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International Journal of Agriculture and Environmental Research
Volume:01,Issue:02
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OXALATE LEVELS IN SELECTED AFRICAN INDIGENOUS
VEGETABLE RECIPES FROM THE LAKE VICTORIA BASIN, KENYA.
1Mr. Wakhanu A. John (MSc), 2Prof. Kimiywe Judith (PhD, CNS),
3Prof. Nyambaka Hudson (PhD)
1Applied Analytical Chemistry, School of Pure and Applied Sciences, Kenyatta University,
P.o Box 43844, Nairobi, Kenya.
2Nutritionist and Consultant, Food, Nutrition and Dietetics Department, Kenyatta University,
P.o Box 43844, Nairobi, Kenya.
3Chairman, Chemistry Department, Analytical/Nutritional Chemistry, Kenyatta University,
P.o Box 43844,Nairobi, Kenya.
ABSTRACT
African indigenous vegetables (AIVs) in Lake Victoria Basin that could provide micronutrients
to fight malnutrition contain oxalates that reduce bioavailability. These can be reduced through
appropriate traditional food processing techniques adopted by households. This study determined
oxalate levels in formulated AIV recipes. Eleven selected AIVs and five AIV mixtures were each
divided into two lots. One lot was boiled and fermented for 48 hours and other lot unfermented.
The unfermented were subjected to three treatments; cooked by boiling in water, cooked by
boiling with cow’s milk and lye and cooked by sautéing. Oxalate levels in recipes were
determined using HPLC. Independent t-test was used to compare the mean oxalate levels
between fermented and unfermented recipes. One-way ANOVA was used to compare mean
oxalate levels between different methods of cooking. Oxalate levels in unfermented recipes
ranged from 2.62-10.17 mg/100g FW and in fermented, 1.54-20.36 mg/100g FW. The mean
levels in some fermented recipes were significantly lower than unfermented (p<0.05). Cooking
methods differently affected oxalate levels. Cooking methods and fermentation do not have a
uniform effect on oxalate level reduction in all AIV recipes but could still be employed as
household procedures in reducing oxalate levels in a number of AIV recipes.
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Keywords: African indigenous vegetables, oxalate levels, fermentation, cooking methods
INTRODUCTION
The African Indigenous vegetables (AIVs) in Vihiga County in Lake Victoria Basin (LVB)
include spider plant, African nightshade, pumpkin, cowpea, amaranths, jute mallow, and slender
leaf (Abukutsa-Onyango 2007). The potential of these AIVs as sources of micronutrients is
limited by the presence of anti- nutrients like phytate, oxalate, tannic acid, Ethylene diamine tetra
acetic acid (EDTA) and sapon in which bind to some micronutrients in the vegetables hence
limiting the micronutrient bioavailability (Makokha and Ombwara, 2005).Currently there is a lot
of research on bioavailability of micronutrients in AIVsand AIV anti-nutrient content and the
effects of these anti-nutrients on human health. It is only very recently that there has been a
significant interest toward Africa’s indigenous vegetables grown in home or backyard gardens
(Abukutsa-Onyango, 2010) otherwise AIVs normally face stiff competition with exotic
vegetables like cabbage, spinach, and lettuce among others (Maundu et al., 1999). The
introduction of exotic vegetables in the African continent had some negative impact on the
consumption and cultivation of indigenous vegetables. During the colonial time, a deliberate
suppression of the indigenous vegetables was done and a lot of efforts were made to promote the
exotic vegetables such as cabbage (Abukutsa-Onyango, 2010).The net effect of such suppression
flowed into the post independent era where the governments perpetuated the agricultural policies
developed by the colonial rulers. Changed food habits in favor of introduced temperate
vegetables lowered the demand of indigenous vegetables, due to the fact that the former fetched
higher prices in local markets (Abukutsa-Onyango, 2010). Indigenous vegetables were
considered out of fashion, poor man’s food that could only be used as a last resort. Thus they
enjoy less social prestige, being associated with the low-income group. As the poor sought to
imitate the eating habits of the affluent and were exposed to more fashionable exotic species, the
indigenous species became neglected (Abukutsa-Onyango, 2010). The neglect and stigmatization
was perpetuated by stakeholders like the policy makers, agricultural training institutions,
researchers, consumers and traders (Mnzava,1997). Having been branded and denoted by the
agriculturalists and researchers as weeds, the tendency was to eradicate them and not conserve
them as it were. However, the potential of AIVs for use in the eradication of malnutrition in poor
households has attracted a lot of research because of the numerous advantages these vegetables
possess over exotic ones.
AIVs adapt easily to harsh or difficult environments and require less input to grow as compared
to other crops. Furthermore AIVs are highly resistant to pathogens and require less attention
(Abukutsa-Onyango et al., 2006). This makes them appropriate for the alleviation of
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malnutrition in people living in areas with high population density in LVB. Their mineral and
vitamin content exceed levels found in exotic vegetables like cabbage (Abukutsa-Onyango,
2010). They also contain ascorbic acid which has been known to enhance iron absorption.
Populations who consume AIVs are less likely to suffer cardiovascular diseases, diabetes and
other diseases and this property is attributed to the presence of non-nutrient bioactive
phytochemicals (Smith and Eyzaguire, 2007). Some studies have also shown that AIVs contain
health promoting compounds such as anti-cancer factors, minerals, vitamins and anti-
oxidants(Abukutsa-Onyango, 2003). This boosts the body’s immune system if consumed. Apart
from providing important nutrients, AIVs can also play an important role in improving income
and subsistence to people (Adebooye and Parody, 2004). However, the potential of AIVs in their
use to fight malnutrition is limited by the anti-nutrients they contain. Anti-nutrients in vegetables
which include phytate, oxalate, tannic acids and hydrocyanic acids are associated with less
bioavailability of zinc, calcium, iron and magnesium in vegetables (Broadhurst and Jones,
1978;Akindahunsi, 2005). Anti-nutrients are organic in nature and chelate with mineral elements
to form insoluble complexes which interfere with absorption and assimilation of these mineral
elements in the human body (Munro and Bassir, 1969). AIVs also contain polyphenols such as
phenolic acids, flavonoids and their polymerization products. They form insoluble complexes
with iron and inhibit iron absorption. Tannin is a phenolic compound (Brown et al., 1990). Anti-
nutrient levels increase with age in stems, roots and seeds (Ekpedema et al., 2000; Weinberger
and Msuya, 2004).One anti-nutrient that has attracted study is the oxalate whose ionposes two
main problems: it reduces the bioavailability of essential elements in the human body such as
calcium, iron and zinc and its crystals block the kidney as kidney stones and also cause gout,
rheumatoid arthritis and vulvodynia (Franceschi and Nakata, 2005). Oxalate is distributed in
plants and this levels range in 3-15 % w/w of their dry weight (Franceschi and Nakata, 2005).
Some plants like rice accumulate oxalate to detoxify aluminium, lead, strontium, copper and
cadmium (Yang et al., 2000; Choi et al., 2001).Oxalate is actually a compound of oxalic acid
(ethanedioic acid); oxalic acid is a colourless and toxic organic compound that belongs to the
family of dicarboxylic acids whose formula is (COOH)2.2H2O. It is soluble in water, alcohol and
ether. It occurs as oxalate in plants and more so in green leafy foods.
Lack of knowledge on the correct choice of food, dietary diversity and anti-nutrient levels in
AIVs has led to underutilization of AIVs (Abukutsa-Onyango, 2003; Waudo et al., 2006;
Kimiywe et al., 2007). Diets in households within the LVB are primarily composed of cereals
and legumes that are high in anti-nutrients that inhibit micronutrient absorption. This can be
reduced through appropriate traditional food processing techniques adopted in households
(Walingo, 2009). The mineral and anti-nutrient content of local foods within LVB needs further
research to identify suitable sources of absorbable minerals and possible suitable dietary
combinations that can contribute towards the reduction of mineral deficiency (Walingo, 2009).
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Traditional food processing methods and diet combinations usually reduce the levels of mineral
anti-nutrients in the plant diets thus increasing mineral bioavailability. Household food
processing methods that promote nutrient content and bioavailability for improved health and
nutrient situation of rural populations whose diets are basically plant based with high anti-
nutrient content should be identified (Walingo, 2009). Some of these processes include thermal
processing, germination, milling/household pounding, microbial fermentation, and soaking.
Cooking has been shown as one of the factors that affect anti-nutrient and nutrient contents of
vegetables. The main methods of cooking in the study area involve boiling in unspecified
amounts of water contributing to nutrient loss and using additives like bicarbonate of soda, lye
(traditional salt),milk, sesame and groundnut paste whose effects are unknown. Cooking in study
area households is done by use of pots rather than pans as pots retain heat and give better
simmering effects (Abukutsa-Onyango, 2010). Also, the covering of the cooking pot is preceded
by sealing it completely with banana leaves and this helps to retain steam, which escapes with
some volatile nutrients and the aroma (Musotsi et al., 2005). Results also show that the recipes in
study area households are based on a mixture of different vegetables (Musotsi et al., 2005).
There is some evidence that boiling vegetables induce losses of 5%-15% of phytate and that
thermal processing can also enhance bioavailability of vitamins and carotenoids by releasing
them from entrapment in the plant matrix (Sandberg, 1991). Microbial fermentation is also one
of the food processing methods employed in the study area. Fermentation can induce phytate
hydrolysis via action of microbial phytase enzymes, which hydrolyze phytate to lower
inositolphosphates that do not affect mineral absorption (Sandberg, 1991). Microbial phytases
originate from micro flora on the surface of cooked food (Sandberg, 1991). Studies also reveal
that the enzyme phytase is affected by anti-nutrients like tannin hence interfere with hydrolysis
of phytate (Sandberg, 1991). Employing both cooking and fermentation of AIVs may contribute
to the increased bioavailability of micronutrients (Gibson et al., 2010). Kimiywe and Waudo
(2007) documented preparation and cooking procedures that could lead to a decrease in the
nutritive value of cooked food. These includes chopping before washing which leads to loss of
vitamin C and vitamin B complex since they are water soluble vitamins, repeated boiling and
frying destroys vitamin C and addition of sodium bicarbonate leads to loss of vitamin B
complex i.e. B1, B2 and niacin.There is a delicate balance between loss of nutrients and
reduction of anti-nutrients by traditional food processing methods adopted in LVB households.
Oke and Bolarinwa (2011) studied the effect of fermentation on oxalate content of cocoyam
flour. It was demonstrated that 48 hour fermentation reduced calcium oxalate significantly by
approximately 58%-65% and that the longer the fermentation period the higher the microbial
population and the higher the reduction of oxalate in the cocoyam. Iwuoha and Kalu (1995)
reported 82.1% and 61.9% oxalate reduction in cocoyam flour produced from boiled and roasted
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cocoyam respectively. The study concluded that high temperatures during cooking significantly
reduced the levels of oxalate in vegetables. Muchoki et al (2010) also demonstrated that high
temperatures reduced oxalate levels in Vigna unguiculata leafy vegetable. The effects of cooking
and microbial fermentation on levels of anti-nutrients needed a study. This study investigated the
oxalate levels in selected African indigenous vegetable recipes from the Lake Victoria Basin,
Kenya.
MATERIALS AND METHODS
Study site
The study was carried out in Vihiga County of Western part of Kenya in the LV Band is located
at longitude 34o 30” East and latitude 00o 11” and 00o 15” North and occupies an area of 563 km2
(CBS, 2001). It has four sub-counties: Emuhaya, Vihiga, Hamisi and Sabatia. It lies between
1300 m and 1500 m above sea level with an equatorial climate and a forest cover of 4 percent
and an annual precipitation of 1900 mm (District Strategic Plan, 2005). Land is arable and
supports a variety of crops (CBS, 1997). It is the third most densely populated County in Kenya
with a population of 595,180 people as per 2005 census. Population density is 975 persons per
sq. km (CBS, 2001). The County is dependent on food relief and its high population growth rate
cannot be sustained by its infrastructure and productivity. Adverse poverty indicators hinder
attainment of food security, as demand grows every year. With an average land size of 0.4
hectares per household, the county can no longer produce enough food. Malnutrition is a
common feature here (Akelola et al., 2007). Land is scarce and 60 percent of the population lives
below the poverty line. The main economic activity by residents is farming of maize, millet, tea,
cassava, sweet potatoes, beans and a variety of vegetables and fruits. Dairy farming is practiced
on small scale, as many people have been restricted to keeping one or two animals because of
inadequate pasture. Uneconomical land use and HIV/AIDS contributes to poverty, and
malnutrition is high due to wide spread poverty, poor feeding habits and over reliance on starchy
foods. Nearly 133 children per 1000 children die before their fifth birthday due to maternal
malnutrition (CBS, 2002). Despite having favorable climate and soils the area is not sufficient in
food production.
SOURCE OF THE AFRICAN INDIGENOUS VEGETABLES
The eleven AIVs selected for use in recipe formulations were spider plant (Cleome gynandra),
pumpkin leaves (Curcubita moschata), cowpea (Vigna unguiculata), green amaranth
(Amaranthusblitum), jute mallow (Corchorus olitorius), sweet potatoes leaves (Ipomea batatas),
African nightshade (Solanum nigrum), and cassava leaves (Manihot esculenta), slender leaf
(Crotolaria ochroleuca), vine spinach (Basella alba) and African kales (Brassica carinata).
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These vegetables were identified and about 1 kg of each purchased from markets within the
study area: Chavakali, Shisejeri, Shamakhokho, Gambogi, Kiboswa, Wemilabi, Luanda and
Majengo. Leaves of sweet potatoes (Ipomea batatas), cassava (Manihot esculenta) and nderema
(Basella alba) were identified and purchased from small vegetable farms in homes within the
study area. The purchased vegetables were de-stalked and the leaves washed with distilled water
to remove dirt. Vegetables of the same species from different markets were mixed to get a
representative sample for each species and transported to Kenyatta University in cool boxes for
recipe preparation. Exactly 1kg of each vegetable sample was weighed and blanched in one litre
of boiling water for two minutes to inactivate enzymes responsible for vitamin degradation
(Mosha et al., 1997 and Nyambaka and Ryley, 1996) and immersed in ice cold water for two
minutes to minimize premature cooking process.
PREPARATION OF LYE
Lye was prepared from pods of beans (Habwe et al., 2009). Pods of green beans were dried after
removing the mature seeds. The dry pods were then burnt over a hot dry pan and the ash
collected after complete burning. The ash was put in a container whose bottom had small holes
and distilled water poured through the ash into another container underneath and filtrate (lye)
collected.
PREPARATION OF RECIPES
There were five vegetable mixtures formulated, each containing 40g of the unmixedAIV as
follows: First mixture (S. nigrum + A. blitum), second mixture (C. ochroleuca + C. olitorius),
and third mixture (C. ochroleuca + C. olitorius + V. unguiculata), fourth mixture (A. blitum + C.
gynandra + S. nigrum) and fifth mixture (C. gynandra + C. moschata).Each of these blanched
five vegetable mixtures and each of the eleven unmixed selected AIVS were divided into two
lots. One lot was fermented and the other lot unfermented. The lot to be fermented was boiled,
cooled and left in the open to allow microbial fermentation to occur for 48 hours. The
unfermented lot was divided into three portions for cooking. One portion was cooked by boiling
in water, another cooked by boiling with lye and milk and the other cooked by sautéing
according to the household cooking procedures commonly employed in the study area. The
unfermented group was refrigerated at – 40C. This generated 16 AIV recipes for study.
Vegetable cooked by boiling in water
Exactly 40 g of the vegetable was boiled in 100 ml of distilled water for 10 minutes at 40ºC and
recipe obtained was cooled to room temperature and sealed in black polythene bags to keep out
light and refrigerated at - 4ºC, awaiting extraction to obtain the sample for laboratory analysis.
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Vegetable cooked by boiling with lye and milk
Exactly 40 g of the blanched AIV to which 30 ml of traditional salt (lye) had been added was
boiled for 7 minutes in 40 ml of distilled water at a moderate temperature of 40ºC. Exactly 30 ml
of fresh milk was added and mixture simmered for three more minutes. The boiled mixture was
left to cool to room temperature, sealed in a black polythene bag and refrigerated at - 4ºC
awaiting extraction to obtain the sample for laboratory analysis.
Vegetable cooked by sautéing
Exactly 20 ml of vegetable cooking oil was transferred by means of clean plastic syringe to a
clean cooking pan and placed on electric cooker plate set at a temperature of 40ºC to heat. One
onion bulb was peeled to remove dry outer skin, washed, sliced and was sautéed in the oil till
golden brown and a pinch of common salt added. Two tomatoes with intact skin were washed
with distilled water, chopped and added to the mixture in the pan. Exactly 40 g blanched AIV
was then added, stirred and mixture heated for 10 minutes. The sautéed AIV was left to cool to
room temperature, sealed in a black polythene bag and refrigerated at - 4ºC awaiting extraction
to obtain the sample for laboratory analysis.
Fermented recipe
Exactly 40 g of the blanched single AIV or AIV mixture was boiled for 10 minutes in 100 ml of
distilled water and left to cool to room temperature, sealed in a black polythene bag to keep out
light and kept in open air for 48 hours to allow microbial fermentation. After the fermentation
period the fermented recipe was refrigerated at - 4ºC awaiting extraction to obtain the sample for
laboratory analysis.
SAMPLE PREPARATION FOR OXALATE ANALYSIS
Glassware was initially washed with a detergent, chromic acid and further washed in 1:1 nitric
acid before rinsing in distilled water. The glassware was dried overnight at 50 0 C. Plastic
containers were washed in 1:1 nitric acid and also rinsed in distilled water before drying them in
an oven at 30 0 C. Standards of oxalic acid were of analar grade and were sourced from Aldrich
Chemicals. Exactly 0.2 g of vegetable sample was homogenized in 1 ml of 0.5 N HCl. The
homogenate was heated at 80 ° C for 10 min with intermittent shaking. To the homogenate 5 ml
of distilled water was added. About 2 ml of the solution was withdrawn and centrifuged at 12
000 g for 10 min. 1 ml of supernatant was passed through a filter (0.45 µm) before HPLC
analysis. Standards were prepared at varying concentrations for quantification.
STANDARDS AND CALIBRATION CURVES
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A stock solution of the standard containing 10 mg/ml of oxalic acid was prepared for calibration.
The peak area was determined and used to obtain the concentration levels of oxalate in the
samples. The regression coefficient (R) was obtained and from it the coefficient of determination
(R2) was worked out.
Figure 1: Calibration curve for oxalate
METHOD VALIDATION
The following performance parameters were evaluated for method validation: linearity domain
of the concentration: limit of detection (LOD), precision (reproducibility), and accuracy (by
recovery tests):
Linearity test of concentration: limit of detection
The linearity of the calibration curve is given by the equation y=mx-c, where the calculated
blank sample absorbance is c and the method sensitivity (the slope) is m and the correlation
coefficient is R. Limit of detection (LOD) was calculated using equation (i) (Eurachem guide,
1998). Absorbance values for 10 replicates of the blank solution were determined and
transformed into concentration values in order to be compared with the data obtained from the
calibration curve. The results are displayed in table 1.
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LOD = X
̄blank + 3Sblank........................................ (1)
X
̄blank is the mean absorbance obtained with the blank solutions:
Sblank is the standard deviations of the blank:
Xi are the values of the blank solutions while n is the number of replicates i.e. n=10.
Table 1: Limit of detection, equation of calibration curve, coefficient of
determination (R2) and regression coefficient (R)
Parameter
LOD(µg/ml)
Equation
R2
R
Oxalate
0.002
Y=3684x+2881
0.997
0.99849
The R2 values hence R values indicate that the established calibration curves are linear over the
respective range of concentrations as R tends to unity. The method detection limits at 3 standard
deviations for all the parameters were < 1 µg/ml which clearly indicates that the method was
reliable for the determination of the levels of oxalates.
Precision
Reproducibility of results was calculated for 10 measurements. Precision was evaluated by
Relative Standard Deviation (%RSD), according to the equation 2 (Eurachem guide, 1998). The
results are given in table 2.
RSD =𝑠
x̄ x 100........................................... (2)
Table 2: Method precision
Parameter
mean
s
%RSD
Oxalate
5.34
0.02
0.37
The obtained results show good precision for the parameter under determination.
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Accuracy
Accuracy was evaluated by recovery test, according to equation3 (Eurachem guide, 1998). The
results are presented in table 3.
% Recovery = 𝐶𝐹−𝐶𝑈
𝐶𝐴 x 100............................................ (3)
Where CU is the concentration in unfortified sample; CA is the concentration of Fortification
(added solution); CF is the concentration determined in fortified sample.
Table 3: Accuracy by Recovery test
Parameter
CU
(mg/g)
CA
(mg/g)
CF
(mg/g)
%Recovery
Oxalate
0.20
0.70
0.89
98.40
The percentage recovery lies within the range (98.40−102.10). This confirms that the method is
accurate and fit for analysis of the parameter.
ANALYSIS OF OXALATE
Analysis was done at Jomo Kenyatta University of Agriculture and Technology’s Home
economics laboratory by reversed-phase HPLC using Hypsil C-18 SUPELCO column (5 µM,
4.6 mmx250 mm) equipped Waters 550 (Waters, MA, USA) as the static phase and the mobile
phase was a solution containing 0.5 % KH2PO4 and 0.5 mM TBA (tetrabutylammonium
hydrogen sulphate) buffered at pH 2.0 with orthophosphoric acid. Flow rate was 1 ml min-1
(Libert, 1981; Yu et al., 2002) and detection wavelength was at 220nm.
DATA ANALYSIS
The data obtained was analyzed by SPSS software (version 17) where it was subjected to
independent t-test to compare whether there was any significant difference in the mean levels of
oxalates between fermented and non-fermented AIVs and one-way ANOVA to compare the
mean values between different AIVs and cooking methods. Where one- way ANOVA showed
significant difference it was followed by multiple range test (Student Newman Keul Test). All
the significance tests were performed at 95% confidence level.
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RESULTS AND DISCUSSION
The mean concentration levels of oxalate in fermented and unfermented AIV recipes
The mean levels of oxalate between fermented and unfermented AIV recipes were compared
(Table 4). Unfermented Amaranthus blitum and C. gynandra+C. moschata recipe formulation
recorded significantly higher mean levels of oxalate than any other unfermented recipes
(p<0.001). Fermented Amaranthus blitum recorded significantly higher mean levels of oxalate
than any other fermented recipes (p=0.107). The mean levels of oxalate in fermented Ipomea
batatas, Solanum nigrum,Manihot esculenta, Cleome gynandra and Basella alba were
significantly lower than in the unfermented ones (p<0.05). The mean levels of oxalate in the five
recipe mixtures that were fermented were significantly lower than in the unfermented ones
(p<0.001). There was no significant difference in the mean oxalate levels between fermented and
unfermented Vigna unguiculata (p=0.569) and Amaranthus blitum (p=0.107) (Table 4). Apart
from 2 AIV recipes (Vigna unguiculata and Amaranthus blitum) all the fermented AIV recipes in
the study had significantly lower mean levels of oxalate than the unfermented ones. However,
when the mean value of all unfermented vegetables was compared with the mean value of all
fermented AIVs (table 5), there was no significant difference (p=0.280, t-test).It was observed
that fermentation reduced oxalate levels in someunmixedAIV recipes and some recipe mixtures.
The observations inthose recipes in which there was a significant reduction in mean levels of
oxalates agreed withfindings of Oke and Bolarinwa (2011) who studied the effect of
fermentation on oxalate content of cocoyam flour. The study showed that 48 hour fermentation
reduced calcium oxalate significantly by approximately 58%-65% and that longer fermentation
period resulted in higher microbial population leading to higher reduction of oxalate
concentration levels.
Table 4: Mean concentration levels of oxalate in fermented and unfermented AIVs
Oxalate
AIVs
Unfermented
(Mean±SE)mg/100g
(n=9)
Fermented
(Mean±SE)mg/100g
(n=9)
P value
Curcubita moschata
9.20±1.61a
6.55±1.53b
0.030
Vigna unguiculata
2.62±.43a
2.24±.01a
0.569
Amaranthus blitum
15.42±1.56b
20.36±.01c
0.107
Corchorus olitorius
10.17±3.73a
3.04±.01a
0.001
Ipomea batatas
3.45±.53a
1.56±.08a
0.038
Solanum nigrum
6.72±.41a
2.69±.02a
<0.001
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Mean ± SE values within the same column followed by the same superscripts are not
significantly different at α=0.05, while values within the same row with p<0.05 are significantly
different, 95% confidence level, independent t-test.
Table 5: oxalate levels in all non-fermented and all fermented recipes
Microbial Fermentation
Mean Oxalate level ±SE
All Non-fermented AIVs
5.48±0.87
All Fermented AIVs
3.88±1.16
p-value
0.280
Independent t test
The mean concentration levels of oxalate in AIVs by different cooking methods
Boiled Curcubita moschata recorded significantly higher mean levels of oxalate than in
Amaranthus blitum, Ipomea batatas, Brassica carinata, Solanum nigrum, Crotolaria ochroleuca
and Cleome gynandra (Table 6). There was no significant difference in mean oxalate levels in
boiled Curcubita moschata compared to boiled Vigna unguiculata, Corchorus olitorius and
Basella alba (p>0.05). Curcubita moschata boiled with lye and milk recorded significantly
higher mean oxalate levels than in Corchorus olitorius, Ipomea batatas, Brassica carinata and
Amaranthus blitum cooked the same way (p<0.05). However, there was no significant difference
in mean oxalate levels in Curcubita moschata boiled with lye and milk when compared with
Vigna unguiculata, Solanum nigrum, Manihot esculenta, Cleome gynandra and Basella alba
cooked the same way (p>0.05). SautéedCorchorus olitorius recorded significantly higher mean
oxalate levels than sautéed, Ipomea batatas, Solanum nigrum, Manihot esculenta, Crotolaria
ochroleuca, Brassica carinata, Cleome gynandra and Basella alba. The mean oxalate levels in
sautéed Amaranthus blitum were not significantly different from sautéed Vigna unguiculata,
Curcubita moschata and Basella Alba (p>0.05).
Manihot esculenta
3.07±.25a
1.94±.08a
0.006
Crotolaria ochroleuca
4.37±.58a
3.01±.00a
<0.001
Brassica carinata
6.10±1.65a
3.18±.06a
0.003
Cleome gynandra
5.12±.09a
1.54±.01a
<0.001
Basella alba
5.24±.52a
2.06±.45a
0.001
S. nigrum+A. blitum
3.04±.00a
2.06±.01a
<0.001
C. ochroleuca+ C. olitorius
2.53±.02a
2.04±.01a
<0.001
C. ochroleuca +C. olitorius+V. unguiculata
3.44±.01a
1.70±.03a
<0.001
A. blitum+C. gynandra+S. nigrum
4.14±.01a
2.30±.01a
<0.001
C. gynandra+C. moschata
5.88±.086b
3.02±.08a
<0.001
p-value
<0.001
<0.001
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Table 6: The mean concentration levels of oxalate in AIVs by different cooking methods
Mean±SE values within the same row followed by the same superscripts are not significantly
different at α=0.05, p<0.001, SNK test.
Sautéed recipes had significantly lower oxalate levels than those boiled with lye and milk except
Amaranthus blitum and Corchorus olitorius (p<0.05) (Table 6).AIVs boiled with water had
significantly lower mean oxalate levels than those boiled with lye and milk except boiled
Brassica carinata which recorded significantly higher mean concentration levels of oxalate than
Brassica carinata boiled with lye and milk (Table 6). The higher levels of oxalate in sautéed
vegetables and in vegetables boiled with lye and milk compared to boiled ones could be
attributed to addition of oxalate to the recipes present in lye, milk, tomatoes and onions during
cooking. The results also showed that high temperatures during boiling and sautéing reduce
doxalate levels in some recipes and not others. The observations in those recipes in which there
was a significant reduction in mean levels of oxalates agreed with findings of Muchoki et al
(2010) that high temperatures reduced oxalate levels in Vigna unguiculata leafy vegetable.
Iwuoha and Kalu (1995) also reported 82.1% and 61.9% oxalate reduction in cocoyam flour
produced from boiled and roasted cocoyam respectively. The mean levels of all boiled AIVs, all
AIVs boiled with lye and milk and all sautéed AIVs were compared (table 7).The results show
that there was no significant difference in the mean values of oxalates by different methods of
cooking(p=0.986, one-way ANOVA).
OXALATE
AIVs
Boiled
(Mean±SE)
mg/100g
(n=3)
Lye+milk
(Mean±SE)
mg/100g
(n=3)
Sautéed(Mean±SE)
mg/100g
(n=3)
Curcubita moschata
7.13±.01a
17.26±.46c
9.86±.18b
Vigna unguiculata
1.66 ±.01a
3.57±.01c
2.15±.06b
Amaranthus blitum
19.78±.01c
9.37±.02a
17.11±.06b
Corchorus olitorius
1.82±.00a
3.66±.01b
25.03±.32c
Ipomea batatas
7.14±.01c
3.53±.01b
1.40±.04a
Solanum nigrum
6.77±.09b
8.09±.02c
5.29±.02 a
Manihot esculenta
2.61± .03b
3.60±.08c
2.00±.02a
Crotolaria ochroleuca
4.04±.02b
6.52±.01c
2.55±.02a
Brassica carinata
15.18±.01c
5.14±.01b
3.22±.03a
Cleome gynandra
5.04±.01b
5.47±.06c
4.86±.01a
Basella alba
4.65±.03a
7.91±.06c
5.28±.01b
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Table 7: mean levels of oxalate in all AIVs by different cooking methods
One-way ANOVA
CONCLUSION
Fermentation of AIVs differently affected oxalate levels. In some recipes a decrease was
observed while in others there was no significant change, suggesting that reduction of oxalate
levels in AIVs may depend on other factors within the recipes other than microbial fermentation.
Cooking methods also differently affected oxalate levels. In some recipes a decrease was
observed while in others there was no significant change, suggesting that the degree of oxalate
degradation may depend on other factors within the recipes apart from heat during cooking.
Cooking methods and fermentation do not have a uniform effect on oxalate level reduction in all
AIV recipes but could still be employed as household procedures in reducing oxalate levels in a
number of AIV recipes.
ACKNOWLEDGEMENTS
The authors would like to thank the following for their contributions during the study: Mr.
Katweo Wambua of Mines and Geology, Nairobi for his support while using the Atomic
Absorption Spectrometer, Paul Karanja (Jomo Kenyatta University Of Agriculture And
Technology) for facilitating the use of HPLC equipment, Denis Osoro, Cornelius Waswa, Kevin
Odhiambo, Eunice Oduor (technicians in the Chemistry Department, Kenyatta University). Ann
Mwangi, John Gachoya and Patrick Kamande (Technicians in the Department of Food, Nutrition
and Dietetics, Kenyatta University) for assisting in recipe preparations from indigenous
vegetables, Dr.Mildred Nawiri, Dr. Ruth Wanjau, Prof Jane Murungi, Dr. Daniel Oyoo, Dr.
Charles Onindo, and Dr. Margaret Ng’an’ga for their advice.
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All AIVs boiled with Lye andMilk
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