Recent progress of soybean protein foods: Chemistry, technology, and nutrition

Abstract

The most important chemical reactions during the process of soybean protein foods are the intermolecular reactions among the residues exposed on the surface of the protein molecules through the denaturation process. In native soybean protein molecules, most amino acid residues responsible for the reactions—such as cysteine (‐SH), cystine (S‐S), and hydrophobic amino acid residues—are buried in the inside region of the molecule, inaccessible to water. These residues become reactable with each other through the exposure from the inside by heat denaturation during processing. The unique textures of soybean protein foods, such as tofu, kori‐tofu, yuba, and texturized products produced by extruder, etc., are the results of both the intermolecular interchange reaction between the exposed ‐SH and S‐S groups and the intermolecular hydrophobic reaction among the exposed hydrophobic amino acid residues. The exposure of amino acid residues is also important for the hydrolysis of soybean proteins by enzymes, through which soy sauce is produced, because the cleavage of the peptide bonds is carried out after binding between the active sites of the enzymes and the enzyme‐specific amino acid residues exposed through denaturation. These facts indicate the importance of the three‐dimensional structures of soybean protein molecules in the technology of soybean protein foods. Recently great progress has been made in the manufacturing techniques of soybean protein foods, such as soy milk, tofu, abura‐age, textured protein products, and soy sauce. The quality of soy milk and tofu was very much improved by controlling the action of the biologically active substances such as lipoxygenases and β‐glucosidases which are contained in soybeans and responsible for the production of off‐flavor. A new abura‐age, whose texture does not deteriorate during frozen storage or drying, was developed by using soybean protein isolate and oil as materials. A new type of textured protein product was also developed: a deep‐fat‐fried nugget with unique texture and flavor. This product is textured through a twin‐type extruder. For soy sauce manufacturing new biotechnology has been applied on the pilot‐plant scale. This is a system of continuous fermentation through bioreactors with the immobilized whole cells of microorganisms, by which the fermentation term is shortened strikingly. New and important discoveries were made on the nutrition of soybean proteins. According to recent experiments using human beings, the amino acid score of soybean proteins is 100 for persons more than 2 years old, indicating that the nutritive value of soybean proteins is equal to animal proteins. Further, it was elucidated that soybean proteins have cholesterol‐lowering action. A discussion is presented on the future of the soybean protein foods.