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Effect of soaking and cooking on nutritional quality and safety of legumes
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Nuzhat Huma, Faqir Muhammad Anjum, Samreen Sehar, Muhammad Issa Khan and Shahzad Hussain Institute of Food Science & Technology, University of Agriculture, Faisalabad, Pakistan Abstract 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 antinutritional 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. Keywords Nutrition, Cooking, Food products, Food safety Paper type Research paper
Nutrition & Food Science Vol. 38 No. 6, 2008 pp. 570-577 # Emerald Group Publishing Limited 0034-6659 DOI 10.1108/00346650810920187
Introduction 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 and 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 metabolic, structural and storage protein. Storage protein 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, environmental
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, whereas starch vary from 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). Soaking and 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 nutritional quality of legumes and further to find out the soaking solution which is the best in terms of leaching of antinutrients. Materials and methods Legumes Sample of legumes; white kidney bean, red kidney bean, lentil, chickpea and 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 temperature before 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.
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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. Proximate composition 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 Total sugars in legumes were determined by Lane and Eynon method given by Ruck (1969). Phytic acid Phytate contents of legumes samples were determined by adopting the method described by Wolfgang and Lantsch (11). Tannins Legumes samples were tested for their tannin contents according to method given in AOAC (1990). Statistical analysis 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, chickpea, 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 between 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).
Table I. Physical characteristics of legumes
Legumes
Weight 100 Volume 100 Seed density Water absorption Swelling capacity seeds (g) seeds mL g/mL capacity (g/100g) (g/100g)
White gram Chickpea Red Kidney bean White kidney bean Lentil
25.96 0.50 22.13 0.90 35.84 0.90 29.73 0.80 2.88 0.50
19.00 1.00 14.00 1.00 29.00 2.00 27.00 1.00 6.00 1.00
1.16 0.05 1.85 0.05 1.23 0.06 1.10 0.01 0.48 0.00
25.60 0.90 23.80 0.50 34.74 0.75 27.80 1.50 15.42 0.50
73.00 1.50 84.53 1.00 74.39 1.90 66.07 0.90 32.72 2.00
Cooking time of unsoaked seeds present a wide variation ranging from 16 to 130 min depending upon size and hardness of seeds 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 gram, chickpea, red kidney bean, white kidney bean and lentil) were subjected to analysis in order to determine their 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 lipids 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 (1981). 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 consistent with the finding of Jadwiga and Anna (1995).
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Table II. Effect of soaking on the cooking time of legumes
White gram Chickpea Red kidney bean White kidney bean Lentil
Cooking time after 2 h soaking min 90.00 2.00 60.00 2.00 105.00 3.00 35.00 1.00 12.00 1.00
Cooking time (un soaked) min
110.00 3.00 75.00 2.00 13.00 3.00 45.00 1.00 16.00 1.00
18.18 0.02 20.00 0.12 19.23 0.02 22.22 0.03 25.00 0.25
Reduction in time % 70.00 2.00 45.00 2.00 85.00 3.00 28.00 1.00 9.00 1.00
Cooking time after 4 h soaking min
36.36 0.10 40.00 0.20 34.61 0.11 37.78 0.2. 43.75 0.09
Reduction in time %
65.00 2.00 42.00 1.00 82.00 3.00 26.00 2.00 8.00 1.00
Cooking time after 6 h soaking min
574
Legumes
40.90 0.05 44.00 0.10 36.92 0.09 42.22 0.10 50.00 0.08
Reduction in time %
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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 Legumes
Protein %
Fat %
Fiber %
Ash %
Total sugars %
19.44 0.12 22.62 0.25 23.69 0.29 22.48 0.11 26.00 0.33
6.69 0.01 6.21 0.02 4.71 0.02 4.83 0.01 3.07 0.01
2.15 0.02 6.78 0.02 1.67 0.01 0.87 0.01 3.20 0.01
3.07 0.05 2.73 0.03 3.91 1.00 3.73 ±0.08 3.07 0.04
7.09 0.06 5.50 0.04 7.32 0.07 8.46 0.08 5.21 0.03
Effect of soaking and cooking
575 White gram Chickpea Red Kidney bean White kidney bean Lentil
Legumes
% protein retained
White gram 13.90 0.60 Chickpea 17.42 0.50 White kidney bean 16.94 0.59 Lentil 18.74 0.67
% loss of protein
% minerals retained
% loss of minerals
% sugars retained
% loss of sugars
21.83 0.60 16.04 0.55 16.50 0.59 20.45 0.67
1.70 0.02 1.88 0.09 2.73 0.03 1.86 0.02
39.50 0.02 31.14 0.09 18.99 0.03 33.33 0.02
4.44 0.05 3.76 0.04 5.60 0.08 2.98 0.06
31.58 0.05 31.68 0.04 26.70 0.08 36.86 0.06
Table III. Chemical composition of legumes
Table IV. Effect of cooking on the nutrients
Figure 1. Effect of soaking solution and cooking on the phytate contents of legumes WG, white gram; CP, cowpea; RKB, red kidney bean; WKB, white kidney bean
Figure 2. 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
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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 NaHCO3 (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/inch2 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 contents of lentil by soaking in sodium chloride solution. References AACC (2000), Approved Methods of the American Association of Cereal Chemists, The American Association of Cereal Chemists, Inc., St Paul, MN. Akinyele, I.O., Onigbinde, A.C., Hussain, M.A. and Omolou, A. (1986), ‘‘Physicochemical characteristics of 18 cultivars of Nigerian cowpea (V. unguiulata) and their cooking properties’’, Journal of Food Science, Vol. 51 No. 6, pp. 1483-5. AOAC (1990), Official Methods of Analysis, 15th ed., Association of Official Analytical Chemists, Arlington, VA. Aykroyd, W.R. and Doughty, J. (1964), Legumes in Human Nutrition, Nutritional Study No. 19, Food and Agriculture Organization United Nations, Rome. Desphande´, S.S., Sathe, S.K. and Salunkhe, D.K. (1984), ‘‘Inter-relationship between certain physical and chemical properties of dry beans’’, Quality of Plant Food Human Nutrition, Vol. 34, pp. 53-65. Duke, A.J. (1981), Handbook of Legumes of World Economic Importance, Plenum Press, New York, NY, Vol. 52, p. 110. Fan, T.Y. and Sosulski, F.W. (1974), ‘‘Dispersibility and isolation of protein from legume flours’’, Journal of Food Science Technology, Vol. 7 No. 2, pp. 256-9. Haug, W. and Hans, J.L. (1983), ‘‘Sensitive method for the rapid determination of phytate in cereals and cereal products’’, Journal of Science Food Agriculture, Vol. 34, pp. 1427-33. Jadwiga, W.J. and Anna, S. (1995), ‘‘Tannin and phytate in grain legumes’’, Technological Chemistry, Vol. 718 No. 14, pp. 249-58. Kon, S. (1979), ‘‘Effect of soaking temperature on cooking and nutritional quality of beans’’, Journal of Food Science, Vol. 44 No. 6, pp. 1329-31. Laurena, A.G., Van-Den, T. and Mendoza, M.A.T. (1984), ‘‘Effect of condensed tannin on the in vitro protein digestibility of cowpea (V. unguiulata)’’, Journal of Agricultural and Food Chemistry, Vol. 32 No. 5, pp. 1045-8. Ogun, P.O., Markakis, P. and Chenoweth, W. (1989), ‘‘Effect of processing on certain antinutrients in cowpea (V. unguiulata)’’, Journal of Food Science, Vol. 54 No. 11, pp. 1084-5.
Onayemi, O., Osibogun, O.A. and Obembeis, O. (1986), ‘‘Effect of different storage and cooking methods on some biochemical, nutritional and sensory characteristics of cowpea’’, Journal of Food Science, Vol. 51 No. 2, pp. 153-6. Phirke, A.V., Chavan, J.K., Jadhar, S.J. and Salunkhe, D.K. (1982), ‘‘Physical properties, chemical composition and biological activity of the saponins isolated from soybean’’, Science, Vol. 120, pp. 224-7. Rehman, Z.U. and Shah, W.H. (1996), ‘‘Effect of soaking and cooking on physical characteristics, tannin contents and protein digestibility of kidney bean (Phaseolus vulgaris)’’, Pakistan Journal of Science Industrial Research, Vol. 39, pp. 60-3. Rincon, F., Ros, G. and Collins, J.L. (1993), ‘‘Mineral loss in cowpea by pressure heating in water’’, Journal of Food Science, Vol. 58 No. 9, pp. 856-8. Ruck, T.R. (1969), ‘‘Chemical methods for analysis of fruit and vegetables products’’, pub. No. 1154, Canada Dept Agric. Res. Station Summer Land. Sefa Dedh, S. and Stanley, D.W. (1979), ‘‘The relationship of microstructure of cowpeas to water absorption and dehulling properties’’, Cereal Chemistry, Vol. 56, pp. 379-81. Sgarbieri, V.C. and Whitaker, J.R. (1982), ‘‘Physical, chemical and nutritional properties of common bean (Phaseolus) protein’’ Advances in Food Research, Vol. 28, pp. 93-166. Silva, C.A.B., Rayes, R.P. and Deng, J.C. (1981), ‘‘Influence of soaking and cooking upon softening and eating quality of black bean (Phaseolus vulgaris)’’, Journal of Food Science, Vol. 46 No. 6, pp. 1716-20. Steel, R.G.D.,Torrie, J.H. and Dickey, D. (1997), Principles and Procedures of Statistics, 3rd ed., McGraw-Hill Book Co. Inc., New York, NY. Wolf, W.J. (1988), ‘‘Effect of agricultural practices, handling, processing and storage on legumes and oil seeds’’, in Karmas, E. and Harris, R.S. (Eds), Nutritional Evaluation Food Processing, AVI Publications, New York, NY, pp. 119-28. Worthington, R.E., Hammons, R.O. and Allison, J.R. (1972), ‘‘Varietial difference and seasonal effects of fatty acids composition and stability of oil from 82 peanut genotypes’’, Journal of Agricultural and Food Chemistry, Vol. 20 No. 4, pp. 727-30. Corresponding author Nuzhat Huma can be contacted at:
[email protected]
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