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Journal of the Science of Food and Agriculture
J Sci Food Agric 79:1325±1330 (1999)
The effects of post-harvest handling on physical,
chemical and functional properties of cowpea
(Vigna unguiculata) seed
JD Abu,1 SS Arogba1* and FM Ugwu2
1
Department of Food Science & Technology, Federal Polytechnic, PMB 1037, Idah, Nigeria
Department of Food Science & Technology, Enugu State University of Science and Technology, Abakaliki Campus, Abakaliki, Nigeria
2
Abstract: Damage by insects and mould, proximate composition, cooking rate and functional
properties of sun-dried and stored cowpea (Vigna unguiculata) seeds were evaluated. Cowpea samples
spread between black polythene ®lms were placed on cement (CS), Wooden (WS) and corrugated iron
sheet (CIS) surfaces, respectively, and sun-dried for 5 h. Storage lasted 6 months in jute and polythene
bags. Results show that sun-drying and storage in polythene bags signi®cantly (p < 0.05) lowered the
extent of insect and mould damage, moisture-loss at drying, emulsion activity and stability, foaming
capacity and foam stability. Treated cowpea seeds could be stored safely for about 5 months. In
contrast, storage in jute bags signi®cantly (p < 0.05) lowered only foam stability. Moisture loss during
sun-drying principally lengthened cooking time as storage duration progressed. Therefore, assessment
of moisture content before and during storage is a preferred quality index of cowpea seeds stored in
polythene bags.
# 1999 Society of Chemical Industry
Keywords: sun-drying; physical properties; chemical properties; functional properties; cowpea; storage
INTRODUCTION
Grain legumes make a signi®cant contribution to the
world's vegetable protein supply. Among the grain
legumes, several varieties of cowpea are largely
cultivated in tropical countries of the world. Nigeria
is the largest producer of dry cowpea seeds with
1.0 106 tonnes of the world's 1.3 106 tonnes.1
Insects, however, constitute a major constraint to grain
storage and utilisation in Nigeria and many of the
Third World countries. The loss in dry weight of about
30% in a 6 month storage has been reported.2
The choice of appropriate drying techniques for
food crops in the tropics is based on cost. Sun-drying is
attractive to the small-scale farmers who are responsible for the production and preservation of the
majority of the food crops in developing countries.
Spreading of soyabean seeds on cement ¯oors covered
with black polythene ®lm in order to enhance the
retention of solar energy during sun-drying is a recent
practice.3 The effect of heat treatment on functional
properties of raw and processed brown bean (Canavalia rosea DC) ¯our has been reported.4
The quality of cowpeas dried on different surfaces,
such as cement, wood and corrugated iron sheet, has
not been investigated. The objectives of the present
study were, therefore, to evaluate the effects of sundrying using the surfaces mentioned on the rate of
insect and mould infestation, and changes in chemical
and functional properties of cowpea (Vigna unguiculata) seeds during storage.
MATERIALS AND METHODS
Sample collection and handling
The black-eyed variety of cowpea (Vigna unguiculata)
seed was purchased from a local market. A lot (15 kg)
was divided into ®ve equal portions. One portion (raw
sample) was immediately subjected to proximate
analysis. Three portions (treated samples) were sundried as described below while the ®fth portion (untreated) served as control. The treated and untreated
samples were subdivided equally and put in jute and
polythene bags and then stored at 26±29 °C and 72±
82% relative humidity for 6 months (March±August).
Analyses were conducted at monthly intervals on the
extent of insect damage, mould growth, moisture
content and cooking time while proximate analysis and
functional properties were conducted at the beginning
and end of storage.
Treatment (sun-drying techniques)
The method of Awonorin and Bamiro3 was modi®ed.
The three portions were spread between two black
polythene ®lms (0.1 mm thick) and placed on corru-
* Correspondence to: SS Arogba, Department of Food Science & Technology, Federal Polytechnic, PMB 1037, Idah, Nigeria
(Received 3 April 1996; revised version received 24 August 1998; accepted 17 December 1998)
# 1999 Society of Chemical Industry. J Sci Food Agric 0022±5142/99/$17.50
1325
JD Abu, SS Arogba, FM Ugwu
Estimation of mould infestation
at 25 °C 2 °C for 60 min before centrifuging at
2000 g for 15 min. Excess water or oil was
decanted by inverting the tubes over absorbent
paper. Samples were allowed to drain. The weight
of water or oil bound was determined by difference.
(d) Bulk density. The method of Giami and Bekebain9
was used. Flour (50 g) was weighed into a 100 ml
graduated cylinder. The cylinder was tapped ten
times against a smooth table surface and the ®nal
volume of the contents was recorded. The tapped
bulk density was expressed as g cmÿ3.
The extent of infestation of 400 seeds randomly picked
were assessed visually.
Cooking rate
gated iron sheet (CIS), wooden (WS) and cement
(CS) surfaces of 0.6 m2 each. The sun-drying lasted for
5 h. Stirring of samples was done at 1-h intervals.
Estimation of damaged seeds
The method of Igbeka (unpublished) was employed. A
total of 400 seeds were randomly picked. The number
of seeds that had insect exit holes were counted.
Sampling was replicated twice and % damaged
seeds = 0.25 (average number of bored seeds).
Chemical analysis
Proximate analysis of powdered samples were carried
out by the AOAC5 methods for moisture, crude fat,
crude ®bre, ash and crude protein. A nitrogen to
protein conversion factor of 6.25 was used. Carbohydrate content was derived by difference.
This was determined by the method of Demooy and
Demooy10 with slight modi®cation. A random sample
(100 seeds) was used on on day 0 and at bimonthly
intervals. Cooking was done at atmospheric pressure
using Tecator block digester that ensured a uniform
and constant temperature during boiling. Cooking was
ended when subsamples, withdrawn periodically,
softened to a uniform mass.
Functional properties
Whole seed ¯our which was passed through a 0.40 mm
sieve was evaluated for the functional properties listed
below. Analyses were replicated twice in each case.
(a) Emulsion activity and stability. These were determined using the methods described by Yasumatsu
et al.6 Flour (1.0 g) and equal volumes (5 ml) of
distilled water and re®ned vegetable oil
(SG = 0.90) were used. The emulsion was centrifuged at 2000 g for 5 min. The ratio of the height
of the emulsi®ed layer to the total height of the
content of the tube was calculated as the emulsion
activity and expressed in percentage. To determine
the emulsion stability, the prepared emulsion was
heated at 80 °C for 30 min in a water-bath. The
heated emulsion was cooled under running tap
water for 15 min and then centrifuged at 2000 g
for 5 min. The emulsion stability expressed as
percentage was determined as the ratio of the
height of emulsi®ed layer to the total height of the
whole layer.
(b) Foaming capacity and stability. These were evaluated using the method of Coffman and Garcia.7
Flour samples (2 g) in 50 ml of distilled water were
mixed in a pre-cooked semi-micro blender jar set
at high speed for 3 min. The resulting mixture was
poured into a graduated cylinder and allowed to
settle for 2 min. The volume of foam was reported
as the foaming capacity. The volume of foam after
standing at 25 °C 2 °C for 60 min was expressed
as a percentage of the initial foam volume, and
reported as the foam stability.
(c) Water and oil binding capacities. These were studied
using the method described by Okaka and Potter.8
Flour (1 g) and 6 ml of distilled water or re®ned
vegetable oil (SG = 0.90) was added. Sample
suspensions were vortexed and allowed to stand
1326
Statistical data
Data obtained were subjected to analysis by the LSD
technique.11
RESULTS
The types of storage material used or surfaces
employed at sun-drying are appropriately termed
`treatment' in this paper.
The temperature recorded between the polythene
®lms used during sun-drying ranged from 40 ° to
66 °C. The rate of damage by insects to cowpea seeds
during storage is shown in Table 1. Appreciable
damage was noticed from month 4 except for treated
samples (CS, WS, CIS) stored in polythene bags,
which remained uninfested up to month 6. Mould
growth (Table 1) was visually detectable in all samples
as from month 4 and with higher density in samples
stored in jute bags. Treated samples in polythene bags
remained mould-free throughout the period of study.
Table 2 shows the proximate composition of treated
and untreated cowpea samples stored for 6 months.
Our data on the raw sample, that is control R,
compared favourably with those of Akpapunam and
Markakis12 which gave the average composition
(g kgÿ1) of 13 cultivars as moisture 96, protein 239,
®bre 41, lipids 13, ash 37 and carbohydrate 577.
Controls P and J, treated samples P and J, and control
R, however, all showed statistically signi®cant differences (p < 0.05) in all parameters analysed except the
`ether extract' content, at the end of the 6 months'
storage. The observed analytical differences were
primarily due to differing moisture contents of samples
studied as in¯uenced by the drying technique, drying
duration of 5 h and nature of packaging materials.
Figs 1 and 2 describe the linear relationship between
J Sci Food Agric 79:1325±1330 (1999)
Post-harvest handling effects on properties of cowpea
Treatment
Drying surface*
1
2
3
4
5
% insect damage
Polythene
0
0
0
0
0
0
0
0
2
0
0
0
8
0
0
0
12
0
0
0
15
0
0
0
Jute
0
0
0
0
0
0
0
0
3
1
1
2
12
4
3
4
18
6
5
7
21
15
10
12
Polythene
±
±
±
±
±
±
±
±
±
±
±
±
‡
±
±
±
‡
±
±
±
‡‡
±
±
±
Jute
±
±
±
±
±
±
±
±
‡
±
±
±
‡‡‡
‡
‡
‡
Packaging material
C
CS
WS
CIS
C
CS
WS
CIS
C
CS
WS
CIS
Table 1. Extent of insect and mould
damage on treated and untreated
cowpea samples during storage
Storage time (months)
C
CS
WS
CIS
6
‡‡‡‡‡ ‡‡‡‡‡‡
‡‡
‡‡‡
‡‡‡
‡‡‡
‡‡
‡‡‡‡
* C, control; CS, cement surface; WS, wooden surface; CIS, corrugated iron sheet surface.
Table 2. Effect of treatments on the proximate composition of sun-dried cowpea sample stored for 6 months
Proximate composition (g kgÿ1)
Treatments
Control R (raw sample, not dried, not stored)
Control P (raw sample, not dried, polythene
packaging)
Treated P* (dried 5 h, polythene packaging)
Control J (raw sample, not dried, jute packaging)
Treated J* (dried 5 h, jute packaging)
SE
LSD(0.05)
Moisture Crude protein
Crude ®bre Ether extract
Ash
Carbohydrate
(by difference)
102.1c
130.5c
236.2a
220.5c
39.0b
32.0e
14.5a
14.4a
32.0d
33.0c
576.2a
567.6b
125.3d
147.1a
142.7b
8.86
2.61
225.7b
216.0d
219.2c
3.94
1.74
37.4c
42.0a
36.0d
1.85
1.19
14.5a
14.5a
14.7a
0.13
0.32
35.0b
36.0a
34.7b
0.81
2.79
561.9b
544.4d
552.7c
6.23
2.19
* Mean values from corrugated iron sheet, wooden and cement sun-drying surfaces Different letters following in a column differ signi®cantly at 5% level by LSD
test.
moisture gain and storage time, notwithstanding the
type of storage material. Computed coef®cients of
variation for the control and treated samples ranged
from (0.90) to (0.98). The respective regression
equations are also indicated on the graphs. It is
apparent from the gradient for the samples stored in
jute bags that the choice of sun-drying surface had no
signi®cant effect (p > 0.05) on the rate of moisture gain
during storage. The treated seeds, however, absorbed
moisture faster (p < 0.05) than the control, as was the
case of samples stored in the polythene bags. Hygroscopicity of samples from polythene bags increased in
the order: control; CS; WS; CIS. The points of
intersection (Fig 2) between surfaces in respect of
storage time:moisture content are (4.9, 118.5) for
control/CS, (4.6, 117.5) for control/WS, (3.4, 117.0)
for control/CIS, (3.1, 108.5) for WS/CS and (0.65,
95.5) for CIS/CS.
J Sci Food Agric 79:1325±1330 (1999)
Cooking rate
Computed coef®cients of variation between cooking
time and storage time ranged from (0.81) to (0.98).
Linear regression equations indicated that cooking
untreated and raw cowpea seeds could last at least
46 min. After storage for 6 months in jute and polythene bags, cooking time doubled (Figs 3 and 4).
Cooking time correlated positively with storage duration as was similarly reported on untreated cowpea
seeds by Onwuka.13 The control and treated samples
stored in jute bags showed signi®cant differences in
cooking time with increased duration of storage (Fig 3)
whereas CS- and CIS-treated samples took appreciably longer to cook than the control and WS-treated
samples stored in polythene bags (Fig 4).
With reference to the control of both storage
materials employed, only the CIS-treated sample from
the jute bag cooked at the lowest rate (Fig 3).
1327
JD Abu, SS Arogba, FM Ugwu
Figure 1. Effect of storage time on moisture content of treated and
untreated cowpea seeds stored in jute bags Key: —— control (CTL)
Y = 7.4x ‡ 104; ~ÿÿÿ~ cement surface (CS), Y = 8.6x ‡ 93;
ÿÿ wooden surface (WS), Y = 9.0x ‡ 91;
&……& corrugated iron sheet surface (CIS), Y = 9.5x ‡ 90.
Figure 3. Effect of storage time on cooking time of treated and untreated
cowpea samples stored in jute bags. Key: —— CTL, Y = 7.4x ‡ 45;
~ÿÿÿ~ CS, Y = 9.0x ‡ 46; ÿÿ WS, Y = 7.8x ‡ 46; &……& CIS,
Y = 6.3x ‡ 46.
Consequently, it appeared that the cooking rates of
samples sun-dried on cement (CS) and wooden (WS)
surfaces inversely correlated with the rate of moisture
uptake during storage (compare Figs 1 and 2 with 3
and 4). Burr et al 14 had reported that the cooking rates
of dry beans were dependent on moisture content,
temperature and time of storage.
Functional properties
Figure 2. Effect of storage time on moisture content of treated and
untreated cowpea seeds stored in polythene bags. Key: —— CTL,
Y = 2.3x ‡ 107; ~ÿÿÿ~ CS, Y = 5.4x ‡ 92; ÿÿ WS, Y = 5.0x ‡ 90;
&……& CIS, Y = 7.0x ‡ 91.
1328
Table 3 shows the effects of treatments on the
functional properties of cowpea samples. Bulk density,
water and oil absorption capacities were not affected
by treatments used in this study. In contrast, the values
for foaming and emulsion properties for control T and
treated samples P and J were signi®cantly lower
(p < 0.05) than those of control R. In this respect it
was noted that the use of a jute bag for storage caused
signi®cant decrease in the emulsion and foam stabilities of the dried samples (compare control T with
treated sample J).
Our data on control T, not shown in Table 3,
indicate that any pair of the underlying sun-drying
surfaces employed in¯uenced either the foaming
capacity, foam stability or emulsion stability of the
cowpea samples to different levels. Sun-drying lasted
5 h and depending on the surface type, maximum
temperatures recorded ranged from 40 ° to 66 °C. It
was implied that the surfaces enhanced drying to
J Sci Food Agric 79:1325±1330 (1999)
Post-harvest handling effects on properties of cowpea
peroxide value, free fatty acid contents, iodine value
and viscosity of the extracted soya oil.
DISCUSSION
Evaluation of physical and chemical properties
Figure 4. Effect of storage time on cooking time of treated and untreated
cowpea samples stored in polythene bags. Key: —— CTL,
Y = 6.4x ‡ 46; ~ÿÿÿ~ CS, Y = 8.3x ‡ 44; ÿÿ WS, Y = 6.5x ‡ 46;
&……& CIS, Y = 7.4x ‡ 48.
different levels by the extent they could re¯ect incident
solar energy back on the black polythene ®lms between
which the samples were spread. This view is supported
by a similar experimental design using cement and
corrugated iron sheet surfaces for sun-drying soyabeans.3 That study recorded maximum temperatures
of 58 ° and 67 °C, respectively, with differences in the
Table 1 indicates that, irrespective of treatments (sundrying technique and type of storage material), cowpea
seeds stored in the dry season (up to April) hardly
experience any physical deterioration. Cowpea seeds
intended for storage during the rainy seasons, however, need to be sun-dried on a surface and preferably
stored in polythene bags.
The results on the proximate composition of cowpea
samples (Table 2) at the end of 6 months' storage were
in¯uenced, singly or in combination, by the drying
technique, drying time, nature of packaging materials
and storage time. In particular, changes in parameters
other than moisture content appeared to re¯ect gain or
loss of water during storage. Sun-drying for 5 h using
cement or wooden surface (though results are not
shown in detail in Table 2) signi®cantly reduced the
moisture content, while using polythene bags for
storage could have caused slower moisture re-absorption. Higher equilibrium relative humidity (ERH) was
therefore attained using jute bags owing to wider
porosity compared to polythene bags.
The rate of moisture re-absorption during storage
was correlated with gradient values (Figs 1 and 2). The
correlation could be associated with the degree of seed
damage observed (Table 1) resulting in the exposure
of the internal tissues.
The signi®cance of choice between any two surfaces
for the purpose of sun-drying cowpea seeds could be
related to gradient values of storage time versus
moisture content graphs (see Fig 2). The preferred
surface should have lower gradient value. Consequently the associated duration of storage for each
storage material for optimum utilisation is derivable
from the point of intersection of control and the
Table 3. Effects of treatments on the functional properties of cowpea samples stored for 6 months
Functional Properties
Treatments
Water
Oil
absorption absorption
Foaming
Foam
Emulsion
Emulsion Bulk density capacity
capacity
capacity (ml) stability (%) activity (%) stability (%)
(g cmÿ3)
(g gÿ1)
(g gÿ1)
Control R (raw sample, not dried, not
stored)
Control T* (dried 5 h, not stored)
Treated P* (dried 5 h, polythene
packaging 6 months)
Treated J* (dried 5 h, jute
packaging 6 months)
SE
LSD(0.05)
23.5a
46.5a
43.5a
40.5a
0.54a
1.09a
0.85ab
21.5b
20.7b
40.3b
40.7b
35.8c
37.0bc
31.8c
32.7bc
0.58a
0.57a
0.97a
1.08a
0.96a
1.21a
21.3b
38.3c
37.7b
34.3b
0.61a
1.08a
1.28ab
0.70
0.82
2.04
1.40
1.98
1.38
2.26
1.47
0.02
0.13
0.03
0.18
0.18
0.34
* Mean values from corrugated iron sheet, wooden and cement sun-drying surfaces. Different letters following in a column differ signi®cantly at 5% level by LSD
test.
J Sci Food Agric 79:1325±1330 (1999)
1329
JD Abu, SS Arogba, FM Ugwu
surface of choice on the graph. Hence, the optimum
keeping quality of cowpea seeds stored in polythene
bags after sun-drying on CIS, WS and CS would be
3.4, 4.6 and 4.9 months, respectively. On the contrary,
the absence of points of intersection between surfaces
and the control (Fig 1), and the extent of mould and
insect damage (Table 1) indicate the insigni®cant role
of drying surfaces when jute bags are employed for
storage of the cowpea seeds.
Cooking consideration and functional properties
Positive correlation of cooking time with storage
duration could signify changes in conformational
structures of macromolecules such as carbohydrate
and protein. The effect of storage material on cooking
rate was confounded by the sun-drying treatment
given to the cowpea seeds. The inference was based on
the different cooking rates of CIS-treated samples
relative to the control (compare Figs 3 and 4).
Furthermore, the levels of physical damage by insects
and mould (Table 1) also had a secondary in¯uence on
the cooking rate of the cowpea samples.
Holes bored in the seeds by insects were expected to
enhance heat and water-absorption during cooking
and to consequently reduce cooking time. Figs 1 to 4,
however, suggest that the amount of moisture-loss
during sun-drying compared to that re-absorbed on
storage was a key factor affecting cooking time, using
the control samples as reference.
Table 3 clearly indicates that the foaming and
emulsion properties of cowpea were adversely affected
by sun-drying, which lasted 5 h, especially on CIS
surface. The situation was aggravated by prolonged
storage. Use of the jute bag particularly affected foam
stability.
Similarity in the oil absorption capacity of the
control (stored) and treated cowpea samples implied
that the conformational structures of molecules such
as starch and protein remained unaltered.4,9 The
observation is not surprising since the lipid component
of these samples was equally unaffected by treatments
(Table 2). The similarity in the water absorption
property could be associated with the moisture gain
during storage.
The sun-drying conditions particularly, however,
could have caused dissociation of some heat-sensitive
stabilising agents (phospholipids, glycolipids) with
respect to emulsion property,15 and change in albumin
and globulin concentrations with regard to foaming
property.16
CONCLUSION
This study has shown that the quality of cowpea seeds
at any period of utilisation is in¯uenced by some postharvest handling operations. Small-scale farmers and
households intending to commercialise or use cowpea
1330
seeds at off-peak seasons are strongly advised to sundry, preferably on a cement or wooden surface, and
store in sealed polythene bags. Although the rate of
insect and mould infestation is drastically reduced,
cooking time lengthens and preparation of products
requiring high emulsion and foaming properties would
need the addition of food grade emulsi®ers and
stabilisers.
ACKNOWLEDGEMENTS
One of the authors, JD, Abu was granted a study
fellowship by his employer.
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J Sci Food Agric 79:1325±1330 (1999)
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