Food Chemistry

Lesson 25. CLASSIFICATION, COMPOSITION AND PHYSICOCHEMICAL PROPERTIES OF LEGUMES

Module 10. Legumes

Lesson 25
CLASSIFICATION, COMPOSITION AND PHYSICOCHEMICAL PROPERTIES

25.1 Introduction

Legumes are an important food crop for both humans and other animals. They are generally low in fat and high in protein, and contain important micronutrients such as folate, potassium, iron and magnesium. Many food additives, such as gums for thickeners (e.g., gum arabic, guar gum and tragacanth gum), are derived from legumes. Soybean derivatives are used extensively in processed foods (such as soybean lecithin).

Many legumes are able to convert atmospheric nitrogen into a form that is usable by other plants. This is accomplished through a symbiotic relationship between the legume plant and special bacteria that live in nodules on the roots of the plant. Because of this ability to ‘fix’ nitrogen, legumes are essential for healthy ecosystems and agriculture. Grain legumes (pulses), complement cereals and make an ideal combination to provide protein quality matching that of animal products. Legumes contribute a major portion of lysine in the vegetarian diet. They are also fairly good sources of vitamins like thiamine, niacine and riboflavin and much needed iron, but relatively poor source of calcium and sulphur containing amino acids.

Legumes are classified in the family Fabaceae, which can be further divided into three sub-families – Faboideae, Mimosoideae and Caesalpinioideae. The sub-families (or families) are based on the form of the flowers, specifically the petal shape.

25.2 Structure

The grains of food legumes are similar in structure but differ significantly from each other in size, shape, colour and thickness of the seed coat. Legume seeds have two major parts; seed coat and the kernel (embryo and cotyledons). Pigeon pea, chickpea, black gram, green gram and horse gram have a seed coat accounting for 12-15 percent of the total weight of the grain where as it is in the range of 8-11 percent for lentils, French bean, kidney bean, pea, soybean and cowpea. On an average, pulses (including soybean) contain 11 percent seed coat, 2 percent embryo and 87 percent cotyledons. The embryo has two parts known as hypocotyl and plumule. Legume proteins are of two types – storage and structural– more versatile and useful in the Indian diets. Storage proteins (70-80 percent) occur within the cells in discrete protein bodies. About 20-30 percent are the structural proteins responsible for cellular activities including synthesis of structural and storage proteins. The cotyledons, account for 93 percent of methionine and tryptophan of the whole seed, while the seed coat is the poorest in these amino acids. The embryo is rich in methionine and tryptophan but it contributes only about 2 percent of their total quantity in the seed. Legume proteins are deficient in methionine and trytophan Starch is the most abundant in legume carbohydrates. Legumes contain an appreciable amount of crude fiber. Cellulose and hemicellulose are the major constituents of crude fiber. Nearly 80-90 percent of crude fiber is present in the seed coat.

25.3 Chemical Composition

The chemical composition of food legumes vary and it is governed by the cultivar, geographical location and growth conditions. Legumes are rich in protein, carbohydrates and oil. They also contain good amount of dietary fiber and mineral. The grain legumes containing low oil (1-5 percent) and high protein (20-30 percent) and carbohydrates (50 percent and more) are called pulses and those having high oil (30-50 percent) and low protein (20-30 percent) are known as oilseeds in India. Soybean and groundnut are the two most cultivated legumes of the world.

25.3.1 Proteins

Proteins can be classified into three basic groups: globulins (70 percent), albumins (15 percent), and glutelins (15 percent). These protein fractions include essential and nonessential amino acids. All legume proteins have less than optimal content of sulphur amino acids, cystine and methionine and in some tryptophan is also deficient. Amino acid deficiency can be met by consuming large amount of legumes or by taking a mixture of legumes or by employing the complementary that exists between high sulphur amino acid cereals and legumes, especially the soybean. Cereals contain 7-14 percent protein whereas legumes have 20-40 percent. Lysine is the first nutritional limiting essential amino acid in most cereals, tryptophan is the second limiting amino acid in maize and the threonine in other cereals. Legumes are deficient in methionine and cysteine. In practice, cereals and legumes are eaten with other foods. The overall protein quality of cereal – legume mixtures is better than that of either protein source alone due to the complementary nature of their amino acid profiles. Digestibility of legume proteins is poor. However, it can be improved through heat-treatments like cooking, autoclaving, roasting, etc. The poor digestibility is due to the presence of protease inhibitors, deficiency of sulphur amino acid, presence of polyphenols and other anti-metabolites and tertiary structure of native proteins. It is important that this less-than optimal digestibility of legume be taken into consideration when one is attempting to meet nutritional requirements of humans with diets which are essentially legume-based. Legumes contain high protein content. It ranges between 17-25 %. Legume proteins are chiefly globulins. Albumins are also present in a few species. The quality of legume proteins is lower than most other classes of proteins. Although grain legumes are rich in protein their nutritive value is limited by the deficiency of sulphur containing amino acid. Low digestibility is another factor contributing to their poor nutritive value.

25.3.2 Carbohydrates

Legumes contains about 60 percent of carbohydrate. Starch is the principal carbohydrate. Minor amounts of lower molecular weight carbohydrates such as sucrose and sucrosyl oligosaccharides are present. The oligosaccharides include raffinose, stachyose and verbascose and they are associated with flatulence. It is the major hindrance to large-scale acceptance of legumes as food. Digestible energy coefficient for most legumes as well as mixed diets containing legumes are generally between 85-90 percent of the gross energy of the dry legume seed whereas metabolizable energy values are 75-85 percent. Soybean contains a considerable amount of carbohydrate such as galactans, pentoses, and hemicelluloses which are poorly utilized.

Legumes are good sources of dietary fibers. Low dietary fiber intake is linked with increased incidence of cancer of the colon and rectum, diverticular disease, coronary heart disease, diabetes and gallstone in affluent societies of the West. A concentrated source of dietary fiber from soybean is obtained by processing de-hulled and defatted soy flakes. It has 65-75 percent dietary fiber. Only 13 gm of soy fiber can provide 10 gm of dietary fiber in food whereas it takes 23 gm of wheat bran; 58 gm of oat bran; 502 gm of apple; and 735 gm of lettuce to provide 10 gm of fiber. The physiological benefits of soy fiber and other dietary fiber sources are increased fecal bulk and its moisture; reduced plasma cholesterol and positive influence on blood glucose and insulin concentration. The hypocholeslterolemic effect is attributed to the dietary fiber fraction of legumes because of its high content of pectins, gums and galactants. Dietary fiber also absorbs bile salt. It is aided by saponins.

25.3.3 Fat

The fat content of legumes is between 1 and 2 percent. Legume fats in general are rich in essential fatty acids.

25.3.4 Other nutrients

Most species of legumes contain only small amounts of provitamin A (50 to 300 International Units of vitamin A per 100g). Fresh legumes have more vitamin A activity. The thiamine content of legumes is approximately equivalent to that of whole cereals. Values range between 0.3 and 1.0 mg per 100 g. Legumes contain little riboflavin. Values range between from 0.1 to 0.4 mg per 100g. They are good source of niacin, containing on the average about 2.0 mg per 100 g. Dry legumes are almost devoid of ascorbic acid. The legumes are considerably richer in calcium than are most cereals. Average calcium content is 100 mg per 100 g. The legumes contain considerable amounts of phytic acid which may affect the absorption and utilization of their calcium. The legumes are good sources of iron. Values range between 2 to 10 mg per 100 g. Un decorticated legumes contain vitamin E in somewhat larger amounts than whole cereals. For pantothenic acid, the reverse relationship holds. Legumes are good sources of folic acid in most common foods.

25.3.5 Anti-nutritional Factors

Most of the legumes in raw form contain a wide variety of anti-nutritional factors or toxic principles like trypsin inhibitor and others. In addition, many of the grain legumes cause flatulence. Fortunately, most of the anti-nutritional factors are heat labile and are destroyed during cooking. These are chemical substances which, although non-toxic generate adverse physiological responses and interfere with the utilization of nutrients. Anti-nutritional factors are protease inhibitors, lectins, goitrogens, antivitamins and phytates, saponins, oestrogens, flatulence factors, allergens and lysinoalanine. Some other anti-nutritional factors are cyanogens, favism factors, lathyrism factors, amylase inhibitors, tannins, aflatoxins and pressor amines. Although only a few legumes may contain all these anti-nutritional factors, many contain a few of them. Most of the anti-nutritional factors are heat labile and are destroyed during cooking. Heat stable compounds such as polyphenols and phytates are, however, not easily removed by simple soaking and heating. These could be reduced by germination and/or fermentation. Legumes are rich source of polyphenolic compounds. Till recently, some of these (e.g. tannins), were considered as anti-nutrients due to their adverse effects on protein digestibility. However, nowadays, there is considerable interest in the antioxidant activity of these compounds and in their potential health benefits, especially in the prevention of cancer and cardiovascular disease. Dark colored legumes like red kidney beans, black beans, black gram and soybean have higher amount of these polyphenolic compounds.

Table 25.1 Chemical composition pulses and legumes (g/100g of edible portion)

(Source: Nutritive value of Indian Foods, National Institute of Nutrition (NIN), Indian Council of Medical Research (ICMR), Hyderabad, 1994)