Improving the umami taste of soy sauce by the addition of bacterial γ-glutamyltranspeptidase as a glutaminase to the fermentation mixture

γ-Glutamyltranspeptidase (GGT) catalyzes the hydrolysis of γ-glutamyl compounds and transfer of their γ-glutamyl moieties to amino acids and peptides. GGT is able to catalyze the hydrolysis of glutamine (Gln) to glutamic acid (Glu), the so called glutaminase reaction. A salt-tolerant wild-type GGT from Bacillus subtilis and D445A mutant GGT, whose transpeptidation activity was abolished, were added to a soy sauce fermentation mixture to increase the amount of Glu in the product. The concentration of Glu in the soy sauce was 36 mM higher with the addition of wild-type GGT than without. Nine out of ten members of a panel recognized a difference and preferred the taste of the soy sauce fermented with the wild-type GGT. These results indicate that wild-type GGT is competent for use in the fermentation of soy sauce as a glutaminase.