**1. Introduction**

Grain legumes are important sources of protein for food at household level and also for applications during food processing. Legume protein has nutritional and techno-functional benefits and uses. In terms of nutrition, the protein is a rich and low-cost source of some of the essential amino acids, which has relevance for food, nutrition security and health. As the world population continues to increase, they are sustainable and affordable alternatives to animal protein. They are also increasingly being used in processed food as functional ingredients to impart desirable characteristics such as texture. This emanates from techno-functional properties of protein such as solubility, emulsification, water holding capacity, foaming, gelation, and oil holding capacity. These properties are important in formulation of food products such as plant based milk products, bakery products, beverages, meat analogues, and other categories of food products which require incorporation of protein as functional ingredient to achieve a optimal product. The functional properties

of legume protein varies, and is influenced by type of legume, cultivar, molecular weight, amino acid composition, and charge distribution, and processing [1, 2].

Grains legumes are contained in pods at harvest, and are used as food either as dry or immature seeds. They are in the class *Leguminosae*. Their major common characteristic is ability to fix nitrogen from the atmosphere as a result of nitrogenfixing microorganisms that is present in their nodules. They have been important sources of food for humans for over a thousand years in most parts of the world. Grain legumes are generally classified as either pulses or oilseed. Pulses include dry beans (*Phaseolus* spp.), dry peas (*Pisum sativum*), dry broad beans (*Vicia faba*), lentil (*Lens culinaris*), chickpea (*Cicer arietinum*), lupins (*Lupinus* spp), dry cow pea (*Vigna unguiculata*), pigeon pea (*Cajanus cajan*), bambara groundnuts (*Vigna subterranea*), vetch (*Vicia sativa*), and other minor pulses. Oilseeds are soybean (*Glycine max*), peanut (*Arachis hypogaea*), rapeseed (*Brassica napus*), sunflower (*Helianthus annuus*), sesame (*Sesamum indicum*) and other minor oilseed crops. They form key part of human food, as they are generally good sources of protein, energy, and micronutrients [3, 4]. Grain legumes generally have a common characteristic of being rich in protein. Their protein content range from about 18 to 50% on dry matter basis [5]. About 40 species of grain legumes exist in the world [6]. Besides their use as food, protein from legumes is also added to food during processing to achieve desired functional properties in the end product. They are therefore good sources of protein for food applications. Soybean and pea are among the major legumes that have been most explored and used in food applications. The other legumes have generally found lesser applications in industrial use. Their extraction and characterization continue to generate more knowledge on their properties and food applications. This chapter discusses protein from grain legumes, their characteristics, functional properties, and extraction for food applications.

## **2. Grain legume composition and protein characteristics**

Grain legumes content of protein, carbohydrates, lipid, and micronutrients vary with type of legume, environment and agronomic factors. The macro-nutrient content of major grain legumes is approximately 18–50%, 0.8–21%. 60–69% and 0.9–7.2%, representing protein, oil, carbohydrates and fiber, respectively [6, 7]. Composition of grain legumes is presented in **Table 1**.

Legume protein are generally classified into three groups, namely storage, biologically active, and structural. Storage protein are the most abundant in legumes. They are a store of nitrogen for germination. Biologically active protein include lectins, enzymes and enzyme inhibitors, while structural proteins are ribosomal, chromosomal and membrane proteins. The storage proteins can further be classified into albumins, globulins, prolamins and glutelins. At the basic level, they can be differentiated by their solubility in various solvents. Albumins are soluble in water, globulins in dilute salt solution, prolamins in 75% ethanol, and glutelins show difficulty to solubilize but are soluble in dilute alkali. They can also be differentiated by their occurrence in monocotyledonous, cereals; and dicotyledonous plants, legumes. Albumins and globulins are the major storage protein in legumes, whereas prolamins and glutelins are the major protein in cereals. Globulin and albumin fractions account for about 70% and 10–20% of total protein in legumes, respectively [34]. The molecular weight of albumins is about 5–80 kDa, and is therefore generally low. Trypsin inhibitors, lectins, and amylase inhibitors may in some cases be part of albumins. As nitrogen source for germination, the storage proteins are rich in nitrogen containing the amino acids asparagine, arginine and glutamine [35].


*Legume Protein: Properties and Extraction for Food Applications DOI: http://dx.doi.org/10.5772/intechopen.100393*

#### **Table 1.**

*Composition of selected legume sources of protein.*

The major classes of globulin proteins in legumes are legumins and vicilins. They are also referred to as 7S and 11S globulins respectively, based on their Svedberg sedimentation values (S) [36]. This is similar for peas, soybean and other grain legumes. But they occur in different ratios in legumes, but both are oligomers. The 11S type is a hexamer made up of six subunits of molecular weight of about 340–360 kDa; whereas the 7S fraction has molecular weight of about 145–180 kDa made up of trimers of molecular weight of about 60–70 kDa [34, 37, 38]. Vicilins generally vary among the legumes in terms of molecular weight. Legume protein is low in cysteine, cysteine, methionine and tryptophan, which are sulfur containing amino acids. Only soybean contains all the essential amino acids required for human nutrition. Its protein also has good digestibility. Thus it has high quality protein almost comparable to protein from animal sources in terms of nutritional quality. But legumes in general have high nutritional value due to their high protein content, and content of other macro- and micro-nutrients.

Protein occur in structures described as primary, secondary, tertiary and quaternary. The primary structure denotes the linear sequence of the amino acid of the protein but providing little information that explains protein functionality. The higher structures are determined by conformational fold and helps in understanding protein molecular characteristics such as net charge, size, shape and other properties.
