88
64.34a
1:1
6
43.5b
77
52.5b
88
52.42°
2:1
6
52.3a
77
66.4a
88
43.45d
Initial
Final
Initial
Final
1:1
6
35c
NA
2:1
6
36c
3:1
6
4:1
HFCS
Duncan groupings: Means with the same letters are not significantly different
44
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Table 3.7:
Average values of mass gain, solids gain and moisture loss for osmotically
dehydrated cranberries using different osmotic agent type and concentration
Osmotic agent
Parameters observed
Concentration
Type
(mass ratio)
Mass loss (%)
Solids gain (%>)
Moisture loss (%)
Sucrose
1:1
-15.71°
28.71°
41.01b
Sucrose
2:1
-9.15°
31.17°
40.4b
Sucrose
3:1
-25.25b
18.78d
33.11°
Sucrose
4:1
-47.72a
6.66°
26.89d
HFCS
1:1
2.95d
37b
40.44b
HFCS
2:1
5.18d
47.8a
50.62a
Duncan groupings: Means with the same letters are not significantly different
The noticeable advantages of HFCS as osmotic agent over granular sucrose can be
seen in the Table 3.7. There are many reasons that can explain this fact, such as higher
mobilily of viscous liquid HFCS compared to solid crystals of granular sugar. Another
reason may also be that the essential component in granular sugar is sucrose
(disaccharide), and in HFCS, fructose (monosaccharide). The size of a molecule may also
have an influence on the permeability and mobility of the molecule. This is confirmed in
works of Karathanos and Kostaropoulos (1995) and Lerici et al. (1985). In both studies
the advantage of monosaccharide (glucose or fructose, respectively) against disaccharide
(sucrose in both experiments) was proven. Argaiz et al. (1994) determined that sugars
with lower molecular weight (glucose) have more profound effect on water activity
depression than polysaccharides (sucrose, maltodextrines) at identical moisture content.
Contreras and Smyrl (1981) concluded that the solid uptake is inversely correlated with
the size of the molecule of osmotic agent.
Mass ratio of agent to fruit also had significant influence on osmotic dehydration.
As expected, higher ratio offered higher moisture removal and higher sugar uptake for
HFCS, but this was not the case with sucrose. Higher ratio of sugar to fruit caused
unanticipated results; lower moisture loss and higher mass loss. One possible reason for
this phenomenon could be a totally different mass transport mechanism occurring for
45
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granular sugar to cranberries when compared to the mechanism occurring during mass
exchange between liquid HFCS and cranberries.
Osmotic dehydration, as the most influential pretreatment on water removal
process had to be tested more profoundly. Three factors (duration of process, agent type
and concentration) were tested with multiple levels. All three aspects had significant
effect on the osmotic dehydration process at a significance level of 0.05. One
combination can be selected as optimal: osmotic dehydration for 24 hours, HFCS as
osmotic agent, and concentration of 2:1 (mass ratio of HFCS to fruit).
3.6 Conclusions
Different methods of pretreatment can have significant influence on subsequent
drying process. In this research, several techniques of pretreatment with various levels
were tested one against another. The effect of different pretreatments can be summarised
as follows:
1. Chemical pretreatment had no significant influence on water transfer
2. Mechanical pretreatment showed substantial increase in moisture loss and sugar
uptake, because the “active” surface area for mass transfer is higher
3. All three factors of osmoticdehydration
(processduration, agent
concentration of sugar solution) showed significant influence,
type and
but the best
conditions cannot be applied due to diminished organoleptic properties such as
aroma, smell, and taste.
Finally, one particular combination of all parameters and pretreatments was
chosen for consequent microwave/vacuum drying experiments:
•
No chemical pretreatment
•
Cutting into quarters as mechanical pretreatment
• HFCS as osmotic solution
• 2:1 mass ratio of HFCS to fruit
•
Duration of osmotic dehydration: 24 hours
46
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3.7 Acknowledgements
The authors of this paper gratefully acknowledge the Natural Sciences and
Engineering Research Council of Canada (NSERC) for their financial support, to Dr
Valerie Orsat, and Mr. Timothy John Rennie for their practical assistance.
3.8 References
Argaiz, A., Lopez-Malo, A., Palou, E., and J. Welti. 1994. Osmotic Dehydration of
Papaya with Com Syrup Solids. Drying Technology. 12(7): 1709-1725.
Barbosa-Canovas, G.V. and H. Vega-Mercado. 1996. Dehydration of Foods. Chapman &
Hall, New York, NY. 330 pp.
Beaudry, C., Raghavan, G.S.V., Ratti, C., and T.J. Rennie. 2003b. Optimisation of skin
pretreatments and osmotic dehydration for cranberries. Submitted for review:
Food Research International.
Boland, F.E. 1984. Fruits and Fruit Product. In: AOAC Official Methods of Analysis.
Edited by Horwitz, W. AOAC, Washington, 413-418.
Bolin, H.R., Huxsoll, C.C., Jackson, R., and K.C. Ng. 1983. Effect of osmotic agents and
concentration on fruit quality. Journal o f Food Science. 48:202-205.
Contreras, J.E. and T.C. Smyrl. 1981. An Evaluation of Osmotic Concentration of Apple
Ring Using Com Syrup Solids Solutions. Canadian Institute o f Food Science and
Technology. 14:310-315.
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Fito, P., Chiralt, A., Barat, J., Salvatori, J.D., and A. Andres. 1998. Some Advances in
Osmotic Dehydration of Fruit. Food Science and Technology International.
4(5):329-338.
Geyer, S., Sunjka, P., and G.S.V. Raghavan. 2003a. Osmotic Dehydration and Microwave
Drying of Guava Fruit, Part I: Optimization of Osmotic Dehydration Parameters.
Submitted for review: The West Indian Journal o f Engineering.
Jia, X., Clements, S., and P. Jolly. 1993. Study of Heat Pump Assisted Microwave
Drying. Drying Technology. 11(7): 1583-1616.
Karathanos, V.T. and A.E. Kostaropoulos. 1995. Air-Drying Kinetics of Osmotically
Dehydrated Fruits. Drying Technology. 13(5-7):1503-1521.
Kiranoudis, C.T., Tsami, E., and Z.B. Maroulis. 1997. Microwave Vacuum Drying
Kinetics of Some Fruits. Drying Technology. 15(10):2421-2440.
Lenart, A. 1996. Osmo-Convective Drying of Fruits and Vegetables: Technology and
Application. Drying Technology. 14(2):391-413.
Lerici, C.R., Pinnavaia, G., Dalla Rosa, M., and L. Bartolucci. 1985. Osmotic dehydration
of Fruit: Influence of Osmotic Agents on Drying Behaviour and Product Quality.
Journal o f Food Science. 50:1217-1219.
Palou, E., Lopez-Malo, A., Argaiz, A., and J. Welti. 1994. The Use of Peleg’s Equation to
Model Osmotic Concentration of Papaya. Drying Technology. 12(4):965-978.
Ponting, J.D. and D.M. McBean. 1970. Temperature and dipping treatment effects on
drying rates and drying times of grapes, prunes, and other waxy fruits. Food
Technology. 24:1403-1406.
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Price, W.E., Sabarez, H.T., Storey, R., and P J. Back. 2000. Role of the Waxy Skin Layer
in Moisture Loss during Dehydration of Prunes. Journal o f Agricultural and Food
Chemistry. 48(9):4193-4198.
Salunkhe, D.K., Bolin, H.R., andN.R. Reddy. 1991. Storage, Processing, and Nutritional
Quality of Fruits and Vegetables, Volume II: Processed Fruits and Vegetables, 2nd
edition, CRC Press, Boca Raton, FI. 190 pp.
Saravacos, G.D., Marousis, S.N., and G.S. Raouzeos. 1988. Effect of ethyl oleate on the
rate of air drying of foods. Journal o f Food Engineering. 7:263-270.
Shi, X.Q. and P.F. Maupoey. 1993. Vacuum Osmotic Dehydration of Fruits. Drying
Technology. 11 (6): 1429-1442.
Tulasidas, T.N., Raghavan, G.S.V., Kudra, T., Gariepy, Y., and C. Akyel. 1994.
Microwave drying of grapes in a single mode resonant cavity with pulsed power.
Presented at ASAE meeting in Atlanta, Georgia. Paper No 94-6547. ASAE, 2950
Niles Rd., St. Joseph. MI 49085-9659. USA.
Venkatachalapathy, K. 1998. Combined Osmotic and Microwave Drying of Strawberries
and Blueberries. Ph. D. Thesis for Agricultural & Biosystems Engineering,
McGill University. 170 pp.
Yongsawatdigul, J. and S. Gunasekaran. 1996a. Microwave-Vacuum Drying of
Cranberries: Part I. Energy Use and Efficiency. Journal o f Food Processing and
Preservation. 20:121-143.
49
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CONNECTING TEXT
In the following chapter, microwave/vacuum drying of cranberries was evaluated.
After optimization of mechanical pretreatment and osmotic dehydration of cranberries,
the next step was to evaluate different process parameters during microwave/vacuum
drying. These drying parameters were microwave power level, microwave power mode
(continuous or pulsed), and absolute pressure applied. The main hypothesis was that high
quality, shelf-stable final product (with moisture of 15 %, wet basis) can be obtained with
microwave/vacuum drying.
It should be noted that the best pretreatment method established in Chapter III
gave cranberries with moisture content of 44 %, and with the same method in the
following chapter it was possible to achieve only moisture content decrease up to 55 %.
The reason for this is different size of the sample: in Chapter III sample size was only 10
g (single-layer osmotic dehydration), and in Chapter IV in order to perform all drying
experiments, chosen sample size was 400 g, resulting in a thoroughly different mass
transport mechanism.
50
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IV. MICROWAVE/VACUUM DRYING TECHNIQUE ON
CRANBERRIES
4.1 Abstract
Mechanically pretreated and osmotically dehydrated cranberries (Vaccinium
macrocarpon) were dried using microwaves (MW) as an energy source, under
subatmospheric pressure. Two MW modes were tested (continuous and pulsed), two
combinations of pulsed MW mode (30 s on/30 s off, 30 s on/45 s off), and three MW
power levels (1.00, 1.25, and 1.50 W/g of initial sample mass). Three vacuum, levels
were compared one to another (3.4, 18.6, and 33.8 kPa of absolute pressure). Several
quality parameters such as time of drying, colour, toughness and others were compared in
order to evaluate different drying conditions. Methods with higher overall MW input and
longer power-off time combined with high vacuum offered high quality dried berries.
4.2 Introduction
One of the requirements of fruit preservation is to prolong shelf life as much as
possible; if possible for the whole year until the next harvest. Another demand is the
maintenance of original fruit properties to the fullest extent, such as appearance, taste,
smell, aroma, nutritional content and many others. Fruit dehydration is one of many
preservation methods used, and drying is the most widely used dehydration method.
The use of microwaves in drying of fruits has been increased in the last few
decades, and it is mainly due to the easy process control, good microwave penetration
into fruit tissues causing volumetric heating, and shorter processing times (Sanga et al.,
2000). Vacuum drying assumes low process temperatures and faster water evaporation,
offering shorter drying times and higher quality of dried product compared to drying
method without vacuum.
51
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4.2.1 Microwave/vacuum drying of food materials
Combination of microwaves as a heat source and subatmospheric pressures has
been a topic of many studies in food processing. Both methods have many advantages,
which can be summarized as follows:
•
“Targeted” heating, i.e. heating of water molecules inside of the product and
therefore heat damage to surrounding tissues is reduced to minimum
•
Instantaneous energy transfer from microwaves to water molecules
•
Lower temperature of process, due to reduction of water boiling temperature with
reduction of absolute pressure
Fruits and vegetables have been used as a material for microwave/vacuum drying
in many studies. Lin et al. (1998) compared quality parameters such as colour, density,
rehydration properties, textural characteristics and nutritional value of carrot in three
methods of drying: microwave/vacuum, hot-air and freeze drying. In all parameters,
MW/vacuum drying showed better than air drying, and equal or better than freeze drying
except in rehydration potential, nutritive value, and appearance. Clary and Ostrom (1995)
dried grapes of Thomson seedless variety under MW/vacuum and concluded that this
method is a viable alternative to conventional air drying of grapes. Bananas were dried in
study of Drouzas and Schubert (1996) using MW and subatmospheric pressure. MW were
used in pulsed mode (10 s on/20 s off), which can slow down burning of a dried product
if continuous mode is used. They proposed equation 4.1 for drying rate:
(M -M j
(M„ - M , )
Where:
M —moisture after time t (ratio)
Me - equilibrium moisture (ratio)
M0 = initial moisture (ratio)
kM= drying constant (If1)
t = time (h)
And the drying constant can be found as:
52
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(4.1)
Where:
ko = empirical constant (min'1), can be estimated by nonlinear regression of the
experimental drying data (Kiranoudis et al. 1997),
Q, Qo= operating and reference pressures (Pa)
P, P0 ~ operating and reference MW power outputs (W)
m ,n = empirical coefficients
Apple, kiwi and pear were MW/vacuum dried in a study by Kiranoudis et al.
1997, and the conclusion is that MW power had significant influence on model
parameters, but this wasn’t the case with vacuum applied. Drouzas et al. (1999) studied
drying kinetics of MW/vacuum drying in fruit gels, and confirmed that drying rate
increased with higher power input and lower absolute pressure in vacuum drying. They
compared the colour of MW/vacuum dried fruit gel with MW/convective dried gel, and
concluded that MW/vacuum drying offered better colour properties of dried product.
Drying of grains was investigated in the study of Wadsworth et al. (1989), where
parboiled rice was dried with MW/vacuum method in order to decrease moisture from
35 % to 14 %. Drying rate and drying efficiency were significantly influenced by MW
power level and vacuum applied. This method of drying is particularly suitable for the
sensitive products, such as spices. Yousif et al. (2000) compared oregano dried using
MW/vacuum, air and freeze drying method. MW/vacuum and freeze dried oregano
showed no significant difference when compared to the fresh oregano plant, whereas airdried samples were darker and with reduced flavour volatiles. Onion is one of appropriate
commodities to be dried under vacuum and MW, because it is very sensitive to high
temperatures and volatile compounds are easily lost in convective drying. Chen and Chiu
(1999) dried onion at 600 W microwave power under 100 torr of absolute pressure, and
deduced that this method of drying can overcome heat and moisture transfer resistance,
have a fast drying rate and high volatile retention.
Yogurt is one of the agricultural products that can be dried using MW and
vacuum. The effective moisture diffusivity was studied (Kim and Bhowmik, 1995) in
53
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yogurt drying using the method of slopes of the drying curve. This method of drying was
a good choice for this type of moisture diffusivity estimation. Kim et al. (1997) used
MW/vacuum drying to dry yogurt and observed lactic acid bacteria (one of quality
parameters for yogurt) survival. The predicted survival of bacteria agreed with
experimental data, and drying at low temperatures offers a high retention of lactic acid
bacteria in yogurt.
Gunasekaran (1999) in his study of pulsed MW/vacuum drying showed results of
several authors for MW/vacuum drying. He recommended that pulsed MW drying can be
more energy efficient than continuous, and that shorter power-on time and longer poweroff time can improve product quality and overall energy efficiency. Proposed factor for
determination of drying type is the pulsing ratio:
n
Cycle Power - on Time + Cycle Power - off Time
Cycle Power - on Time
PK —----------------------------------------------------------------------
( 4 .3 )
For PR= 1, the drying method is continuous, higher PR signifies shorter power-on and
longer power-off times of MW power input.
Microwave/vacuum drying of cranberries has been the main topic of research in
the study of Yongsawatdigul and Gunasekaran (1996a,b), where they used osmotically
pretreated cranberries, and dried them under continuous and pulsed MW power mode,
using two levels of absolute pressure. Pulsed mode was more energy effective than
continuous, and gave the product with higher quality parameters such as colour, texture,
rehydration properties and others. These quality parameters showed better when
compared to air dried berries and store-bought. Mechanically pretreated and osmotically
dehydrated cranberries were dried using different methods (Grabowski et al., 2002).
These methods were freeze drying, vacuum drying, and convective drying in four dryer
types. The criteria for comparison among these methods were energy consumption and
product quality. Regarding product quality, freeze drying showed the best characteristics
in anthocyanins content, rehydration ratio, colour and taste. Energy consumption was
higher in vibrated fluid bed and the pulsed bed dryer than in other dryer types.
54
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4.3 Objectives
The objectives of this study can be summarized as follows:
•
To establish the best drying conditions for cranberries using microwaves as an
energy source, with application of low absolute pressure in order to reduce
process temperature.
•
To compare the best cranberries obtained with MW/vacuum drying against those
obtained in convective and MW/convective drying with similar process
conditions.
The hypothesis is that with microwave/vacuum drying can lead to a better quality
product, with good retention of original fruit characteristics.
4.4 Materials and Methods
4.4.1 Cranberry pretreatment
Mechanically pretreated (cut in quarters) thawed cranberries of Stevens cultivar
harvested by hand in Quebec were subjected to osmotic dehydration with High Fructose
Com Syrup for 24 hours. The factors of osmotic dehydration process were: 2:1 mass ratio
of syrup to fruit, HFCS used was 77 °Brix - Invertose 2655, and process temperature was
23±1°C. Initial moisture of cranberries is 88 %, and during osmotic dehydration it has
reduced to 55±1 %.
4.4.2 Microwave/vacuum drying
In this study the influence of several process parameters on the quality of final
product will be tested. Parameters such as MW power level, MW power mode, and the
pressure applied. Microwave/vacuum drying equipment is shown in Figure 4.1:
55
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Desiccator
PC
Vacuum meter
Vacuum pump
MW absorber -
MW meters
--------|Li Tuning screws
MW generator i
“7
/
Vacuum chamber
MW chamber
jI
Data collector
Figure 4.1: Schematic representation of MW/vacuum drying equipment
After osmotic dehydration, samples of 100±1 g were dried. This sample size was
used in order to facilitate subsequent calculations. Moisture of these samples was
55±1 %. Samples were placed in a thick-wall glass jar, used as a vacuum chamber. This
container was placed on a balance inside of MW chamber. Container was connected with
a desiccator and vacuum pump (John Scientific Inc., Canada). Desiccator with anhydrous
CaS0 4 served to remove the moisture, because it is dangerous if moisture enters the
pump. The real-time temperature of a sample was monitored using the optical fibre
(Fisher Scientific, Canada). A laboratory-scale microwave oven was modified and used to
perform the tests. Microwaves were generated by a 750 W, 2450 MHz microwave
generator, whose power can be modulated, and traveled through rectangular wave-guides
to the microwave cavity. A circulator ensured the absorption of reflected microwaves
within the main cavity and tuning screws inserted in the top of the wave-guide assembly
permitted to maintain the reflected power around zero during the process.
Three power levels were tested: 100, 125 and 150 W, or 1.00, 1.25 and 1.5 W/g of
initial sample mass, respectively. Three MW power modes were applied: continuous, 30 s
on/30 s off, and 30 s on/45 s off. The effect of pressure on product quality parameters was
tested under three levels of absolute pressure: 3.4,18.6, and 33.8 kPa.
56
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4.4.3 Quality evaluation
Tested quality parameters were colour, textural properties, rehydration ratio, and
water activity. Time of process was used as one of the process parameters, and it served
to calculate the overall drying efficiency using equation 4.4 (modified from
Yongsawatdigul and Gunasekaran, 1996a):
(4.4)
Where:
DEb = drying efficiency (kgwater/MJ)
ton~ total time of M W power-on (s)
P = MW power input (W)
mi = initial mass (kg)
Mi = initial moisture content (ratio)
Mf= final moisture content (ratio)
Colour was measured using chromameter (Model CR-300X, Minolta camera Co.
ltd., Japan) equipped with a 5 mm diameter measuring area. The results were expressed as
Hunter L (whiteness/darkness, ranged from 0 to 100, 100 being the lightest), a (redness
for the positive value, greenness for the negative ones) and b* (yellowness for the positive
value, blueness for the negative ones) (McGuire, 1992). Then the data obtained were
converted to hue angle (h°) and Chroma C* (color saturation) and overall colour
difference AE were calculated, following these expressions:
h° = arctan
b*
(4.5)
(4.6)
(4.7)
Where:
AL* = L * -L * St
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(4.8)
Aa* = a * -a *st
(4.9)
Ab* = b * -b * ,t
(4.10)
L*sh a*st, and b*st are Hunter values of the standard (fresh cranberries).
Textural properties were measured on the Instron Universal testing machine
(Series IX Automated Materials Testing System 1.16), showed in Figure 4.2. For small
fruits such as cranberries, the probe element was Kramer shear press; shown in Figure
2.8. A 50 kN load cell was used and the speed of crosshead was 160 mm/min. The
parameters obtained were Young’s modulus (MPa) and toughness (MPa).
Figure 4.2: Instron Universal Testing Machine
Rehydration ratio was calculated using the following expression:
RR = 10-
mrh •(l 00 - M m)
mdh - ( 1 0 0 - M j ,
Where:
RR = rehydration ratio (ratio)
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(4.11)
mrh and m^h = mass of rehydrated and dehydrated (dried) sample (g)
Min and Mjh = moisture contents of the sample before and after drying (% wet basis)
4.4.4 Experimental design
All experiments were conducted in three replicates, and the data obtained were
subjected to Analysis of Variance (ANOVA) using Statistical Analysis Software (SAS
Institute Co.) version 8.0. Experimental design was completely randomised block design.
Duncan’s multiple range tests for pairwise comparison of each variable was used for
comparison among the treatments. Differences were determined as significant or non­
significant at a significance level of 0.05 in all cases.
4.5 Results and Discussions
4.5.1 Energy efficiency
Energy efficiency was determined using equation 4.4, and the data are
summarized in Tables 4.1 and 4.2:
Table 4.1:
MW Power
Total drying
Power-on drying
time (min)
time (min)
Drying efficiency
1
MW mode
1
(W/g)
Average values of total drying time, power-on drying time and drying
efficiency for cranberries dried at 3.4 kPa of absolute pressure
1
Continuous
33.75e
33.75
0.238°
1
30s on/30s off
88.50b
44.25
0.182b
1
30s on/45s off
152.753
61.10
0.132a
1.25
Continuous
19.00f
19.00
0 .3 3 9 ^
1.25
30s on/3 0s off
36.50e
18.25
0.352e,f
1.25
30s on/45s off
53.25°
21.30
0.302d
1.5
Continuous
13.508
13.50
0.397f
1.5
30s on/3 0s off
32.25°
16.13
0.332de
1.5
30s on/45 s off
43.25d
17.3
0.310d,°
Duncan groupings: Means with the same letters are not significantly different
59
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MW power had a significant influence on drying time and drying efficiency, as
well as MW mode applied. But, pressure didn’t have any significant influence on the
drying time and drying efficiency, and following results were obtained:
Table 4.2:
Average values of total drying time, power-on drying time and drying
efficiency of cranberries dried with 125 W power level and MW mode of
30 s on/30 s off, under different absolute pressures
Absolute pressure
Total drying time
Power-on drying
Drying efficiency
(kPa)
(min)
time (min)
(A A J/kgw ater)
3.4
36.50a
18.25
0.352a
18.6
37.25a
18.63
0.345a
33.8
39.75a
19.88
0.324a
Duncan groupings: Means with the same letters are not significantly different
4.5.2 Colour parameters
Colour parameters were evaluated for each drying method, and the results showed
that both MW power and MW mode had a significant influence on lightness L*, redness
a*, hue angle h°, chroma value C*, and colour difference AE. The only colour parameter
without any significant influences was yellowness b*. It should be noted that methods
with higher power-off time (30 s on/45 s off) showed better values for colour difference
AE for all MW power levels.
Table 4.3: Average colour values for cranberries dried under different MW power levels
and MW modes, at 3.4 kPa
MW Power
MW mode
L'
a
1
Continuous
30.18b
1
30s on/30s off
1
*
b*
h°
C*
AE
29.9d
13.77a
24.77a,b
32.94e
9.78a
32.39a,b
35.15b,c
14.92a
23.05a,b
38.20b,c
5.99ab,c
30s on/45 s off
36.95a
39.63a
14.683
20.30bab
42.29a
5.90a,bc
1.25
Continuous
31.02a,b
30.21d
14.4T
25.46a
33.49d,e
9.30a,b
1.25
30s on/3 0s off
31.59a,b
31.26dc
14.448
24.90a,b
34.46c,d,e
9.04ab
1.25
30s on/45s off
33.94a,b
35.3 l b
15.57a
23.82a,b
38.63a,b
4.00°
1.5
Continuous
30.47b
29.52d
13.96a
25.33a
32.66e
9.87a
1.5
30s on/30s off
31.26a,b
34.5 l b>c
13.99a
22.23a,b
37.31b,c,d
7.98a,b
1.5
30s on/45s off
32.9 l a>b
35.74b
14.58a
22.1 l a’b
38.68a,b
5.29b,c
(W/g)
Duncan groupings: Means with the same letters are not significantly different
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Table 4.4 shows the influence of applied vacuum on the colour parameters, but
this parameter had influence only on three colour values: redness a*, hue angle h°, and
chroma value C*. Other values (L*, b \ and AE) were not significantly affected with
changed operating pressure. The absolute pressure of 3.4 kPa showed significantly higher
redness, lower hue angle and higher chroma value, but compared to standard (fresh
cranberries) pressure didn’t have any significant influence.
Table 4.4: Average colour values of cranberries dried with 125 W power level and MW
mode of 30 s on/30 s off under different absolute pressures
Absolute pressure
(kPa)
L*
a
*
b'
h°
C*
AE
3.4
18.6
33.8
32.39a 35.15a 14.92a 23.05b 38.193 5.98a
31.32“ 30.87b 15.58a 26.80a 34.59b 8.02a
31.79a 28.86b 15.41a 28.1 l a 32.72b 9.30a
Duncan groupings: Means with the same letters are not significantly different
4.5.3 Textural properties
Textural properties are very important characteristics of dried fruits. For each
drying method, two textural properties were assessed: Young’s modulus and toughness,
both expressed in MPa.
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MW Power (W/g)
' "
3 30s on/45s o ff * 30s on/30 s off O Continuous
Figure 4.3: Average values of Young’s modulus for cranberries dried under different
MW powers and MW modes at 3.4 kPa of absolute pressure
Duncan groupings: Means with the same letters are not significantly different
From Figure 4.3, it is observed that methods with 1.25 W/g and pulsed MW
modes, as well as method with 1.5 W/g and pulsed mode of 30 s on/45 s off had
significantly lower Young’s modulus, meaning that they have lower stiffness. Both MW
power and MW modes had significant effect on Young’s modulus. On the other hand,
toughness of cranberries was not significantly influenced by any of methods applied, as
can be seen in Figure 4.4.
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MW Pow er (W/g)
15
0 30s on/45s o ff ■ 30s on/30s o ff □ Continuous
Figure 4.4: Average values of Toughness for cranberries dried under different MW
powers and MW modes at 3.4 kPa of absolute pressure
Duncan groupings: Means with the same letters are not significantly different
Different pressures did not have significant influence on both texture parameters,
and this can be seen in Table 4.5:
Table 4.5: Average values of Young’s modulus and toughness of cranberries dried with
125 W power level and MW mode of 30 s on/30 s off under different
absolute pressures
Absolute pressure
Young’s modulus
(kPa)
(MPa)
Toughness (MPa)
6.36a
3.4
0.0176a
18.6
6.66a
0.0156a
6.22a
33.8
0.0145a
Duncan groupings: Means with the same letters are not significantly different
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4.5.4 Rehydration ratio
One of the most important parameters of dried product quality is its ability to
rehydrate, or to infuse water again. Cranberries are often used in cereals and they must
have high rehydration ratio. Rehydration ratios of dried berries were significantly
influenced by all three process parameters: MW power, MW mode, and pressure.
30s on/45s o ff
30s on/3 Os o ff
M W mode
Continuous
M W P ow er (W/g)
Figure 4.5:
[® C ontinuous ■ 30s on/30s o ff □ 30s on/45s off]
Average values of rehydration ratios for cranberries dried under different
MW powers and MW modes at 3.4 kPa of absolute pressure
Duncan groupings: Means with the same letters are not significantly different
Table 4.6: Average values of rehydration ratios for cranberries dried with 125 W power
level and MW mode of 30 s on/30 s off under different absolute pressures
Absolute pressure (kPa)
Rehydration ratio
3.4
2.64a
18.6
2.51a,b
33.8
2.44b
Duncan groupings: Means with the same letters are not significantly different
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It can be concluded that MW power level of 1.25 W/g offers the best conditions for high
rehydration ratio, and that MW mode of 30 s on/45 s off is also very beneficial to
rehydration properties, as well as higher vacuum.
4.6 Conclusions
MW drying combined with subatmospheric pressures can be used to dry
cranberries more rapidly than conventional air-drying. This method also offers product
with higher quality parameters, such as colour, texture and rehydration characteristics.
Several conclusions can be made:
• The majority of values used to describe colour were significantly influenced with
MW power level and MW power mode. On the other hand, pressure didn’t show
any effect on cranberry colour. In order to obtain colour of dried berries similar to
fresh cranberries, the method with higher MW power level should be used (1.25
and 1.5 W/g) and with pulsed MW mode with shorter power-on and longer poweroff time (30 s power-on/45 s power-off).
• Textural characteristics had different response to drying conditions for the two
tested parameters. Young’s modulus which represents product’s stiffness was
significantly influenced by MW power level and MW mode, but not with pressure
applied. Toughness of product was not affected with any of applied conditions.
With the purpose of obtaining a product of good textural characteristics, the
drying method should be with higher MW power levels and pulsed MW modes of
shorter power-on and longer power-off time, similar to the requirement observed
for colour characteristics.
• Rehydration ratios were significantly affected by all three process parameters:
MW power level, MW mode, and pressure applied. Higher RR values were
observed with MW power level of 1.25 W/g, and partially 1.5 W/g. The same
pulsed MW modes should be applied as for colour and texture optimization: 30 s
power-on/45 s power-off. This was the only parameter on which vacuum had
significant influence, and preferential treatment is with lower absolute pressure
(higher vacuum) of 3.4 kPa.
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•
For drying efficiency DE, the situation was very similar to other parameters: both
MW power level and MW mode had a significant influence, but pressure didn’t.
The conclusion can be similar, higher power levels and modes with longer poweroff time can offer lower energy consumption. It must be noted that DE was
calculated only using power obtained from MW, and not from vacuum pump. This
has been done in order to compare the values for DE with results from Beaudry et
al. (2003a,c) and Yongsawatdigul and Gunasekaran (1996a).
•
Undesirable feature of MW drying is low uniformity of dried product. This was
obvious in all experiments, especially in those with higher MW power levels and
continuous mode, causing burned, almost carbonized spots in the centre of
vacuum container. Composite foods such as cranberries have low thermal
conductivity and can make “thermal avalanches” making temperature control
difficult (Gunasekaran, 1999). This problem can be solved by placing the sample
on a rotating tray or moving belt. In addition, bigger samples and larger MW
chamber should be used, in order to enhance the distribution of microwaves.
Overall conclusion is that quality dried cranberries can be obtained by
MW/vacuum drying, using higher power levels (shorter drying time), and pulsed MW
modes with shorter power-on and longer power-off time. Longer power-off time is
beneficial for water redistribution inside the fruit, giving more uniformly dried product,
with better rehydration properties. Higher vacuum is preferable, guarantying lower
process temperature. Therefore, MW/vacuum drying is a feasible method for drying of
osmotically dehydrated cranberries.
4.7 Acknowledgements
The authors would like to express their gratitude towards Natural Sciences and
Engineering Research Council of Canada (NSERC) for their financial support, to Dr
Valerie Orsat, Mr. Timothy John Rennie, and Mr. Yvan Gariepy for their
recommendations and technical assistance during this research.
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4.8 References
Beaudry, C., Raghavan, G.S.V., and T.J. Rennie. 2003a. Optimization of microwave
drying of osmotically dehydrated cranberries. Submitted for review: Drying
Technology.
Beaudry, C., Raghavan, G.S.V., Ratti, C., and T.J. Rennie. 2003b. Optimisation of skin
pretreatments and osmotic dehydration for cranberries. Submitted for review:
Food Research International.
Beaudry, C., Raghavan, G.S.V., Ratti, C., and T.J. Rennie. 2003c. Effect of four drying
methods on the quality of osmotically dehydrated cranberries. Submitted for
review: Drying Technology.
Chen, Su-Der and E-Mean Chiu. 1999. Kinetics of Volatile Compound Retention in
Onions during Microwave Vacuum Drying. Food Science and Agricultural
Chemistry. l(4):264-270.
Clary, C.D. and A.S. Ostrom Gwynn. 1995. Use of Microwave Vacuum for Dehydration
of Thompson Seedless Grapes. CATIPublication No 950405.
Drouzas, A.E. and H. Schubert. 1996. Microwave Application in Vacuum Drying of
Fruits. Journal o f Food Engineering. 28(2):203-209.
Drouzas, A.E., Tsami, E., and G.D. Saravacos. 1999. Microwave/Vacuum Drying of
Model Fruit Gels. Journal o f Food Engineering. 39:117-122.
Grabowski, S., Marcotte, M., Poirier, M., and T. Kudra. 2002. Drying Characteristics of
Osmotically Pretreated Cranberries - Energy and Quality Aspects. Drying
Technology. 20(10):1989-2004.
67
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Gunasekaran, S. 1999. Pulsed Microwave-Vacuum Drying of Food Materials. Drying
Technology. 17(3):395-412.
Kaminski, W. and T. Kudra. 2000. Equilibrium moisture relations for foods and
biomaterials. In: Drying Technology in Agriculture and Food Sciences. Edited by
A.S. Mujumdar. Science Publishers, Inc. Enfield, NH. 313 pp.
Kim, S. S. and S.R. Bhowmik. 1995. Effective Moisture Diffusivity of Plain Yoghurt
Undergoing Microwave Vacuum Drying. Journal o f Food Engineering. 24:137148.
Kim, S. S., Shin, S. G., Chang, K. S., Kim, S. Y., Noh, B. S., and S.R. Bhowmik. 1997.
Survival of Lactic acid Bacteria during Microwave Vacuum-drying of Plain
Yoghurt. Lebensm.-Wiss. Und Technol., 30:573-577.
Kiranoudis, C.T., Tsami, E., and Z.B. Maroulis. 1997. Microwave Vacuum Drying
Kinetics of Some Fruits. Drying Technology. 15(10):2421-2440.
Lin, T.M., Durance, T.D., and C.H. Seaman. 1998. Characterization of vacuum
microwave, air and freeze dried carrot slices. Food Research International.
31(2):111-117.
McGuire, R.G. 1992. Reporting of Objective Color Measurements. Hortscience.
27(12):1254-1255.
Sanga, E., Mujumdar, A.S., and G.S.V. Raghavan. 2000. Principles and Applications of
Microwave Drying. In: Drying Technology in Agriculture and Food Sciences.
Edited by A.S. Mujumdar. Science Publishers, Inc. Enfield, NH. 313 pp.
Tulasidas, T.N., Raghavan, G.S.V., and A.S. Mujumdar. 1995. Microwave Drying of
Grapes in a Single Mode Cavity at 2450 MHz - II: Quality and Energy Aspects.
Drying Technology. 13(8&9): 1973-1992.
68
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Wadsworth, J.I., Velupillai, L., and L.R. Verma. 1989. Microwave-Vacuum Drying of
Parboiled Rice. ASAE/CSAE Meeting Presentation No 89-6101.
Yongsawatdigul, J. and S. Gunasekaran. 1996a. Microwave-Vacuum Drying of
Cranberries: Part I. Energy Use and Efficiency. Journal o f Food Processing and
Preservation. 20:121-143.
Yongsawatdigul, J. and S. Gunasekaran. 1996b. Microwave-Vacuum Drying of
Cranberries: Part II. Quality Evaluation. Journal o f Food Processing and
Preservation. 20:145-156.
Yousif, A.N., Durance, T.D., Seaman, C.H., and B. Girard. 2000. Headspace Volatiles
and Physical Characteristics of Vacuum-microwave, Air, and Freeze-dried
Oregano (Lippia berlandieri Schauer). Journal o f Food Science. 65(6):926-930.
69
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CONNECTING TEXT
After determining of the appropriate drying conditions such as microwave power
density, microwave power mode and pressure applied, the next step was to compare
microwave/vacuum drying with similar process: microwave/convective drying. Several
final product quality parameters such as colour, texture, and organoleptic properties were
assessed, and special emphasis was given to process energy efficiency. The conditions of
drying pretreatment methods were similar, and pretreated cranberries were with
comparable features.
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V.
MICROWAVE/CONVECTIVE AND
MICROWAVE/VACUUM DRYING OF CRANBERRIES:
A COMPARATIVE STUDY
5.1 Abstract
In this comparative study two drying methods of cranberries (microwave/vacuum
and microwave/convective) are reviewed, and their advantages and disadvantages
regarding the quality of dried product and the process are presented. Mechanically and
osmotically pretreated cranberry fruits (Vaccinium macrocarpori) of the Stevens cultivar
were subjected to drying and quality evaluation. Quality parameters are colour (in Hunter
L*, a*, b* coordinates) measured with a chromameter, textural characteristics (toughness
and Young’s modulus) measured with an Instron universal testing machine, and
organoleptic properties (colour, texture, taste, and overall appearance) judged by a panel
of six untrained judges. Special emphasis was given to the energy efficiency of the
process, monitoring of the real-time temperature profile during processing, and the total
microwave power-on time. Two microwave power densities are assessed, such as 1.00
and 1.25 W/g of initial sample mass, as well as different microwave power-on/power-off
cycling periods.
In almost all observed parameters, microwave/vacuum drying exhibited enhanced
characteristics when compared to microwave/convective drying. Colour values were quite
similar, but in comparison of textural properties MW/vacuum exhibited better results.
Drying efficiency results (defined as mass of evaporated water per energy unit) showed
that microwave/vacuum drying is more energy-efficient than microwave/convective.
Tasting panel results exhibited slight preference in all aforementioned parameters for
microwave/convective dried samples.
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5.2 Introduction
The modem food industry sets strict conditions on each process performed today.
Increasing awareness of the importance in energy savings can result in fundamental
changes in traditional processes. Drying is one of the most energy demanding processes
(together with distillation), and there is no exception. In the last decade, many studies
have focused on the improvement of convective drying, combining it with other
processes, or total replacement by another method.
Cranberry fruit and its products are a healthy and beneficial addition to every-day
diet. High vitamin C content and proven health benefits make this fruit a highly
recommended dietary constituent, especially for people with urinary tract diseases (Avom
et al., 1994). Dehydration of cranberries can make it available year-round, and guarantee
a long shelf-life; this is also possible by freezing cranberries, the traditional way of long
term storage for these berries. High moisture content of fresh cranberries (app. 88 %, wet
basis) must be reduced to between 15 and 20 % (wet basis), with water activity (aw) o f
less than 0.7.
The objective of this study is to compare available data on cranberry drying,
particularly on two drying methods: microwave/convective and microwave/vacuum.
Special importance will be given to energy aspects for each drying method, as well as
sensory evaluation of obtained products.
5.3 Drying Methods
Drying is one of the oldest food preservation methods. Water in foods is the main
culprit for its deterioration, and the purpose of drying is to reduce the moisture of food
low enough to make it shelf-stable and less prone to microbial or enzymatic changes.
There are many methods for drying of food (Grabowski et al., 2003; Barbosa-Canovas
and Vega-Mercado, 1996), and here will be described only microwave/convective as
enhancement of convective drying, and microwave/vacuum as further improvement.
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5.3.1 Microwave/convective drying
The use of microwaves (MW) overcomes the usual problem of poor heat transfer
in conventional hot-air drying. In microwave-assisted drying, heat is not transferred to,
but generated in the tissues. The energy transfer rates are much higher than in
conventional drying operations, especially during the falling drying rate period (Beaudry
et al., 2003a,c). Microwaves can be used as an energy source, and therefore lower
temperatures of hot air can be applied. Microwave heating is characterised with better
penetrating effect than most of the other methods, with “targeted” heating (i.e. MW
energy is absorbed mostly by water in the product), and it is easier to control than heating
using hot-air if the dryer is designed properly. MW technique is suitable to combine with
other methods, such as hot-air, vacuum, freezing equipment, heat pump (Sanga et al.,
2000). Drying time can be reduced and final product quality improved using MW, and the
drying time can be reduced up to eight times. Schiffrnann (1987) reported that these
drying systems are capable of handling 300 lb of product per hour with 60 kW of
microwave energy at 915 MHz. One of the problems related to MW drying equipment is
certainly high initial investment that can go up to $3,500 per kW of power (Schiffrnann,
1987).
Beaudry et al. (2003c) dried cranberries using MW and hot air and showed that
product quality is equivalent to freeze-dried cranberries. It is suitable to combine
microwaves with hot air because it improves both drying efficiency and economics of the
process, exhibited in study of Tulasidas et al. (1995a,b), where they dried grapes using
hot-air and microwaves and concluded that it improves both quality and economics of the
process.
5.3.2 Microwave/vacuum drying
Further drying improvements can be obtained by using subatmospheric pressures.
Water evaporation takes place at lower temperatures under vacuum, and hence the
product processing temperature can be significantly lower, offering higher product
quality. Many comparisons have been made between MW/vacuum drying and other
systems, mainly focusing on hot air and freeze drying.
The MW/vacuum dehydration was first used for concentration of citrus juice
(Decareau, 1986). In the food industry, MW/vacuum drying is used for the drying of
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pastas, powder and many porous materials. McDonnell Company has built a MW/vacuum
drying system (MIVAC®) to dry grains, and absolute pressure ranged from 3.4 to 6.6 kPa,
and moisture could be evaporated at temperatures of 26 to 52°C. However it was not
commercially successful due to economics. Evaporated water from product in this
MW/vacuum drying system is removed usually by condensing using cooling system,
which uses water as a cooling medium (Sanga et al. 2000).
5.3.3 Drying efficiency
One way to evaluate the feasibility of the process is to calculate the drying
efficiency with the following equation, modified from Yongsawatdigul and Gunasekaran
(1996a):
(5.1)
Where:
DEa = drying efficiency (kg of water evaporated/MJ)
ton= total time of MW power-on (s)
P = MW power input (W)
rrii = initial mass (kg)
Mi = initial moisture content (ratio)
Mf= final moisture content (ratio)
Equation 5.1 only considers the efficiency of MW systems, it does not consider
energy required to heat air, or energy required for vacuum pump.
Pressure and power level must be correctly chosen to maximise the efficiency.
Drouzas et al. (1999) showed that the drying rate was significantly raised with increase of
the pressure or the MW power level, but the final quality of dried banana slices was
lower. The same trend was observed in Wadsworth et al. (1996), drying efficiency of
parboiled rice was significantly influenced by both microwave power level and dryer
operating pressure.
Kiranoudis et al. (1997) suggested following mathematical model, representing
moisture at any given time:
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M = M 0 ■e~kM*
(5.2)
Where:
M= average material moisture content at any given time (ratio, wet basis)
Mo = initial moisture content (ratio, wet basis)
kM= drying constant (h'1)
t = time (h)
The drying constant can be determined by using equation 5.3:
(5.3)
Where:
Q = operating pressure (Pa)
P = microwave power level (W)
ki (i=0, 1, 2) = empirical coefficients that can be estimated by fitting the total model
employed to the experimental drying curves (nondimensional)
One way to counter disadvantages of MW drying such as non-uniform heating is
to operate in a pulsed mode, by alternating between MW power-on and power-off. This
permits better redistribution of the temperature and the moisture profile within the
product during power-off times. For a given product, the MW power-on time and the
pulsing ratio should be optimised. Pulsed application of microwave energy combined
with vacuum to dry cranberries has been found more efficient than continuous application
(Yongsawatdigul and Gunasekaran, 1996a). In pulsed mode, shorter power-on time and
longer power-off time provided a higher drying efficiency, where energy utilisation
coefficient of pulsed mode ranged from 0.53 to 0.95, and for continuous MW application
it was significantly lower, ranging from 0.43 to 0.67. Concerning the quality properties,
continuously dried samples had a higher redness and undesirable tougher texture than the
samples dried with pulsed mode
75
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5.4 Materials and Methods
For both microwave/convective and microwave/vacuum drying methods the same
equipment was used, shown in Figure 4.1. This equipment consisted of air blower, air
heaters, magnetron device, part that monitored and controlled reflected MW power, wave
guides, vacuum pump, pressure and MW power measuring instruments, MW chamber
with balance, temperature measuring devices such as thermocouples and optical fibres,
and data analysis system.
5.4.1 Microwave/convective drying
Cranberries (Vaccinium macrocarpon) of the Stevens cultivar were harvested by
hand in Quebec and frozen. After thawing, they were cut in halves and subjected to
osmotic dehydration using high fructose com syrup (76°Brix) for 24 hours at room
temperature and 1:1 syrup to fruit mass ratio. This resulted in osmotically dehydrated
cranberries with 57±1 % moisture content. Samples of 125 g were subjected to
microwave drying using hot air. MW power densities were 1.00 and 1.25 W/g of initial
sample mass, and MW modes were 30 s on/30 s off and 30 s on/60 s off. All
combinations were replicated three times. Heated air was continuously blown on the
sample holder by a 0.2 kW blower placed below the drying cavity and three 2 kW air
heaters. Properties of the air were 62°C and speed of 1 m/s (average values throughout the
process). Temperatures of the air inlet, air outlet, and the sample were monitored using
type T thermocouples throughout the experiment. Cranberry mass was recorded with
balance, temperature recorded using optical fibre (Fisher Scientific, Nepean, ON) and
they were dried until moisture content reached 15 %.
5.4.2 Mlcrowave/vacuum drying
Cranberries from the same source as above were used for MW/vacuum drying, but
they were cut in quarters and subjected to osmotic dehydration with 2:1 syrup to fruit
mass ratio. This concentration was used because of the significant difference it makes, as
proven in Chapter III. Nevertheless, their initial properties were almost the same as for
cranberries used for MW/convective drying, with moisture content of 55±1 %. MW
power densities were 1.00 and 1.25 W/g per total sample mass, and MW modes similar to
76
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MW/convective: 30 s on/30 s off and 30 s on/45 s off. Vacuum were maintained at
3.4 kPa of absolute pressure by means of vacuum pump (John Scientific Inc., Canada)
whose power was 0.7 kW. Temperature of the sample was recorded using optical fibre
thermometer during the whole experiment.
5.4.3 Quality evaluation
The quality of dried cranberries depends on the initial sample quality, drying
method, and the drying conditions used. Tested quality parameters were colour and
textural properties (toughness and Young’s modulus). Colour was measured in L*a*b*
coordinates, where L* is the lightness (0 for black, 100 for white), a* for the red-purple
(positive values) to the bluish-green (negative values) and b* indicates the yellowness
(positive values) and blueness (negative values) (McGuire, 1992), using a Minolta
Chromameter Model CR-300X (Minolta camera Co. Ltd., Japan). Three derived colour
parameters - hue angle h°, Chroma value C*, and colour difference AE were calculated
using the following equations:
h° = arctan
(b*
ya * I
<5-4)
C* = \ a * f +{b*)2Y2
(5.5)
AE = V(aZ *)2 + (Aa * f + (Ab * f
(5.6)
Where:
AL* = L * -L * st
(5.7)
Aa* = a * -a * sl
(5.8)
Ab* = b * - b * sl
(5.9)
Where st subscript represents I*, a , and b* values of a standard cranberry (fresh
cranberry of Stevens cultivar).
The textural properties (toughness and Young’s modulus in MPa) of cranberry
samples were determined by using the Instron Universal Testing Machine (Series IX,
Automated Materials Testing System 1.16).
Drying efficiency was calculated using equation 5.1.
77
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Sensory analysis was performed using a tasting panel of six untrained judges that
compared colour, texture, taste and overall appearance. They assigned marks to three
samples (MW/convective dried, MW/vacuum dried, and hot-air dried) ranging from 1 to
5, where 1 represented unacceptable quality, 2 - poor quality, 3 - medium, 4 - good and
5 - excellent quality. Judges were asked to give their remarks about each of the samples.
5.5 Results and Discussion
5.5.1 Colour and textural properties
Results of the colour parameters are presented in Table 5.1:
Table 5.1:
Comparison of colour values of MW/convective and MW/vacuum dried
cranberries
Drying
MW density
MW mode
method
(W/g)
(s on/s off)
1.00
MW/
convective
L*
a
30/30
31.0
1.00
30/60
1.25
*
b*
h°
C*
AE
27.4
11.9
23.3
29.8
5.9
33.2
32.0
13.8
23.3
34.9
2.4
30/30
32.5
25.0
12.5
26.5
27.9
6.4
1.25
30/60
29.6
26.8
10.8
21.9
28.9
6.6
1.00
30/30
32.4
35.2
14.9
23.1
38.2
6.0
MW/
1.00
30/45
37.0
39.6
14.7
20.3
42.3
5.9
vacuum
1.25
30/30
31.6
31.3
14.4
24.9
34.5
9.0
1.25
30/45
33.9
35.3
15.6
23.8
38.6
4.0
From Table 5.1 can be seen that cranberries dried with longer power-off time (45
or 60 seconds) had lower colour differences (AE) in three of four times than those dried
with shorter power-off times (30 seconds). They also showed lighter colour in three of
four times (I*). Higher power level exhibited lowering of L* value, giving darker
cranberries, and AE increased with increasing of MW power density. MW/vacuum
cranberries were redder (higher a* value) and yellower (higher b* value).
Textural properties are presented in Table 5.2:
78
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Table 5.2: Toughness and Young’s modulus for MW/convective and MW/vacuum dried
cranberries
Young’s
Drying
MW density
MW mode
Toughness
method
(W/g)
(s on/s off)
(MPa)
1.00
30/30
0.0198
10.9
MW/
1.00
30/60
0.0208
12.1
convective
1.25
30/30
0.0240
11.6
1.25
30/60
0.0214
11.4
1.00
30/30
0.0166
8.14
MW/
1.00
30/45
0.0158
7.91
vacuum
1.25
30/30
0.0176
6.36
1.25
30/45
0.0171
6.30
modulus
(MPa)
As can be seen in Table 5.2, MW/vacuum drying offers cranberries with softer
texture (lower toughness) and with lower Young’s modulus representing chewier
samples.
5.5.2 Temperature during drying
Temperature profiles for MW/convective, MW/vacuum and convective dried
cranberries can be seen in Figure 5.1. Convective drying was performed only for
comparison purpose and temperature of the heated air was 62°C, with speed of 1.0 m/s.
Convective drying lasted for almost four hours, however only the temperature profile
during first 50 minutes is presented here because the temperature remained constant
thereafter. Oscillations during both MW processes are expected, because temperature
increases during MW power on time, and decreases during MW power off time. Average
temperature during MW/vacuum process is slightly lower than during MW/convective,
which is expected, because MW/convective process also had hot-air heat added. The
expected cranberry temperature of approximately 27°C (temperature of water evaporation
at 3.4 kPa, Moran and Shapiro, 1996) was not noticeable, probably because of internal
heat build-up and increased temperature of the sample, indicating that the supplied MW
power was too high. This temperature of approximately 27°C can be obtained with
precise control of MW power input and vacuum regulation. Heat build-up is evident
79
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because cranberries, as most of complex systems, have poor heat diffusivity and have
non-uniform temperature profiles.
T em perature profiles
120
100
"■ ■ ■ ■ ■ • ■ • ■ ■ ■ • • ■ ■ ■ ■ ■ a
AA A
A
a*
AA
■
I
*A i
AA
.♦
A A
a ♦
A»
20
■ MW/convective
0
+
0
5
A
MW/vacuum
♦ Convective
,
,
F
,
,
,
,
,
,
10
15
20
25
30
35
40
45
50
T im e (m in)
Figure 5.1: Typical temperature profile during MW/convective, MW/vacuum and
convective dried cranberries
Peak of 80°C in MW/vacuum process temperature profile can be explained with
high initial moisture of cranberries, where higher water content is responsible of
absorbing more MW energy. As moisture content decreases, the temperature of sample
stabilizes.
5.5.3 Energy aspects
Drying efficiencies of two drying methods were calculated using equation 5.1.
This equation is not precise, because the total power input is not only by microwaves, but
also includes air blower and air heater in MW/convective process, and the vacuum pump
in MW/vacuum process. However for these calculations only MW power input is used, in
order to compare these results with those from Yongsawatdigul and Gunasekaran
(1996a).
m
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Similar approach was used in Grabowski et al. (2002) where they suggested
following equation for instantaneous energy efficiency etn (ratio):
_ energy used for evaporation at time t
;
;
input energy at time t
1AX
£ in —
(y.lU)
And cumulative energy efficiency s (ratio) over a given time interval:
1 t
s = --^s(t)d t
(5.11)
* o
Mass and energy balances for MW/convective and MW/vacuum drying systems
can be set as follows:
(5-12)
Where:
msamj and msam2 = initial and final mass of the sample (kg)
mevw= mass of evaporated water in order to reduce moisture from initial to final (kg)
Energy balance in case of MW/convective drying can be set as follows:
^M W
tMWon
^ s a m l ’ ^saml ’ ^in
i^ b l
^ \ e ) ' A f — ^ s a m l ' ^ saml ' ^'fin
^ e v w ' ip
P'loss
(5 -1 3 )
And energy balance in case of MW/vacuum drying is:
P m W ' tMWon + m sam\ ’ Csam\ ' T ir t+ P v p wt !ol = M sana ' Csam2 ' P fin
m evw ' l p + P,loss
(5 -1 4 )
Where:
P mw
= MW power input (kW)
tMWon = total MW power-on time (s)
Csami and csam 2 = specific heats of the sample at initial and final stage of drying, complex
values, depend on material temperature and moisture content (kJ/kg-°C)
T in
and Tfm = initial and final temperatures of the sample (°C)
Phi and Pfa = power inputs from the air blower and air heaters (kW)
Pw = power of the vacuum pump (kW)
81
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tm - total drying time (s)
ip
= specific enthalpy of water vapour at process pressure and exit temperature (kJ/kg)
Eioss= energy losses during process (kJ)
Three of the values from the equations above are very difficult to determine, these
are specific heats of the samples csami and csam 2 and energy losses Eioss. Specific heat
depends on the sample temperature and composition (moisture content). Yongsawatdigul
and Gunasekaran (1996a) recommended the following equation for the specific heat of
cranberries:
ccb=l + m-Tcb+ n-M cb
(5.15)
Where Tch and Mcb represent temperature (°C) and moisture content (%, wet basis) of
cranberries, and /, m, and n are empirical coefficients. The same authors determined that
coefficient n is much larger than m, indicating much stronger effect of moisture content
on specific heat than temperature in the observed processing range. They calculated
specific heat of fresh cranberries as 3.77 kJ/kg °C, which is close to the value reported in
literature 3.78 kJ/ kg-°C by Hayes (1987).
In Table 5.3 drying efficiencies (DEb) are presented for two compared drying
methods:
Table 5.3:
MW power-on times and drying efficiencies for cranberries dried with
MW/convective and MW/vacuum method
Drying
MW density
MW mode
MWpower-
Drying efficiency
method
(W/g)
(s on/s off)
on time (min)
( k g w a te r / M J )
1.00
30/30
75.3
0.11
MW/
1.00
30/60
71.8
0.12
convective
1.25
30/30
65.4
0.10
1.25
30/60
58.6
0.11
1.00
30/30
44.3
0.18
MW/
1.00
30/45
61.1
0.13
vacuum
1.25
30/30
18.3
0.35
1.25
30/45
21.3
0.30
82
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MW/vacuum drying is apparently more energy-efficient than MW/convective
because DE values are higher. It is interesting to notice that drying efficiency for
MW/convective increases in MW mode with longer power-off time, and decreases in
corresponding MW modes in MW/vacuum drying. These values for drying efficiency are
comparable to those obtained by Yongsawatdigul and Gunasekaran (1996a), which
ranged from 0.2 kg/MJ for continuous MW mode to the 0.38 kg/MJ for MW mode with
the longest power-off time in MW/vacuum drying process.
5.5.4 Sensory evaluation
Sensory evaluation was performed by a tasting panel comprised of six untrained
judges, where they evaluated colour, texture, taste and overall appearance for three
cranberry samples: MW/convective dried, MW/vacuum dried and hot-air dried. Statistical
analysis were carried out using Kruskal-Wallis’ test for nonparametric statistics
(Mendenhall et al., 1986). Significance level was 0.05, experimental design was
completely randomised design, and testing of hypotheses was done using/2 distribution.
Table 5.4:
Organoleptic analysis of three cranberry samples dried using three different
drying methods
Overall
Drying method
Colour
Texture
Taste
MW/vacuum
3.0
2.8
3.3
2.8
MW/convective
3.2
3.3
3.7
3.0
Convective
4.0
3.7
4.0
4.2
appearance
Drying method does not have significant influence on any of the parameters
tested. Some differences can be observed, especially in the overall appearance where
convective drying ranked the best, and this can be explained with small number of judges
or too narrow ranking system (only five units). Judges commented MW/vacuum dried
sample as with tough texture, with noticeable “caramelised” odour and burnt taste,
probably because of the non-uniform dried sample. Non-uniformity in all parameters
(above all colour) was the main remark for MW dried samples, especially for
MW/vacuum dried samples.
83
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5.6 Conclusions
Several differences were detected between MW/convective and MW/vacuum
drying and they can be summarised as follows:
•
Colour parameters for both methods are quite similar. It can be seen that MW
power level and MW mode have more influence on the colour than application of
vacuum or hot air.
•
Textural properties can depend on drying method, and MW/vacuum dried
cranberries showed softer texture and were less tough than MW/convective.
•
Total power input was much higher during MW/convective drying, confirming
better energy consumption characteristics of MW/vacuum process, confirmed
with higher drying efficiency values.
•
Organoleptic analysis showed that although no significant difference was detected
in all tested parameters (colour, texture, taste, overall appearance) MW/convective
dried cranberries were more appreciated by judges than MW/vacuum dried, but
both of MW drying methods were beaten by ordinary hot-air dried cranberries.
Non uniformity of MW dried samples is very serious problem that needs to be
addressed.
5.7 Acknowledgements
The authors of this research paper would like to express their appreciation towards
Natural Sciences and Engineering Research Council of Canada (NSERC) for their
financial support and to Mr. Yvan Gariepy for his constructive recommendations during
course of this work.
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5.8 References
Avom, J., Monane, M., Gurwitz, J.H., Glynn, R.J., Choodnovskiy, I., and L.A. Lipitz.
1994. Reduction of Bacteriuria and Pyuria After Ingestion of Cranberry Juice.
Journal o f the American Medical Association. 271:751-754.
Barbosa-Canovas, G.V. and H. Vega-Mercado. 1996. Dehydration of Foods. Chapman &
Hall, New York, NY. 330 pp.
Beaudry, C., Raghavan, G.S.V., and T J. Rennie. 2003a. Optimization of microwave
drying of osmotically dehydrated cranberries. Submitted for review: Drying
Technology.
Beaudry, C., Raghavan, G.S.V., Ratti, C., and T J. Rennie. 2003b. Optimisation of skin
pretreatments and osmotic dehydration for cranberries. Submitted for review:
Food Research International.
Beaudry, C., Raghavan, G.S.V., Ratti, C., and T J. Rennie. 2003c. Effect of four drying
methods on the quality of osmotically dehydrated cranberries. Submitted for
review: Drying Technology.
Decareau, R.V. and R.A. Peterson. 1986. Microwave Processing and Engineering. Ellis
Horwood Series in Food Science and Technology, Deerfield Beach, FL. 224 pp.
Drouzas, A.E., Tsami, E., and G.D. Saravacos. 1999. Microwave/Vacuum Drying of
Model Fruit Gels. Journal o f Food Engineering. 39:117-122.
Geyer, S., Sunjka, P., and G.S.V. Raghavan. 2003b. Osmotic Dehydration and
Microwave Drying of Guava Fruit, Part II: Microwave Convective and
Microwave Vacuum Drying. Editing for publication in: The West Indian Journal
of Engineering
85
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Grabowski, S., Marcotte, M., and H.S. Ramaswamy. 2003. Drying of fruits, vegetables,
and spices. In: Handbook of Postharvest Technology -
Cereals, Fruits,
Vegetables, Tea, and Spices. Edited by Chakraverty, A., Mujumdar, A.S.,
Raghavan G.S.V., and H.S. Ramaswamy. Marcel Dekker, Inc. New York, NY. pp.
653-695.
Grabowski, S., Marcotte, M., Poirier, M., and T. Kudra. 2002. Drying Characteristics of
Osmotically Pretreated Cranberries - Energy and Quality Aspects. Drying
Technology. 20(10):1989-2004.
Gunasekaran, S. 1999. Pulsed Microwave-Vacuum Drying of Food Materials. Drying
Technology. 17(3):395-412.
Hayes, G.D. 1987. Food Engineering Data Handbook. Longman Scientific and Technical,
New York, NY. 183 pp.
Kaminski, W. and T. Kudra. 2000. Equilibrium moisture relations for foods and
biomaterials. In: Drying Technology in Agriculture and Food Sciences. Edited by
A.S. Mujumdar. Science Publishers, Inc. Enfield, NH. 313 pp.
Kiranoudis, C.T., Tsami, E., and Z.B. Maroulis. 1997. Microwave Vacuum Drying
Kinetics of Some Fruits. Drying Technology. 15(10):2421-2440.
McGuire, R.G. 1992. Reporting of Objective Color Measurements. Hortscience.
27(12): 1254-1255.
Mendenhall, W., Scheaffer, R.L., and D.D. Wackerly. 1986. Mathematical Statistics with
Applications. PWS Publishers, Duxbury Press, Boston, MA.
Moran, M.J. and H.N. Shapiro. 1996. Fundamentals of Engineering Thermodynamics, 3rd
edition. Chapter VIII. John Wiley & Sons, Inc. New York, NY. 859 pp.
86
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Sanga, E., Mujumdar, A.S., and G.S.V. Raghavan. 2000. Principles and Applications of
Microwave Drying. In: Drying Technology in Agriculture and Food Sciences.
Edited by A.S. Mujumdar. Science Publishers, Inc. Enfield, NH. 313 pp.
Schifimann, R.F. 1987. Microwave and Dielectric Drying. In: Handbook of Industrial
Drying. Edited by Mujumdar A.S. Marcel Dekker, Inc. New York, NY.
Tulasidas, T.N., Raghavan, G.S.V., and A.S. Mujumdar. 1995a. Microwave Drying of
Grapes in a Single Mode Cavity at 2450 MHz - I: Drying Kinetics. Drying
Technology. 13(8&9):1949-1971.
Tulasidas, T.N., Raghavan, G.S.V., and A.S. Mujumdar. 1995b. Microwave Drying of
Grapes in a Single Mode Cavity at 2450 MHz - II: Quality and Energy Aspects.
Drying Technology. 13(8&9):1973-1992.
Wadsworth, J.I., Velupillai, L., and L.R. Verma. 1989. Microwave-Vacuum Drying of
Parboiled Rice. ASAE/CSAE Meeting Presentation No 89-6101.
Yongsawatdigul, J. and S. Gunasekaran. 1996a. Microwave-Vacuum Drying of
Cranberries: Part I. Energy Use and Efficiency. Journal o f Food Processing and
Preservation. 20:121-143.
Yongsawatdigul, J. and S. Gunasekaran. 1996b. Microwave-Vacuum Drying of
Cranberries: Part II. Quality Evaluation. Journal o f Food Processing and
Preservation. 20:145-156.
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VI. GENERAL DISCUSSIONS AND CONCLUSIONS
Drying as one of the food preservation methods has been extensively researched
in the past few years. This fact confirms the importance of drying, and need to improve
and develop this process to the highest extent. Fruits are very sensitive commodities,
regarding their processing behaviour, and fruit drying has to be especially designed to suit
this produce. Cranberries are widely used in Canada’s food industry, and one of the main
preservation methods of cranberries besides freezing is drying. Therefore, experiments
were performed in order to determine the changes in the final product quality and to
optimise process efficiency.
Different pretreatments such as chemical, mechanical and osmotic dehydration
were tested with the intention of improving drying rates in the subsequent process and
preserving the initial fruit quality. Chemical pretreatment consisted of dipping of
cranberries into solutions of ethyl oleate and sodium hydroxide, but it showed no
significant difference on the pretreatment and was not used in later experiments. A
mechanical skin pretreatment, to be exact cutting of berries in halves or quarters showed
significant moisture removal improvement. Cutting in quarters was significantly better
than halves, and thus was applied before osmotic dehydration. Osmotic dehydration as the
main drying pretreatment had to be tested more profoundly, and different process
parameters were tested, such as osmotic agent type (granular sucrose and high fructose
com syrup) and concentration (from 1:1 to 4:1 agent to fruit ratio for granular sucrose and
1:1 and 2:1 for high fructose com syrup), and process duration (12, 24, 36, and 48 hours).
High fructose com syrup exhibited higher moisture removal values than granular sucrose,
and higher concentration of 2:1 was significantly better than 1:1 concentration. Process
duration of 24 hours was chosen because it offered sufficiently high moisture removal
and product quality when compared to processes of 36 and 48 hours.
Upon identifying the best pretreatment method (method that offers the highest
moisture loss possible), cranberries were subjected to microwave drying under
subatmospheric pressures. Various process conditions were evaluated in order to find the
most appropriate method of cranberry drying. Microwave power densities were 1.00,
1.25, and 1.50 W/g of initial total sample mass, microwave power modes were continuous
88
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and pulsed, where pulsed mode consisted of two combinations: 30 s power-on time/30 s
power-off time, and 30 s power-on time/45 s power-off time. Three levels of vacuum
were tested: 3.4, 18.6, and 33.8 kPa of absolute system pressure. Tested product quality
parameters were colour, texture properties (toughness and Young’s modulus), rehydration
ratios, and special emphasis was given to energy efficiency. Colour of cranberries was
significantly influenced by both microwave power and microwave mode factor, but the
pressure applied had no significant influence. Higher power levels (1.25 and 1.50 W/g)
and modes with longer power-off time (45 s) should be used in order to obtain the product
with lower colour difference from fresh cranberries. Young’s modulus responded in the
same way, having values closer to fresh fruit when dried with higher microwave power
level with pulsed microwave cycle with longer power-off time, and without significant
influence from the applied vacuum. Toughness of cranberries was not affected with any
of the aforementioned process conditions. The only parameter on which vacuum had
significant influence was rehydration ratio, and this parameter was higher when the
absolute system pressure was lower. Higher microwave power (1.25 and 1.50 W/g) and
pulsed microwave mode with longer power-off time (30 s power-on/45 s power-off)
increased rehydration ratio significantly. Drying efficiency responded similar to product
quality parameters, showing improved values for higher microwave power densities (1.25
and 1.50 W/g), pulsed microwave mode with longer power-off time (30 s power-on/45 s
power-off), but did not respond notably to vacuum factor. In general, for obtaining higher
quality dried cranberries, it can be concluded that higher microwave power densities
should be applied, pulsed modes with longer power-off time (30 s power-on/45 s poweroff time), and lower system pressure (3.4 kPa of the absolute pressure) should be applied.
In the final chapter, the appropriateness of microwave/vacuum method was tested,
comparing it with microwave/convective drying. Parameters such as colour values,
texture characteristics, sensory properties (colour, texture, taste, and overall appearance),
drying efficiencies, and temperature during process were compared for both of the drying
methods. Colour values of the final product were quite similar for both methods, but
texture properties (toughness and Young’s modulus) were better in microwave/vacuum
dried cranberries. Total microwave power-on time was longer for microwave/convective
process, resulting in lower drying efficiency. Although significant statistical differences
were not found, taste panel showed higher preference for microwave/convective dried
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samples than for microwave/vacuum, remarking that later samples were with “burnt”
odour and caramelised taste.
Overall, this work proves that fruits should be pretreated before drying in order to
optimise efficiency of process and improve final product quality. Cranberry
microwave/vacuum drying method demonstrated good characteristics, and small fruits
such as berries can be dried using this hybrid process. The high value of dried berries
might prevail over high cost of this process, making it economical.
Further research in this area should incorporate more thorough exploration of
chemical changes (especially vitamin and anthocyanin content) during storage of
cranberries after different drying methods. In addition, microwave/drying method should
be tested on different commodities, especially sensitive fruits such as berries.
90
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REFERENCES
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