Gamma-glutamylation of the white particulates of sufu and simultaneous synthesis of multiple acceptor amino acids-containing γ-glutamyl peptides: Favorable catalytic actions of glutaminase

White particulates, observed on the surface of sufu, were mainly composed of tyrosine (Tyr, 83.12 g/100 g) and phenylalanine (Phe, 5.12 g/100 g). This issue was tackled through a γ-glutamyl transfer reaction catalyzed by l-glutaminase of Bacillus amyloliquefaciens. This novel approach can simultaneously improve the solubility and flavor characteristics of white particulates, while synthesizing kokumi-active γ-[Glu](n = 1,2,3)-Tyr and γ-[Glu](n = 1,2)-Phe peptides using glutamine (Gln) as the γ-glutamyl donor and white particulates as acceptors (with consumption of 68.5% of Tyr and 70.89% Phe). For the first time, the glutaminase was found to catalyze the synthesis of kokumi-active multiple acceptor amino acids-containing γ-glutamyl peptides (γ-[Glu](n=1,2,3)-Phe, γ-[Glu](n=1,2,3)-Tyr, γ-[Glu](n=1,2,3)-Met and γ-[Glu](n=1,2,3)-Val). The resultant post-enzymatic reaction mixture contained short chain γ-glutamyl peptides and could serve as a flavor enhancer e.g. at 6 mg/mL to enhance umaminess, mouthfulness and thickness of commercial chicken broth with minimal astringency and bitterness involved. Therefore, it is worth adding glutaminase to the brines to catalyze the transfer of insoluble amino acids to water-soluble and kokumi-active γ-[Glu](n=1,2,3)-Tyr and γ-[Glu](n=1,2)-Phe in sufu fermentation.