Divalent magnesium cation concentrations determine the formation of tofu-like precipitates with differing urea solubilities

Different coagulant concentrations can induce urea-soluble precipitates (USPs) and urea-insoluble precipitates (UIPs) during tofu-like precipitate formation. In this study, MgCl(2) concentration-dependent changes in USPs were quantified using a method based on urea solubility differences in order to investigate the factors affecting USP versus UIP formation. The addition of various Mg salts revealed that anions influence the solubility of proteins in both USPs and UIPs. Moreover, addition of MgCl(2)/NaCl mixtures, in which the Cl(−) concentration was held constant, demonstrated that Mg(2+) was essential for UIP formation but not for USP formation and that Cl(−) was inconsequential for both USP and UIP formation. NaCl addition showed that the driving force for USP formation was salting-out due to the presence of cations. Overall, our data indicated that the Mg(2+) concentration determined the separation of USPs and UIPs. These results will help elucidate the molecular mechanisms mediating the separation of silken tofu and regular tofu.