Baijiu is an important fermented product in China. A yeast named YX3307 that is capable of producing a large amount of ethyl caproate (EC) was isolated from Daqu, a crude fermentation starter for Baijiu. This yeast was identified as Clavispora lusitaniae on the basis of its morphological properties, physiological and biochemical characteristics, and 26S rDNA sequence. Single-factor experiments were conducted to obtain the optimum fermentation conditions for EC production by YX3307. The highest EC yield (62.0 mg/L) from YX3307 was obtained with the following culture conditions: inoculum size 7.5% (v/v), seed cell age 30 h, sorghum hydrolysate medium (SHM) with a sugar content of 10 Brix and an initial pH of 6.0; incubation at 28°C with shaking at 180 rpm for 32 h; addition of 10% (v/v) anhydrous ethanol and 0.04% (v/v) caproic acid at 32 and 40 h, respectively, static culture at 20°C until 72 h. YX3307 synthesized more EC than ethyl acetate, ethyl lactate, ethyl butyrate, and ethyl octanoate. An intracellular enzyme or cell membrane enzyme was responsible for EC synthesis. YX3307 can produce many flavor compounds that are important for high-quality Baijiu. Thus, it has potential applications in improving the flavor and quality of Baijiu.

Daqu is a saccharifying and fermentation agent for Baijiu production. It provides ingredients, flavours or flavour precursors, microorganisms and enzymes that influence Baijiu quality and character. Here, a systematic investigation was performed on the physicochemical properties, volatile flavour compounds and microbial community structures of Chinese Guojing roasted sesame‐like flavour Daqu. The results show that the Daqu exhibited high saccharifying ability, but low liquefying and fermenting ability. Analysis by headspace solid‐phase microextraction gas chromatography–mass spectrometry showed that the Daqu and a simulated solid‐state fermentation sample contained 28 and 49 volatile components, including alcohols, esters, aldehydes, benzodiazepines, alkenes and nitrogen containing compounds. The main volatiles were aldehydes, esters and alcohols in Daqu, while the esters, alcohols and phenols were relatively higher in the solid‐state fermentation sample. With the microbial flora, Bacillus, Actinobacteria_norank, Thermoactinomyces, Enterobacteriaceae_norank, Enterobacter, Actinopolyspora, Kroppenstedtia, Pseudomonas and Sphingobium were the main prokaryotic microorganisms. The dominant fungi were Thermomucor, Trichocomaceae_Incertae Sedis, Absidia, Eurotiomycetes_Incertae Sedis, Mucorales_Incertae Sedis and Eurotiales_Incertae Sedis. By systematically characterising the physicochemical properties, flavour compounds and microorganisms in Daqu, this study increases the understanding of Daqu and provides information for improvements in Baijiu production and quality. © 2019 The Institute of Brewing & Distilling

We identified physicochemical, volatile compound, and microbial community differences between three grades of roasted-sesame-like flavored Daqu. The Daqu grades had different physicochemical characteristics. The concentrations and proportions of various esters reflected sensory evaluations of the Daqu classes but no rule linking the flavor compound types and concentrations to Daqu grade was identified. The prokaryotic microorganism communities were similar in premium-and first-grade Daqu but very different in general-grade Daqu. The eukaryotic microorganism communities in premium-and general-grade Daqu were similar, but the genus abundances were more similar in the premium-and first-grade Daqu samples than in premium-and general-grade Daqu. ARTICLE HISTORY

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Background: Daqu, the saccharification, fermentation, and aroma-producing agents for Baijiu brewing, is prepared using a complex process. Aging is important for improving the quality of Daqu, but its impact has rarely been studied. This study investigated changes in the physicochemical properties, flavor compounds, and microbial communities during aging of Daqu with a roasted sesame-like flavor. Results: The physicochemical properties changed continuously during aging to provide a high esterifying activity. Aging removed unpleasant flavor compounds and helped to stabilize the flavor compounds in mature Daqu. A high-throughput sequencing approach was used to analyze the changing composition of the microbial communities during aging. Aging helped to modify the microbial population to produce better Baijiu by eliminating low-abundance microbial communities and optimizing the proportion of predominant microbial communities. Nine genera of prokaryotic microbes formed the core microbiota in Daqu after aging. Regarding eukaryotic microbes, Zygomycota, the predominant community, increased in the first 2 months, then decreased in the third month of aging, while Ascomycota, the subdominant community, showed the opposite behavior. Absidia, Trichocomaceae_norank and Rhizopus were the predominant genera in the mature Daqu. Conclusions: Significant correlations between microbiota and physicochemical properties or flavor compounds were observed, indicating that optimizing microbial communities is essential for aging Daqu. This study provides detailed information on aging during Daqu preparation.