MAJOR SOYBEAN PROTEINS
The simplest criterion used for the characterization of proteins is their solubility in various media. As in all legumes, the bulk of soybean proteins are globulins, characterized by their solubility in salt solutions. The solubility of soybean proteins in water is strongly affected by the pH. Close to 80 % of the protein in raw seeds or unheated meal can be extracted at neutral or alkaline pH. As the acidity is increased, solubility drops rapidly and a minimum is observed at pH 4.2 to 4.6. This is the isoelectric region of soybean proteins taken as a whole.
The pH dependence of solubility is used in the manufacture of isolated soybean protein, whereby defatted, unheated meal is extracted with water at neutral or slightly alkaline pH, and the protein is then precipitated from the filtered extract by acidification to the isoelectric region.
More precise and detailed fractionation of the proteins can be carried-out by techniques such as ultracentrifugation, gel filtration and electrophoresis.
Since the classical work of W. Wolff, it has become customary to characterize the soybean protein fractions by their sedimentation constants.
Four major fractions, known as 2 S, 7 S, 11 S and 15 S have been studied extensively. (S stands for Svedberg units. The numerical coefficient is the characteristic sedimentation constant in water at 20oC. The figures are not exact but nominal. Thus the 11 S globulin has a sedimentation constant of 12.3). The 11 S and 7 S fractions constitute about 70% of the total protein in soybeans. The ratio 11 S/7 S is a varietal characteristic and may vary from 0.5 to 3.
The 2 S fraction consists of low molecular weight polypeptides (in the range of 8000 to 20000 daltons) and comprises the soybean trypsin inhibitors (see below).
The 7 S fraction is highly heterogeneous. Its principal component is beta-conglycinin, a sugar containing globulin with a molecular weight in the order of 150000. The fraction also comprises enzymes (beta-amylase and lipoxygenase) and hemagglutinins (see below ).
The 11 S fraction consists of glycinin, the principal protein of soybeans. Glycinin has a molecular weight of 320000-350000 and is built of 12 sub-units, associated through hydrogen bonding and disulfide bonds. The ability of soy proteins to undergo association-dissociation reactions under known conditions, is related to their functional properties and particularly to their texturization.
The 15 S protein is probably a dimer of glycinin. Conglycinin and glycinin are storage proteins and they are found in the protein bodies within the cells of the cotyledons.