Coagulation and gelation of soy protein isolates induced by microbial transglutaminase

The reaction process and corresponding mechanism of coagulation and gelation of native soy protein isolates (SPIs) induced by microbial transglutaminase (MTGase) were investigated. The protein constituents of SPIs, including a majority of subunits of beta-conglycinin and acidic subunits of glycinin, could be polymerized by MTGase to form high weight molecular (WM) biopolymers. Both the coagulation and gelation reactions of native SPI solutions induced by MTGase were dependent upon the initial protein substrate concentration ([C](0)). In the coagulating reactions, the turbidity of SPI solutions continually increased with increasing [C](0) in the range from 0.25 to 3.0%. As for the gelation reactions, with the concentration increasing from 3 to 8% (w/v), the onset time of gelation of native SPIs induced by 0.8 units/mL of MTGase at 37C shortened by similar to 5-fold, and the storage modulus (G') of finally formed gels (after 4 h) increased from similar to 1 to 1300 Pa. Both the coagulation and gelation reactions of SPI solutions were promoted remarkably by increasing the enzyme concentration. Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis analysis showed that the protein constituents of MTGase-induced aggregates of SPI (2% w/v) were mainly composed of basic subunits of glycinin and some of newly cross-linked high MW biopolymers. The solubility analysis of protein constituents indicated that the covalent cross-linkage, hydrophobic and H bindings and disulfide bonds were mainly involved in the coagulation of SPI induced by MTGase.