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Nutrition & Food Science
Preliminary studies on the microbiological and physico-chemical characteristics of
fermented pounded yam
E.T. Otunola, R.S. Ogunbiyi,
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E.T. Otunola, R.S. Ogunbiyi, (2005) "Preliminary studies on the microbiological and physico‐chemical
characteristics of fermented pounded yam", Nutrition & Food Science, Vol. 35 Issue: 3, pp.135-142, https://
doi.org/10.1108/00346650510594877
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Preliminary studies on the
microbiological and physicochemical characteristics of
fermented pounded yam
Characteristics
of pounded yam
135
Downloaded by California State University Fresno At 09:31 25 October 2017 (PT)
E.T. Otunola
Department of Food Science and Engineering, Ladoke Akintola
University of Technology, Ogbomoso, Nigeria
R.S. Ogunbiyi
Institute of Basic and Applied Sciences, Kwara State Polytechnic,
Ilorin, Nigeria
Abstract
Purpose – The aim of the paper is to evaluate the microbiological and physico-chemical
characteristics of fermented pounded yam, a unique and popular food of the Igbominas of Nigeria.
Design/methodology/approach – Fresh pounded yam was prepared in the laboratory by a
modification of the traditional method, and then fermented for varying periods of time (0-96 hrs), also
essentially by the traditional method. Analysis was carried out on the products following
documented and established procedures.
Findings – Data obtained indicated noticeable increases in the populations of all the groups of
microorganisms investigated with increases in fermentation time, except coliforms which declined in
population after 24 hours. Heating of the samples after each fermentation interval resulted in
significant reductions in microbial populations such that coliforms could not even be detected after
24 hours of fermentation. Lactic acid production, an indication of flavour development, and pH drop
were highest at 48 hours of fermentation. The samples also became softer with increases in
fermentation period. The heated samples were considered safe microbiologically, since they contained
neither coliforms nor faecal coliforms.
Research limitations/implications – The consumption of fermented pounded yam is encouraged
since it is safe microbiologically and develops a desired flavour, while further research should be
carried out on the nutritional benefits.
Practical implications – The findings here suggested that adequate heating after fermentation is
necessary to eliminate any potential microbial contamination, and for full flavour development.
Originality/value – The results of this research contribute to the knowledge of fermented foods,
especially those that are indigenous to Nigeria and West Africa.
Keywords Fermentation, Acidity, Food products, Nigeria
Paper type Research paper
Introduction
Yam (Dioscorea spp) constitute one of the major staple foods in Nigeria and many West
African countries. The origin of this important root crop is believed to be in the south
eastern Asia but later introduced into the tropical zones of Africa and America (Philips,
1959; Coursey, 1967; Ustimanko - Bukumousky, 1983).
The genus Dioscorea is known to have several species, but only about ten species
are cultivated for food uses while another six species are cultivated for pharmaceutical
and other industrial purposes (Coursey, 1967, 1983; FAO, 1994). Their availability is
however usually seasonal, being more abundant in the middle of the raining season
than in any other part of the year (Coursey, 1967; Onwueme, 1978). Moreover, storage
Nutrition & Food Science
Vol. 35 No. 3, 2005
pp. 135-142
# Emerald Group Publishing Limited
0034-6659
DOI 10.1108/00346650510594877
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136
as at now is mainly by the traditional methods as practiced by the subsistence farmers
of the tropics, and hardly lasts for six months after harvest without spoilage (Coursey,
1983; Olorunda and Adesuyi, 1973; Asiedu, 1989).
The various ways by which yam tubers are processed for consumption include
boiling, roasting, frying and conversion into pounded yam. Of all these, conversion into
pounded yam, though very laborious, appears to be the most popular and most
cherished especially in West Africa, and particularly among the Yorubas of the sub
region. Here it is even very much tied to the culture of the people such that it is
generally regarded as the most important and sometimes the most critical and most
acceptable meal during cultural festivals and traditional wedding ceremonies. Its
consumption is always accompanied, with various types of soups such as okra,
spinach, melon, or a combination of these and assorted types of meat or fish. Pounded
yam is locally known as ‘‘iyan’’ by the Yorubas of West Africa, but may be called a
variety of names such as ‘‘fufu’’ in non-Yoruba speaking areas and ‘‘foutou’’ in parts of
French speaking West Africa countries especially in those parts where the Yorubas
have migrated over the years (Asiedu, 1989).
It has long been established that fermented foods play an important role in local
diets all over the world. It is also widely acknowledged that food fermentation is a
relatively efficient, low-energy preservation process that can normally prolong product
life, and more importantly, reduces the need for refrigeration or other energy- intensive
operations in food preservation. It is also believed to be the oldest means of food
preservation. Furthermore, the art of food fermentation, which may be almost as old as
the origin of man, can be regarded as lying between traditional arts and modern
science. It is also widely known that the classes of fermented foods produced in a
particular part of the world, not only reflect the diets of the people of that region, but
also reflect, to a large extent, the culture and tradition of such people. This is especially
true of all parts of Africa and particularly all parts of Yoruba land of West Africa,
where there is a large dependence on fermented foods obtained from various classes of
food crops and in various forms.
The Igbominas constitute a prominent and large group among the Yoruba tribe of
Nigeria and other West African countries. They are found mainly in Irepodun, Ifelodun
and Isin local government areas of the present Kwara State, as well as in Ila Local
government area of the present Osun State, all of which are in the south western part of
Nigeria.
One of the unique and well cherished traditional practices of the Igbominas is the
consumption of fermented pounded yam. In the traditional process, excess pounded
yam is deliberately prepared the previous day to take care of the immediate need of the
following days. This excess is allowed to undergo natural fermentation for varying
periods of time depending on the intended ultimate use. At the end of the desired
period of time, it is then spread evenly on the bottom of a flat earthenware plate and
heated until steam emerges for a few minutes. It is then served for consumption.
Part of the recognised advantages of this practice is the reduction in preparation
time as a lot of time is saved in the mornings to allow farmers to go to their farms and
school children to go to school early enough. The other benefit is the development of a
desired and highly cherished flavour the nature of which has hitherto not been
investigated. The other salient and usually unrecognized, and yet important benefit is
preservation.
In consonance with situations in similar fermented foods, various species of
microorganisms are expected to be involved in the fermentation process and the
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attendant flavour development. The physico- chemical properties of the pounded yam
would also definitely undergo some changes. In similar traditional fermented foods
such as ‘‘agidi’’, and reconstituted cassava flour, such changes have been found to be
significant (Oluwamukomi, 1986; Adeyemi and Oluwamukomi, 1989; Oyelade et al.,
2003).
Information available from literature so far has not revealed any such studies on
fermented pounded yam in spite of its popularity and importance to the Igbominas of
Nigeria. This study therefore aims at investigating the microbiological and physicochemical properties of this unique fermented food.
Characteristics
of pounded yam
137
Materials and methods
Materials
Yam tubers, wooden pestles and mortar were purchased from Ipata market in Ilorin,
Nigeria.
Methods
Preparation of pounded yam. Pounded yam balls were prepared in the laboratory by a
modification of the traditional procedure as shown in Figure 1. Healthy yam tubers
were carefully selected and washed thoroughly with tap water. They were then peeled
Yam Tubers
Washing
Peeling
Dicing
Washing
Boiling
Pounding/mashing
Figure 1.
Packaging into balls
Flowchart for the
production of fresh
pounded yam
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138
and diced into desirable sizes and washed again with tap water. Boiling was done on a
hot plate for about thirty minutes. The boiled yam pieces were then pounded with
wooden pestles in the wooden mortar with intermittent addition of hot water until a
stiff glutinous dough was obtained. At this stage, the granular structure of the starch
molecules would have become considerably disrupted. The dough obtained was then
moulded into many ball-like portions for further studies and analyses.
Fermentation of the pounded yam balls. Some portions of the pounded yam balls as
obtained above were selected for fermentation. Fermentation was carried out
essentially by the traditional method. The portions were separately covered in bowls
and were allowed to ferment naturally at room temperature for varying periods of time,
ranging between 24 and 96 hours. At the end of each interval a portion was selected
and used for required analysis.
In order to investigate the effect of heat, as being practiced traditionally, on the
microbial populations, portions from each lot were selected at the end of each
fermentation period, spread evenly on the bottom of a flat earthenware plate to a
thickness of about 4 mm, and then heated to an internal temperature of about 110 C.
The population of each type of microorganism was then determined after the heating
period.
Determination of the populations of the various groups of microorganisms. The
populations of the various microorganisms encountered during fermentation, both
before and after heating were determined by the procedures of ICMSF (1978). Nutrient
agar, MacConkey agar and Man-de-Rogosa and Sharpe (MRS) agar were used for total
aerobic bacterial, coliform and lactic acid bacterial counts respectively, while acidified
potato dextrose agar was used to determine the populations of yeasts and molds.
All counts were made with a Gallenkamp electronic colony counter (model cx 300).
All determinations were made in at least duplicates and the mean value recorded in
each case.
Determination of the physico-chemical characteristics. The pH of each sample was
determined by the AOAC (1974) method using a Pye-Unicam pH meter, while titratable
acidity was determined by the method of Adeyemi and Oluwamukomi (1989). The
results expressed as per cent lactic acid.
Syneresis was determined by the procedure described by Radley (1976) as modified
by Oluwamukomi (1986). The gel strength of each sample was determined by the
method of Haighton (1959), using a cone penetrometer (Stanhope-Seta 1700), while the
water absorption capacity was measured by the procedure described by Quinn and
Paton (1979).
All determinations were in triplicates and the mean value recorded in each case.
Results and discussion
Populations of the different groups of microorganisms in the fermented but
unheated samples
Data on the pattern of changes that occurred in the populations of the different groups
of microorganisms before heating the fermented pounded yam samples are as
presented in Table I. It was observed that all the groups investigated increased in
population with corresponding increases in fermentation period. The total aerobic
bacterial increased from 0 in the fresh and unfermented pounded yam to 250 103
cfu/gm after 96 hours of fermentation. Similar trends were observed with respect to the
populations of the lactic acid bacteria, yeasts and molds (Table I). The trend with
respect to coliform counts was however different in the sense that after an initial sharp
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increase from 0 to 110 103 cfu/gm within the first 24 hours of fermentation, the
population declined to 52 103 cfu/gm at 96 hours of fermentation.
The increases observed thru ought the fermentation period with respect to all the
groups of microorganism, except the coliforms is not unexpected since yam, from
which the pounded yam was obtained is essentially a carbohydrate food providing
adequate sources of carbon and energy for the various microorganisms. This situation
therefore has the potential to lead to increases in the populations of the
microorganisms involved. Furthermore the prevailing room temperature of
fermentation is known to be very favourable to the growth of these microorganisms.
The lactic acid bacteria and some yeasts, which are generally lacking in starch
degrading enzymes, even showed increases in their populations. This could be
attributed to the fact that the presence of molds and other starch degrading
microorganisms could make simple sugars available for the use of non-starch
degrading microorganisms in the fermenting environment.
The different trend observed with respect to coliform population could be explained
on the basis of the fact that the initial increase was due to availability of nutrients. The
sharp decline after 24 hours of fermentation was probably due to the unfavourable
environment later created by the presence of lactic acid bacteria. Lactic acid bacteria
are generally known to do this by producing lactic acid as shown in this work (Table II).
Lactic acid in turn lowers the pH of the medium and at the same time on its own
inhibits the growth of many enteric bacteria including coliforms and associated
pathogenic microorganisms (Steinkrraus 1989; Jay, 1978; Frazier and Westhof, 1978).
Moreover the lactic acid bacteria are also known to produce some anti-bacterial
substances which are active mostly against coliforms and pathogenic bacteria in
fermented foods. The situation here may not be an exception.
Characteristics
of pounded yam
139
Populations of microorganisms (103) cfu/gm
Total aerobic
bacteria
Lactic acid
bacteria
Period of
fermentation
(hrs)
UH
HE
UH
HE
UH
HE
UH
HE
UH
HE
0
24
48
72
96
0
159
200
218
250
0
10
15
18
30
0
110
96
60
52
0
42
0
0
0
0
120
150
170
200
0
6
10
16
20
0
35
50
80
120
0
7
4
2
0
0
50
95
125
150
0
8
5
3
0
Coliforms
Molds
Yeasts
Notes: UH=Unheated, HE=Heated after fermentation
Fermentation
period (hrs)
0
24
48
72
96
Titratable acidity
(%Lactic acid)
PH
5.54
5.00
4.46
5.02
5.86
±
±
±
±
±
Table I.
Changes in the
populations of various
groups of
microorganisms with
fermentation period
0.08
0.01
0.14
0.08
0.15
0.23
0.27
0.68
0.54
0.50
±
±
±
±
±
0.01
0.01
0.03
0.02
0.04
Table II.
Changed in the pH and
titratable acidity with
fermentation period
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140
The flavour usually enjoyed in fermented pounded yam is most likely to be due to
the production of lactic acid and some other flavouring substances produced by the
lactic acid bacteria and yeasts. Lactic acid bacteria and yeasts are known to produce a
number of flavouring substances such as acetoin and diacetyl in addition to lactic acid
( Jay, 1978; Frazier and Westhof, 1978; Steinkraus, 1989). These substances, usually
produced from carbohydrate foods are also likely to contribute to the flavour enjoyed in
fermented pounded yam.
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Populations of microorganisms in fermented and heated pounded yam samples
As indicated in Table II, the populations of the various groups of microorganism
reduced drastically after heating the fermented pounded yam samples, regardless of
the length of fermentation. Such reductions were particularly noticeable with respect to
coliforms, which could not be detected at all after 24 hours of fermentation, as well as
yeasts and molds neither of which could not be detected after 72 hours (Table
II).
These observations are not surprising since the temperature of heating (110 C) has
the potential to destroy virtually all microorganisms especially those in the vegetative
forms. A few that can survive do so by means of spore formation as in molds and some
yeasts, or some other means of heat tolerance as usually exhibited by some lactic acid
bacteria. The seemingly initial survival of some groups of microorganisms, especially
within the first 48 hours, even after heating as observed here could be due to the
protective effects of some foods against the lethal effects of heat on some
microorganisms. Some food materials, especially carbohydrate foods, are known to
confer such protective effects on some microorganisms in them ( Jay, 1978; Frazier and
Westhof, 1978).
pH and titratable acidity of the fermented pounded yam samples
The pH values, all of which though remained in the acidic range, decreased from 5.54 in
the unfermented sample to 4.46 within the first 48 hours of fermentation, but however
increased again to 5.86 when fermentation was increased to 96 hours (Table II). The
trend with respect to titratable acidity was in the reverse, in the sense that it increased
sharply from 0.23 per cent (lactic acid) in the unfermented sample to 0.68 per cent
within the first 48 hours, and dropped again to 0.50 per cent after 96 hours of
fermentation (Table II). Both of these observations nonetheless implied increases in
acidity within the first 48 hours of fermentation, which is in agreement with
expectation since fermentation of carbohydrate materials normally results in increased
acidity as indicated by drops in pH and increase in titratable acidity. In this study, these
also resulted in decreases in the population of some of the microorganisms, notably the
coliforms as indicated in Table I. The increase in pH and decrease in titratable acidity
with further fermentation however appears unexpected. A possible explanation could
Table III.
Changes in the physical
properties of fermented
pounded yam with
fermentation time
Fermentation
period (hrs)
Syneresis
(mm)
0
24
48
72
96
0.0
1.0
3.0
6.0
10.0
±
±
±
±
±
0.0
0.0
0.1
0.0
0.2
Gel strength
(mm)
22.4
30.0
31.6
32.0
36.0
±
±
±
±
±
0.2
0.1
0.2
0.2
0.1
Water absorption
capacity (%)
233.3
173.3
166.7
160.0
153.3
±
±
±
±
±
2.0
1.8
2.5
1.0
0.8
Moisture
content (%)
70.0
72.5
76.8
80.0
84.0
±
±
±
±
±
1.1
0.8
0.5
0.7
0.2
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be that part of the carbohydrate content might be converted into protein or some other
nitrogenous material by fermentation, the metabolism of which can later lead to the
formation of ammonia which could in turn increase the pH or reduce the acidity of the
medium. Suggestions on such conversion of carbohydrate had earlier been made with
regards to similar observations in cereal and legume fermentations (Otunola and
Ogunrombi, 1998; Sunny-Igweji et al., 1998). This aspect may however need further
investigation.
The lactic acid produced as observed here is likely to contribute significantly to
flavour development and its production is likely to be at its best when pounded yam is
fermented for 48 hours as indicated in Table II.
Changes in the physical characteristics of fermented pounded yam
The values of syneresis and moisture contents showed decreases with increases in the
fermentation period, while those of water absorption capacity and gel strength
indicated a reverse trend (Table III). The values of syneresis as observed here are
generally very low compared with the observations of earlier workers on similar
starchy foods (Oluwamukomi, 1986; Ade-Omowaye et al., 2001; Adeyemi and
Oluwamukomi, 1989; Otunola et al., 2003). This could however serve as an indication
that very little water was squeezed out between associated polymers of amylose and
amylopectin on prolonged cooling of the samples. The rate of this squeezing however
increased with increases in fermentation period (Table III).
The decreasing values of the gel strength as indicated by increasing depth of
penetration (Table III) serves to prove that the samples became softer with
fermentation period. This was further supported by the increasing values of moisture
content, and corresponding decreases in those of water absorption capacity (Table III).
Conclusion
The results of this work have revealed that fermentation could have significant effects
on the microbiological and physico-chemical attributes of pounded yam. Also, the
fermented product, in addition to acquisition of desirable flavour is likely to be free of
coliforms and pathogenic microorganisms especially when subjected to heat treatment
after the desired period of fermentation. Its consumption may therefore be encouraged
and the production procedure may be modified to reflect modern scientific technology
to improve product quality. In addition, further investigation may need to be done on
the food product, especially on the nutritional characteristics.
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