Protein content in legume seeds is governed both by genotype and by environment. Seed protein levels can vary across varieties of a given species and even among seeds on an individual plant. In general, however, food legumes contain 20-30% protein by proximate analysis (Table 2). The exceptions to this are soybean and winged bean, which contain up to 37 and 45% protein, respectively.

Legume proteins are primarily of two types: storage proteins, which account for approximately 70% of total seed nitrogen, and enzymatic, regulatory, and structural proteins, which are present for normal cellular activities, including the synthesis of storage proteins. Legume storage proteins are soluble in dilute salt solutions but insoluble in water and therefore fall into the classical globulin group of protein fractions. Legume protein types are further characterized by their sedimentation coefficients, which in most species approach 11S and 7S; these are commonly referred to as the legumins and vicilins, respectively. Most legumes contain both types of storage protein, but the proportion of the two types varies from species to species.

In terms of protein quality, as defined by an optimal proportion of amino acids required by humans, legume proteins are deficient in the

Table 2 Protein contents of food legume seeds


Protein range (% dry weight)

Broad bean




Common bean




Grass pea


Horse gram




Moth bean


Mung bean






Pigeon pea


Rice bean




Urd bean


Winged bean


Source: Salunkhe DK, Kadam SS and Chavan JK (1985) Postharvest Biotechnology of Food Legumes. Boca Raton, FL: CRC Press.

Source: Salunkhe DK, Kadam SS and Chavan JK (1985) Postharvest Biotechnology of Food Legumes. Boca Raton, FL: CRC Press.

sulfur-containing amino acids and tryptophan but are rich in lysine. Cereals, on the other hand, are relatively deficient in lysine; thus, the combination of legumes with cereals often can improve the overall protein quality of the mixed foods. The nutritive value (or biological value) of legume proteins has been investigated quite extensively and has been shown to be rather low in some legumes, with the amount of utilizable protein ranging from 32 to 78%. In other words, not all of the protein available in a given legume (see Table 2) is converted into new protein when consumed by humans. The reasons for this are the general deficiency of essential amino acids (sulfur-containing and tryptophan) and the presence of many inhibitors of protease activity that are found in legume seeds. These enzyme inhibitors are primarily proteinaceous in character, and many have an effect on the digestive enzymes trypsin or chymotrypsin. The inhibition of these enzymes leads to a reduction in protein digestibility and thus the gut's ability to absorb amino acids. Fortunately, because many of these inhibitors are proteinaceous, cooking, heating, fermenting, and, in some cases, germination can inactivate and significantly lower their inhibitory effect. However, not all of the inhibitors found in legume seeds are proteins (e.g., other inhibitors include tannins and polyphenols).

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