**2.1. Soybean protein**

Soybeans contain 35% protein as storage protein, which is used for nutrition during germina‐ tion. That storage protein is stored in granules, called protein bodies, of about 5–8 μm diameter. Soluble soy protein is extracted from insoluble protein bodies that are burst during soy milk and *Tofu* production. All soy protein is stored in the protein body [1]. Other proteins exist as nonstorage proteins, containing important physiological proteins such as trypsin inhibitors. When an animal ingests a trypsin inhibitor, a digestive enzyme, trypsin activity is inhibited by combination of the trypsin inhibitor specifically with trypsin. Consequently, the pancreas works excessively to secrete and supplement trypsin activity [1]. However, after heating, the inhibitor loses its inhibitory activity and does not bind with trypsin.

Protein digestibility-corrected amino acid score (PDCAAS) values are used to evaluate the protein quality based on the amino acid requirements of humans and their ability to digest it. It was long thought that the amount of amino acid requirements of humans dictate a low score for soybeans because methionine and cysteine residues, sulfur amino acids, in soybean storage proteins have a low composition. The score was only 86 points based on the amino acid requirements of a developing rat. However, in 1985, the score was modified to 100 points, the same as milk and eggs, based on the amino acid requirements of humans. Soy storage proteins are rich in nutrition for human needs [4].

Throughout the world, the recently improving healthy image of soy protein is interesting. In particular, the health benefits of soy foods attract attention in the United States. Health claims are authorized by the Food and Drug Administration (FDA) in the United States: foods containing 6.25 g of soy protein or more can be said by manufacturers to reduce the risk of heart disease if a consumer ingests 25 g/day of soy protein [5]. In Japan, some soybean containing foods are manufactured as *Tokuho*: government-approved foods for specified health benefits, as for hypocholesterolemic activity in this case.

The taxonomy of soybean storage protein has been conducted according to the sedimenta‐ tion coefficient by an ultracentrifugal fraction as 2S globulin, 7S globulin, 11S globulin, and 15S globulin. Yamauchi [1] reported details of soybean proteins: 2S globulin contained αconglycinin, 7S globulin composed with β-conglycinin and γ-conglycinin, and 11S globulincontaining glycinin. In addition, the 11S globulin composes hexamer. It is a 350,000 Da protein. Furthermore, their proteins are composed with five subunits as G1–G5; their subunit was 10 polypeptides as A1a, A2, A1b, A3, A5A4, A4, B2, B1b, B4, and B3. Their polypeptides are combined specifically as A1aB1b, A1bB2, A2B1a, A3B4, and A5A4B3. In fact, β-conglycinin, called 7S globulin, combines a dimer protein and a monomer protein, which are 150,000– 200,000 Da, or an average of 180,000 Da. They are composed of three subunits: an αsubunit of 63,000 Da, an α′-subunit of 67,000 Da, and a β-subunit of 48,000 Da. The protein has a low concentration of sulfur amino acids. In particular, the β-subunit does not contain methionine, cysteine, and tryptophan [6].
