Free amino acid (FAA) is the important component of vinegar that influences quality perception and consumer acceptance. FAA is one of the major metabolites produced by microorganisms; however, the microbial metabolic network on FAA biosynthesis remains unclear. Through metagenomic analysis, this work aimed to elucidate the roles of microbes in FAA biosynthesis during Monascus rice vinegar fermentation. Taxonomic profiles from functional analyses showed 14 dominant genera with high contributions to the metabolism pathways. The metabolic network for FAA biosynthesis was then constructed, and the microbial distribution in different metabolic pathways was illuminated. The results revealed that 5 functional genera were closely involved in FAA biosynthesis. This study illuminated the metabolic roles of microorganisms in FAA biosynthesis and provided crucial insights into the functional attributes of microbiota in vinegar fermentation.

Dry-cured meat products are considerably popular around the world due to unique flavor. Proteolysis is one of the enzymatic reactions from which flavor substances are derived, which is affected by endogenous proteases. The purpose aimed to reveal the potential relationship between endogenous proteases and key flavor substances in dry-cured pork coppa in this paper. The dynamic changes of endogenous proteases activity, free amino acids, and volatiles during dry-cured pork coppa processing were characterized. The results showed that 5 kinds of free amino acids, Glu, Lys, Val, Ala, and Leu, were identified as significant contributors to taste. Meanwhile, key volatiles, such as hexanal, nonanal, octanal, benzaldehyde, 3-methyl butanoic acid, 2-methyl propanoic acid, and ethyl octanoate, greatly contributed to the flavor characteristics of dry-cured pork coppa. Further partial correlation analysis was performed to better elucidate the relationship among parameters. The results revealed that close relationship between endogenous proteases and key substances. RAP not only significantly affected the accumulation of key active-amino acids, but also affected the accumulation of ethyl octanoate, 2,3-pentanedione, and 2,3-octanedione by regulating the accumulation of octanoic acid and Leu. In addition, cathepsin B and D, DPP Ⅱ, DPP Ⅳ and RAP notably affected accumulation of hexanal.