An electronic nose, gas chromatography-mass spectrometry (GC-MS) and gas chromatography-olfactometry (GC-O) were used to explore the changes of aroma compounds in vinegar prepared with Monascus-fermented rice during its acetic acid fermentation process. Different multivariate statistical analysis, such as cluster analysis, principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were used to investigate the differences in volatile compounds between different stages of fermentation and to identify the characteristic volatile components. The results showed that the electronic nose could clearly discriminate samples from different acetic acid fermentation stages. A total of 54 volatile compounds were identified by GC-MS and GC-O during the acetic acid fermentation process. The characteristic aroma components were n-octanol, isobutanol and ethyl valerate for the early stage; benzoic acid, ethyl palmitate, n-hexanol, 2,4-di-tert-butylphenol and ethyl lactate for the middle stage; propyl acetate, ethyl lactate and isobutyl acetate for the mid-tolate stage; L(+)-2,3-butanediol and ethyl heptanate for the late stage. The results of this study provide a theoretical basis for aroma regulation and flavor improvement of vinegar prepared with Monascus-fermented rice.

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.