Alteration of microbial community for improving flavor character of Daqu by inoculation with Bacillus velezensis and Bacillus subtilis

Daqu, an important saccharification and fermentation starter, provides microbes, hydrolytic enzymes, and flavor compounds, and plays a crucial role for production of Chinese Baijiu. To evaluate the feasibility of bioaugmentation inoculated with Bacillus velezensis and Bacillus subtilis for improving the flavor character of Daqu, microbial community, enzyme activity, and volatile compound were analyzed and compared between traditional Daqu (T Daqu) and fortified Daqu (F Daqu). Bioaugmentation inoculation increased the abundance of Bacillus, Lactobacillus, and Candida. Additionally, analysis of enzyme activity showed that liquefying power, saccharifying power, and esterifying power were significantly higher (P < 0.05) in F Daqu than T Daqu. Meanwhile, the contents of volatile compounds including esters, pyrazines, and alcohols significantly increased (P < 0.05) after bioaugmentation inoculation. In particular, the contents of tetramethylpyrazine and phenethyl alcohol in F Daqu significantly increased (P < 0.05) by 90.8% and 69.5% compared to T Daqu, respectively. Moreover, many enzymes involved in saccharification, ethanol fermentation, and aroma generation were predicted, and the abundance of their encoding genes obviously increased in F Daqu. Taken as whole, bioaugmentation inoculation with B. velezensis and B. subtilis altered the microbial community for regulating their metabolic activities, and subsequently, improved the flavor character of Daqu.