Studies On The Mechanism Of Soymilk Coagulation By An Alkaline Proteinase-Alcalase

For its high soymilk-coagulating activity, Alcalase, an alkaline protease, was selected from thirteen proteases to study the coagulating properties and mechanisms of protease-treated soymilk coagulum. The main conclusions were obtained as follows:1. It was found that the coagulating activity of Alcalase, papain and bromelain were stronger among all of the proteases, although most of proteases used in this study possessed such activity. The proteases derived from plants and microorganisms were more active in coagulating soymilk than that from animals; Alkaline proteases were stronger than neutral ones; While acidic proteases had no coagulating activity. For these proteases, the optimal temperatures to coagulate soymilk were usually about 20 ℃ higher than that to hydrolyze soy proteins.2. Many processing factors had profound effects on the clotting time, adhesiveness and strengthness of soymilk coagulum. With the incubation temperature increasing from 30 ℃ to 70 ℃, the soymilk clotting time and adhesiveness decreased, while coagulum strengthness increased; So did the clotting time, adhesiveness and coagulum strengthness as the pH value decreased from 7.0 to 6.0; The soymilk treated with protease could not form coagulum as long as the protein concentration in soymilk was lower than 2.5%; When the protein concentration was excess of 2.5%, increment of the protein concentration will increase the coagulum strengthness; However, the adhesiveness and cohesiveness showed no significantly difference with the increment of protein concentration; the increment of enzyme dosage decreased the clotting time, but did not affect the physiological properties of soymilk coagulum; Addition of calcium and magnesium ions to soymilk resulted in limited decrement of clotting time, increment of coagulum strengthness and fall of adhesiveness of soymilk coagulum.3. The coagulum from soymilk treated with Alcalase had a smooth mouth feel and bitterless taste. However, comparing with the gel from soymilk treated with CaCl2, it was of low strengthness and high adhesiveness, and moreover, the whey could not easily drain from the coagulum.4. Hydrogen bonds and hydrophobic interaction played an important role in the formation of coagulum from Alcalase-treated soymilk, ion bonds and disulfide bridges had no significant effect on the formation of the coagulum. The formation of new covalent bonds was not observed during the formation of coagulum.5. Due to the lower degree of hydrolysis and the high average peptide chain length, glycinin treated with Alcalase was more easily to form coagulum than β -conglycinin treated with Alcalase, and the glycinin coagulum was stronger than β -conglycinin one. Reduction of α - Helixes and increment of random coil were observed during the formation of Alcalase-treated soymilk coagulum, which were helpful for the formation of hydrogen bonds between soy protein molecules and the exposure of hydrophobic amino acid residues.6. Conglycinin is a glycoprotein containing approximately 5% carbohydrate. The carbohydrate moietiesinhibited the coagulation of soymilk treated with Alcalase, and the deglycosidation increased the gel-forming ability of β -conglycinin.7. Addition of lipid to soy protein isolates was beneficial to the formation the coagulum of soy protein isolates treated with Alcalase.8. The gelation mechanisms of Alcalase-treated soymilk were summarized as follows: Hydrolysis of soy proteins treated with Alcalase increased the surface hydrophobicity of soy proteins. The hydrophobic interaction predominantly resulted in random aggregation of the soybean proteins led to the formation of protein particles. The aggregated particles were then further aggregated to large granules, and at last formed coagulum. The hydrogen bonds, formed during the aggregation of the building particles of soybean proteins, enhanced the strengthness and stability of the coagulum structure.