Effect of soaking and cooking on nutritional quality and safety of legumes
Purpose – Legumes are widely grown and are consumed as a source of plant protein throughout the world. They rank second after cereals with respect to their consumption order. Legumes have anti-nutritional factors which make their uses limited. This study aims to check the effect of soaking and cooking on the anti-nutrient contents and nutritional quality of the legumes. Design/methodology/approach – Five legumes (white kidney bean, red kidney bean, lentil, chickpea, and white gram) frequently used by the masses were selected for soaking and cooking trials. Legumes were tested for their weight, volume, density, swelling capacity and water absorption capacity before soaking and cooking. Legumes were soaked in simple water, 2 per cent sodium chloride solution, acetic acid and sodium bicarbonate and cooked in a beaker with 1?:?5 seed water ratio to uniform soft mass. After soaking and cooking, legumes were tested for anti-nutrients (phytic acid and tannin) and their nutritional quality. Findings – The statistical analysis of the study results revealed that dark colour legume (red kidney bens) has a high level of phytic acid and tannin compared with light colour (white kidney beans and white grams). Soaking and cooking of legumes result in significant reduction in phytic acid and tannin contents. Maximum reduction of phytic acid (78.055) and tannin (65.81 per cent) was found for sodium bicarbonate soaking followed by cooking. These treatments also result in a slight reduction in nutrients such as protein, minerals and total sugars. Practical implications – Soaking and cooking of legumes reduce their anti-nutrients; phytic acid and tannin significantly. These treatments may be used domestically as well as commercially to increase the nutrients' availability from legumes to meet the problem of protein and minerals deficiencies. Originality/value – Along with water different soaking solutions which are easily available in the market were used to test out their effect on the nutritional quality and safety. These may be used by the common people to raise their nutritional status.
Nutrition & Food Science
Vol. 38 No. 6, 2008
# Emerald Group Publishing Limited
Effect of soaking and cooking on
nutritional quality and safety of
Nuzhat Huma, Faqir Muhammad Anjum, Samreen Sehar,
Muhammad Issa Khan and Shahzad Hussain
Institute of Food Science & Technology, University of Agriculture,
Purpose ? Legumes are widely grown and are consumed as a source of plant protein throughout the
world. They rank second after cereals with respect to their consumption order. Legumes have anti-
nutritional factors which make their uses limited. This study aims to check the effect of soaking and
cooking on the anti-nutrient contents and nutritional quality of the legumes.
Design/methodology/approach ? Five legumes (white kidney bean, red kidney bean, lentil,
chickpea, and white gram) frequently used by the masses were selected for soaking and cooking
trials. Legumes were tested for their weight, volume, density, swelling capacity and water absorption
capacity before soaking and cooking. Legumes were soaked in simple water, 2 per cent sodium
chloride solution, acetic acid and sodium bicarbonate and cooked in a beaker with 1 : 5 seed water
ratio to uniform soft mass. After soaking and cooking, legumes were tested for anti-nutrients (phytic
acid and tannin) and their nutritional quality.
Findings ? The statistical analysis of the study results revealed that dark colour legume (red kidney
bens) has a high level of phytic acid and tannin compared with light colour (white kidney beans and
white grams). Soaking and cooking of legumes result in significant reduction in phytic acid and
tannin contents. Maximum reduction of phytic acid (78.055) and tannin (65.81 per cent) was found for
sodium bicarbonate soaking followed by cooking. These treatments also result in a slight reduction in
nutrients such as protein, minerals and total sugars.
Practical implications ? Soaking and cooking of legumes reduce their anti-nutrients; phytic acid
and tannin significantly. These treatments may be used domestically as well as commercially to
increase the nutrients? availability from legumes to meet the problem of protein and minerals
Originality/value ? Along with water different soaking solutions which are easily available in the
market were used to test out their effect on the nutritional quality and safety. These may be used by
the common people to raise their nutritional status.
Keywords Nutrition, Cooking, Food products, Food safety
Paper type Research paper
Leguminous plants that produce edible part are referred to as food legumes. The word
pulse is used to describe legumes that bear edible dry seed that is directly used by man.
Family leguminacese consist of 600 genera an d 13,000 species (Aykroyd and Doughty,
1964). Legumes are widely grown and consumed as a source of plant protein
throughout the world. These are considered as one of the cheapest and richest source of
dietary protein, which are used as a substitute or supplement in the relatively
expensive animal protein in human diet. Beside proteins, carbohydrates, minerals and
vitamins are also present in legumes (Wolf, 1988).
Protein contents of legumes vary between 17 and 34 per cent, which include meta bolic,
structural and storage protein. Storage pr otein is made up to 80 per cent of the total
protein (Sgarbieri and Whitaker, 1982). Lipid content is in the range of 1-6 per cent and
mainly depends upon variety, origin, location of growth, climate, season, envir onmental
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Effect of soaking
factors and soil type (Worthington et al., 1972). Lipids found in legumes are mainly
neutral lipids i.e. triglycerids, di and monoglycerides, free fatty acids, sterols and sterol
esters. Major carbohydrates in legumes are starch and numerous others sugars. Sugars
ranges between 6 and 12 per cent, wher eas starch vary fr om 24 to 41 per cent. Peas and
bean are poor source of aft soluble vitamin but contain moderate amount of water soluble.
Peas and beans are high in phosphorous and very low in sodium. Phosphorous is second
highest mineral that exist in several forms. In chickpea, phosphorous is distributed as
acid soluble (74 per cent), inorganic (11 per cent), phytates (45 per cent), phospjatides (16
per cent) and others (10 per cent) (Wolf, 1988).
The most commonly used legumes are kidney beans, chickpeas, pigeon pea, cowpea,
mung bean, moth bean, black gram and green gram. Utilization of legumes in food is
often limited due to presence of several factors. The development of the hard to cook
defect reduce the palatability and cooking quality of legumes. Seed size, swelling
capacity, seed coat and texture of certain legumes are also associated with cooking
quality. Legumes are also under utilized because of enzymes inhibitors, proteins of low
quality due to their deficiency in sulfur containing amino acids, flatulence factors, phytic
acid which reduces the bioavailability of divalent minerals, toxic factors such as tannins
and haemaglutinins. Previously, many attempts have been made to reduce the level of
these antinutrients from legumes. Soaking in simple water and salt solution is a common
practice to soften texture and hasten the cooking process (Silva et al.,1981).Soakingand
cooking improve nutritional quality of legumes. Beside improvement in nutritional
quality most of valuable nutrients are lost during processing. Losses of minerals and
vitamins have been found more significant as compare to protein and other nutrients.
In this context, research was designed to find out the effect of soaking and cooking
on the nut ritional quality of legumes and further to find out the soaking solution which
is the best in terms of leaching of antinutrients.
Materials and methods
Sample of legumes; white kidney bean, red kidney bean, lentil, chickpea an d white
gram were procured from the local market. Samples were clean in two stages i.e. first
with laboratory seed cleaner for removing extraneous material and then manually
cleaned which were stored in polythene bag at room temp erature befo re use.
Physical characteristic of seeds
Each of physical character was determined according to their specified method.
Weight, volume and density. Seed weight and volume was determined according to
method described by Phirke et al. (1982). Seed density was calculated from the values
obtained for weight and volume of seeds.
Swelling capacity. 50 seeds of each sample were weighed followed by cooking in
boiling water for 20 min. Seeds were drained and reweighed and their swelling
capacity was determine by method as described by Akinyele et al. (1986).
Water absorption capacity. Water absorption capacity of legume were determined
by soaking 10 g seed in water at a seed to water ratio of 1:5 by following the method of
Sefa Dedeh and Stanley (1979).
Soaking of legumes. Legumes were soaked in simple water and 2 per cent solution of
sodium chloride, acetic acid and sodium bicarbonate prior to cooking.
Cooking time. Pre-soaked whole seeds were cooked in a beaker in water five times to
the seeds weight. Samples were periodically checked for their softness by pressing them
between fingers and thumb. Samples were considered cooked when soft to uniform mass.
Samples of legumes were analyzed for moisture, crude protein, crude fiber, ash and
crude fat contents by following their respective methods as described in AACC (2000).
Total sugars in legumes were determined by Lane and Eynon method given by Ruck
Phytate contents of legumes samples were determined by adopting the method
described by Wolfgang and Lantsch (11).
Legumes samples were tested for their tannin contents according to method given in
The data obtained were statistically analyzed by using the analysis techniques
described by Steel et al. (1997).
Results and discussion
Five legumes namely white gram, chic kpea, red kidney bean, white kidney bean and
lentil were test for their physico-chemical properties and nutritional quality and safety
during the processing.
Physical characteristics of legumes
Physical parameters are important in determining the quality of legume and also their
acceptability by the consumers. Higher values for seed volume and 100 seeds weight
were found for red kidney bean while lowest values were exhibited by lentil for these
two parameters as depicted in Table I. Water absorption and swelling capacity of
legume varied betwe en 15.24 (lentil) to 34.74 per cent (red kidney bean) and 32.72
(lentil) to 84.53 per cent (chickpea), respectively. Apparent seed density of these legumes
was found to 0.48 (lentil) to 1.85 g/mL (chickpea). It is apparent from results that seed
size influence the physical characteristics particularly the water absorption capacity of
legume. These results are in line with the findings of Deshpande
et al. (1984).
White gram 25.96 ? 0.50 19.00 ? 1.00 1.16 ? 0.05 25.60 ? 0.90 73.00 ? 1.50
Chickpea 22.13 ? 0.90 14.00 ? 1.00 1.85 ? 0.05 23.80 ? 0.50 84.53 ? 1.00
Red Kidney bean 35.84 ? 0.90 29.00 ? 2.00 1.23 ? 0.06 34.74 ? 0.75 74.39 ? 1.90
White kidney bean 29.73 ? 0.80 27.00 ? 1.00 1.10 ? 0.01 27.80 ? 1.50 66.07 ? 0.90
Lentil 2.88 ? 0.50 6.00 ? 1.00 0.48 ? 0.00 15.42 ? 0.50 32.72 ? 2.00
Effect of soaking
Cooking time of unsoaked seeds present a wide variation ranging from 16 to 130 min
depending upon size and hardness of seed s as shown in Table II. Soaking of legumes
seeds in water for 2 and 4 h reduced the cooking time by 18.18 and 25.00 per cent,
respectively. Prolonged soaking did not show further distinct reduction in cooking time
of the legumes. Reduction in cooking time could be the result of sufficient amount of
water from the soaking media that ultimately result in reduced hardness of legumes.
Onayemi et al. (1986) found 23.00-25.00 per cent reduction in cooking time for cowpea
whereas 55.26 per cent reduction in cooking time for various cultivars of dry bean is
reported by Phirke et al. (1982).
Chemical composition of legumes
Nutrients in legumes. Legumes (white g ram, chickpea, red kidney bean, white kidney
bean and lentil) were subjected to analysis in order to determine t heir nutrient profile
mainly crude protein, starch, total sugars, total lipids minerals and fiber as shown in
Table III. Protein and starch are the main components of these legumes. Protein of
legumes on dry weight basis varied between 19.44 (white gram) and 26.0 per cent
(lentil), whereas total sugars and lipi ds ranged from 5.21 (lentil) to 8.46 per cent (white
kidney bean) and 3.06 (lentil) to 6.69 per cent (white gram), respectively. Higher values
for minerals contents were found for red kidney bean (3.91 per cent) that varied up to
2.73 per cent (chickpea). These values for the nutrient composition of legumes are
confirmation of research studies carried out by Wolf (1988), Fan and Sosulski (1974)
and Duke (198 1).
Cooking and soaking significantly affect the nutrients composition of the legumes as
shown in Table IV. All legumes showed a significant loss in their nutrients contents;
protein showed 14.78-21.83 per cent reduction in the total amount of proteins present
when cooked in pressure cooker. These losses are attributed to partial removal of
essential as well non-essential amino acids with other nitrogenous compounds formed as
results of chemical degradation of protein into water soluble amino acids due to high
temperature and pressure. Minerals contents of legumes also showed significant
reduction (18.99-39.50 per cent) during the process of cooking due to leaching of minerals
into cooking water. Rincon et al. (1993) investigated the loss of micro and macro minerals
during the process of cooking. The findings of his studies are strengthening force for our
studies results that nutrients are lost during the cooking process. During cooking of
legumes sugars are lost significantly in higher amount as compare to proteins and
minerals. Data represented in Table IV showed that sugar lost varied between 26.70 and
36.86 per cent and this loss is mainly due to the solubility of sugars in water. Cooking
result in separation of some bean cells rather than breaking which in turn release the cell
contents (protein, minerals, sugars etc.) to the surrounding media and consequently
caused reduction in the nutrients of bean (Kon, 1979).
Antinutrients in legumes
Phytic acid and tannic acid are the two main antinutrients present in legumes. The
amount of phytic acid ranged from 233.33 to 599.67 mg /100 g, whereas range for
tannic acid is in between 164.70 and 371.67 mg/100 g of the legumes used in current
study. The amount of these antinutrients is higher in pigmented legumes (red kidney
bean, chickpea, lentil) as compare to white colored legumes (white kidney bean, white
gram). However, amount of phytic acid are distinctly higher than tannin contents in
these legumes. Results are consiste nt with the finding of Jadwiga and Anna (1995).
Effect of soaking on the
cooking time of legumes
Legumes Cooking time
(un soaked) min
after 2 h
in time %
after 4 h
after 6 h
in time %
White gram 110.00 ? 3.00 90.00 ? 2.00 18.18 ? 0.02 70.00 ? 2.00 36.36 ? 0.10 65.00 ? 2.00 40.90 ? 0.05
Chickpea 75.00 ? 2.00 60.00 ? 2.00 20.00 ? 0.12 45.00 ? 2.00 40.00 ? 0.20 42.00 ? 1.00 44.00 ? 0.10
Red kidney bean 13.00 ? 3.00 105.00 ? 3.00 19.23 ? 0.02 85.00 ? 3.00 34.61 ? 0.11 82.00 ? 3.00 36.92 ? 0.09
White kidney bean 45.00 ? 1.00 35.00 ? 1.00 22.22 ? 0.03 28.00 ? 1.00 37.78 ? 0.2. 26.00 ? 2.00 42.22 ? 0.10
Lentil 16.00 ? 1.00 12.00 ? 1.00 25.00 ? 0.25 9.00 ? 1.00 43.75 ? 0.09 8.00 ? 1.00 50.00 ? 0.08
Effect of soaking
Soaking of legumes in different soaking solution followed by cooking significantly affect
the phytic acid contents as depicted in Figure 1. Significant reduction (53.43-61.11 per
cent) in phytic acid contents of legumes was found as results of soaking followed by
cooking for 15 min in pressure cooker. Maximum reduction in phytate was observed in
Effect of soaking solution
and cooking on the
phytate contents of
legumes WG, white gram;
CP, cowpe a; RKB, red
kidney bean; WKB, white
Effect of soaking solution
and cooking on the tannin
contents of legumes WG,
white gram; CP, cowpea;
RKB, red kidney bean;
WKB, white kidney bean
Table IV .
Effect of cooking on the
% loss of
% loss of
% loss of
White gram 13.90 ? 0.60 21.83 ? 0.60 1.70 ? 0.02 39.50 ? 0.02 4.44 ? 0.05 31.58 ? 0.05
Chickpea 17.42 ? 0.50 16.04 ? 0.55 1.88 ? 0.09 31.14 ? 0.09 3.76 ? 0.04 31.68 ? 0.04
White kidney bean 16.94 ? 0.59 16.50 ? 0.59 2.73 ? 0.03 18.99 ? 0.03 5.60 ? 0.08 26.70 ? 0.08
Lentil 18.74 ? 0.67 20.45 ? 0.67 1.86 ? 0.02 33.33 ? 0.02 2.98 ? 0.06 36.86 ? 0.06
Chemical composition of
Legumes Protein % Fat % Fiber % Ash % Total sugars %
White gram 19.44 ? 0.12 6.69 ? 0.01 2.15 ? 0.02 3.07 ? 0.05 7.09 ? 0.06
Chickpea 22.62 ? 0.25 6.21 ? 0.02 6.78 ? 0.02 2.73 ? 0.03 5.50 ? 0.04
Red Kidney bean 23.69 ? 0.29 4.71 ? 0.02 1.67 ? 0.01 3.91 ? 1.00 7.32 ? 0.07
White kidney bean 22.48 ? 0.11 4.83 ? 0.01 0.87 ? 0.01 3.73 ?0.08 8.46 ? 0.08
Lentil 26.00 ? 0.33 3.07 ? 0.01 3.20 ? 0.01 3.07 ? 0.04 5.21 ? 0.03
white gram by the combination of two treatments. Reduction in phytic acid is attribute to
its leaching in soaking and cooking water as observed by Ogun et al. (1989) while
cooking cowpea. Soaking of legume for 2 h before cooking in different soaking solution
also show significant reduction in phytate as compare to soaking in simple water as
indicated in Figure 1. Higher reduction in phytate was observed in 2 per cent NaHCO
(69.24-78.05 per cent) soaked legumes followed by acetic acid (61.25-71.06 per cent) and
sodium chloride (57.90-67.31 per cent) soaking before cooking. Tannin contents also
significant reduction as a result of combined treatment of soaking and cooking as shown
in Figure 2. Cooking of soaked legumes in pressure cooker at 151 lb/inch
for 15 min
show 25.23-50.09 per cent reduction in tannin contents of legumes. Laurena et al. (1984)
also observed reduction in tannic acid of legumes by the process of leaching during
cooking. Soaking solutions also have significant effect on the tannin contents of the
legume. Maximum reduction in tannin was observed for sodium bicarbonate except
lentil that shows maximum reduction while soaked in acetic acid solution. The results
regarding the reduction of tannin contents by the combination soaking and cooking are
consistent to finding Rehman and Shah (1996) who found partial reduction in tannin
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