Revealing the Microbial Origins of N-Lactoyl Amino Acids in Soy Sauce: Synthesis Conditions, Potential Enzymes, and Utilization Preference

The flavor contribution of N-lactoyl amino acids (Lac-AAs) to soy sauce is gradually gaining attention. However, research on their formation mechanism is limited. In this study, the production of Lac-AAs of 12 species of yeasts commonly detected in soy sauce was investigated, and Candida orthopsilosis, Candida parapsilosis, and Millerozyma farinosa were identified as key yeasts because of their productivity and high correlation coefficients with peak areas of Lac-AAs (r > 0.8). According to synthesis experiments, the formation of Lac-AAs was catalyzed by endoenzymes, with an optimal pH of 5–7 and a temperature of 50–60 °C. After BLAST comparison, cytosolic nonspecific dipeptidase 2 (CNDP2) or other similar metallodipeptidases may be potential enzymes for Lac-AAs synthesis. According to preference experiments, 13 amino acids were exploitable to form the corresponding Lac-AAs by selected yeasts, except Gly, Ala, Ser, Pro, and Asp, presumably related to the structure of side chains. Among them, nonpolar amino acids were preferred by yeasts. These findings enrich knowledge on taste substances and microorganisms and can help to regulate the microbial community for flavor improvement of soy sauce.