Novel Umami-, Salty-, and Kokumi-Enhancing γ-Glutamyl Tripeptides Synthesized with the Bitter Dipeptides from Defatted Peanut Meal Protein Hydrolysate

Defatted peanut meal protein hydrolysates (DPMHs) usually have a bitter taste. γ-Glutamylation by Bacillus amyloliquefaciens l-glutaminase was introduced to DPMH to reduce its bitterness and generated a γ-glutamylated product (DPMH-G). Extra l-glutamine (l-Gln) (5% w/w) was added to DPMH, and the mixture was then γ-glutamylated (DPMH-G-Q). Results showed that γ-glutamylation decreased the bitterness of the products and also enhanced their kokumi, umami, and salty taste, especially for DPMH-G-Q. Bitter amino acids and bitter peptides were found to be substrates (acceptors) of the synthesized γ-[Glu](1,2)-AAs and γ-Glu-AA-AAs, respectively. The production yield of γ-[Glu](1,2)-AAs was only 0.69/100 g for DPMH-G and 2.30/100 g for DPMH-G-Q, which was much lower than that of γ-Glu-AA-AAs (5.73/100 g for DPMH-G and 18.72/100 g for DPMH-G-Q). The improvement in taste attributes of DPMH might mainly be due to the consumption of bitter dipeptides and the production of γ-Glu-AA-AAs. In DPMH-G-Q, eight γ-Glu-AA-AAs were identified, including γ-Glu-Ile-Lys, γ-Glu-Ala-Ile, γ-Glu-Leu-Leu, γ-Glu-Phe-Leu, γ-Glu-Thr-Leu, γ-Glu-Ile-Met, γ-Glu-Val-Leu, and γ-Glu-Ser-Tyr, which were first time reported. They all can enhance umami, salty, and kokumi taste with a threshold value between 1.61 ± 0.21–2.16 ± 0.19, 1.65 ± 0.19–2.23 ± 0.20, and 0.67 ± 0.21–1.00 ± 0.22 mM, respectively. Insufficient l-Gln restricted the formation of γ-glutamyl peptides, and this was why DPMH-G had a lower yield and variety than DPMH-G-Q. This also suggested that l-glutaminase is selective to different substrates. Overall, this study provides a new method to reduce the bitterness of protein hydrolysates and also improve the taste by synthesizing γ-glutamyl tripeptides.