Soybean Proteins

This chapter focuses on soybean proteins. Soybeans contain a variety of proteins with unique properties for the germinating seed. The characteristics of soybean foods are attributable in part to the proteins from the bean. The largest mass of the seed protein is the storage proteins, glycinin and β-conglycinin, which do not have biological activity other than as amino nitrogen stores for the germinating seed. The structures of these two proteins were relatively conserved across many legume species and other related plants in major plant gene families of the legumins and vicilins. The other important soybean seed proteins that are discussed in the chapter have biological activity and include the lipoxygenases, the trypsin inhibitor family, including Kunitz trypsin inhibitor and Bowman-Birk inhibitor, and the soy lectins. The seed proteins represent between 30 and 50% of seed mass with the storage proteins accounting for 65–80% of the seed protein. Soybean proteins have excellent amino acid profiles for humans although they are deficient in sulfur amino acids for livestock and rodents. In addition to soy's protein nutritional qualities, other biological activities are attributed to them, including cholesterol-lowering abilities and anti-cancer activities. The storage proteins possess major flavor-binding ability that is a challenge in preparing bland soy protein products for Western tastes as well as formulation properties unique among food proteins.