Summary Daqu provides hydrolytic enzymes that saccharifying the macromolecular carbohydrate in fermenting grains. The process of saccharification is the premise of Baijiu production. But little is known about the protein composition associated with the activity of saccharifying enzymes in daqu. This study investigated related proteins between nongxiangxing daqu and jiangxiangxing daqu by metaproteomics. We present that the amylase system (α‐amylase, glucoamylase and α‐glucosidase) and cellulase system (cellobiohydrolases, endoglucanase and β‐glucosidase) are regarded as the main saccharifying enzymes. Combined with protein‐based taxonomic annotation, the key proteins involving in amylase system were mainly from Kroppenstedtia, Lichtheimia, Byssochlamys and Thermomyces, while Thermoascu contributed the most to cellulase system. Moreover, it was found that the up‐regulated amylase proteins at thermophilic fermentation period in jiangxiangxing daqu in comparison with nongxiangxing daqu were affiliated to Aspergillus, Rhizomucor, Byssochlamys and Thermomyces, along with Thermoascu contributing to cellulase system. These findings will provide clues for improving saccharification efficiency in Baijiu production.

BACKGROUND: The saccharification function of daqu is usually characterized by two indicators: saccharification power and liquefaction power. Daqu provides diverse microbial saccharifying enzymes for hydrolyzing carbohydrate in Baijiu fermenting grain. Obviously, the composition of microbial communities and enzymatic genes in different types of daqu cultured at varied temperatures is different. However, these differences in saccharification function are not fully understood. RESULTS:The findings suggested that the saccharification power and liquefaction power of jiangxiangxing daqu were lower than those of nongxiangxing daqu throughout the production process. We employed metagenomics to find evidence that a mode of multiple saccharifying enzymes involving amylase, cellulase and hemicellulase originating from various microbes exists in daqu. Moreover, a totality of 541 related differential genes were obtained, some of which, annotated to genera of Aspergillus, Lactobacillus and Weissella, were significantly enriched (P < 0.05) in nongxiangxing daqu, while others, annotated to thermophilic genera of Virgibacillus, Bacillus, Kroppenstedtia and Saccharopolyspora, showed a higher relative abundance in jiangxiangxing daqu (P < 0.05).CONCLUSION: Various microbial communities of daqu showed diverse saccharification capacity during cultivation of different parameters. These findings are helpful in comprehending the saccharification functional genes of daqu.

Chinese Jiang-flavor Baijiu is the most widely consumed liquor. Jiang-flavor Daqu, a fermentation starter, is important sources of key flavors of Jiang-flavor Baijiu. Some microbes play significant roles in flavor formation of Daqu. In order to clarify the microbial population that promotes the formation of Daqu flavor, we use high throughput sequencing technology combined with headspace solid-phase microextraction gas chromatography-mass spectrometry to investigate microbial population and volatile compounds in Jiang-flavor Daqu. In addition, the dynamic changes of physicochemical factors and enzyme activities in Jiang-flavor Daqu were investigated. Correlations between microbial population, volatile compounds, physicochemical factors, and enzyme activities of Jiang-flavor Daqu were disclosed by redundancy analysis and Spearman correlation analysis. A total of 66 volatile compounds were identified and 14 primary microorganisms were selected. Results showed that high temperature environment could promote the formation of acids, aldehydes and ketones, phenols, furans by affecting the growth of Monascus, Trichomonascus, Cutaneotrichosporon, Wallemia, Millerozyma, Nigrospora, Cladosporium, Bacillus, and Pediococcus in the early fermentation stage. While high nitrogen environment was more suitable for the growth of Virgibacillus and Kroppenstedtia, who could promote the formation of pyrazines in the late fermentation stage. Practical applicationsThis study has provided a scientific basis for the directed regulation of Daqu fermentation through physicochemical factors, developed scientific basis for artificially constructing Daqu microbial population and obtaining an easy-to-operate, reproducible fermentation system for Daqu production.

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BACKGROUND The ester‐synthesis enzymes influenced by environmental factors during Daqu‐making process largely determine the flavor of Chinese liquor, but the main ester‐synthesis enzyme and its key influencer remain unclear. Here, the volatile ester profiles over the whole Daqu‐making process, under different treatments, for at least 90 days, were carefully analyzed, and the potential ester‐synthesis enzymes, as well as their dependently environmental factors, were explored. RESULTS In the detected 46 volatile esters, only the short‐chain (C4–C8) and medium‐chain (C9–C13) ester content obviously changed, as the primary contributor discriminating different samples. Their trends were both consistent with that of the alcohols and the primary metabolism, which included alcohol acyltransferases (AATs) reaction with alcohols and acyl‐CoAs as the substrates. Among the potential ester‐synthesis enzymes, the typical AAT activity also exhibited the highest correlation with the short‐ and medium‐chain esters (r > 0.78, P < 0.05). The Mantel test between environmental factors and ester production showed that temperature of Daqu was directly correlated with the short‐chain esters (r = 0.58, P < 0.01) and AAT activity (r = 0.56, P < 0.01). Further, the short‐ and medium‐chain ester content in Daqu under the treatment nearer to the reported optimal temperature of 40–50 °C of AATs reaction was overall higher than that of the other treatment Daqu. CONCLUSION This study revealed that the temperature‐dependent AATs reaction was the main enzymatic method producing the short‐ and medium‐chain esters over the whole Daqu‐making process. The results could contribute to the flavor improvement of Baijiu. © 2022 Society of Chemical Industry.