Properties And Compositions

A common feature of the structure of soybean seed is storage of the bulk of the protein and oil in distinct membrane-bound, subcellular organelles called protein bodies and lipid bodies, (spherosomes) as illustrated in Figure 1. Although not shown in Figure 1, the protein bodies contain inclusions referred to as globoids (>0.1 //m) that are storage sites for phytate and cations such as potassium, magnesium, and calcium. During germination, the contents of the storage organelles are mobilized and utilized by the growing seedling. Soybeans grow on erect, bushy annual plants, 6.3-2.0 m high with hairy stems and trifoliolate leaves. The flowers are white or purple or combinations of white and purple. Growing season varies with latitude (120-130 days in central Illinois). Seeds are produced in pods, usually containing three almost spherical to oval

Figure 1. Transmission electron micrograph of a section of a mature, hydrated soybean cotyledon. Protein bodies (PB), lipid bodies (LB), and cell wall (CW) are identified. Source: Ref. 1.

seeds weighing 0.1-0.2 g. Commercial varieties have a yellow seed coat plus two cotyledons, plumule, and hypocotyl-radicle. Cotyledons contain protein and lipid bodies.

On a moisture-free basis, soybeans contain approximately 8% hulls, 20% oil, 43% protein (N X 6.25), 5% ash, and 52% protein in dehulled, and defatted meal (variable depending on efficiency of dehulling and oil extraction, variety of seed, and climatic conditions during growth). The proteins found in soybeans are complex mixtures consisting of four characteristic fractions with molecular weights of ca 8,000-700,000, as illustrated by the ultracentrifuge pattern (Fig. 2). The 7 S and 11 S fractions usually predominate. The 7 S and 11 S fractions are considered to be storage proteins and are located in the protein bodies. The principal portion of soybean 7 S fraction, yS-conglycinin, consists of at least seven isomers resulting from various combinations of three subunits (a, a', and/?) afi, a'fi2, aa'f!, a2ji, a'a2, «3, and /?3. Based on the sizes of the subunits, the /?-conglycinin isomers have molecular weights between 126,000 and 171,000.

The 11S molecule, also called glycinin, is more complex than /J-conglycinin. It consists of ca 12 polypeptides, half of which have acidic isoelectric points (molecular weight ca 37,000-44,000) and half of which have basic isoelectric points (molecular weight ca 17,000-22,000); six different acidic and five different basic polypeptides have been separated and identified. Some acidic and basic subunits are linked in nonrandom pairs through disulfide bonds. They are apparently synthesized as single-chain proteins that are subsequently modified by proteolysis to form the acidic and basic polypeptide chains.

In addition to the storage proteins, soybeans contain a variety of minor proteins, including trypsin inhibitors, hemagglutinins, and enzymes (eg, urease and lipoxygenase). Amino acid compositions of soybeans are given in Table 1 along with the essential amino acid pattern for a high-quality protein that meets human requirements as established by the Food and Nutrition Board of the National

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