Dynamic succession of microbial community in Nongxiangxing daqu and microbial roles involved in flavor formation

Daqu, the fermentation starter of Chinese baijiu, is produced by barley and/or peas during open fermentation using natural inoculation. However, the succession of microbial communities and their roles during daqu manufacture remain unclear, which are closely related to the flavor and quality of baijiu. In this study, the dynamics of physicochemical properties, microbial communities, and volatile compounds were investigated at both fermentation (FM) and storage (ST) stages during the manufacture of Nongxiangxing daqu. High-throughput sequencing analysis showed that Weissella and Lactobacillus were the predominant bacterial genera, at the end of storage, the proportion was 31.45% and 13.14% respectively. Thermoascus, Rhizopus, Pichia and Rhizomucor were dominant fungi in daqu, and Thermoascus, Pichia and Rhizopus still occupied a large proportion at the end of daqu storage, accounting for 67.65%, 23.94% and 7.05% respectively. Redundancy analysis was employed to investigate the relationship between microorganisms and physicochemical indexes, and the results showed that acidity and Lactobacillus were positively correlated, saccharification power was positively correlated with Aspergillus, Rhizomucor, Rasamsonia and Thermoascu. The correlation networks between microorganisms and volatile substances were analyzed, a total of 60 compounds including 42 esters, 4 alcohols, 5 ketones and 5 aldehydes, 2 acids, 1 amine, 1 pyrazine were detected, the results suggested that esters are the main aroma components in daqu, and most of esters are positively correlated with fungi. In addition, key enzymes involved in carbohydrate hydrolysis, ethanol fermentation, and flavor formation were revealed. This study may further improve our understanding concerning the microbial system and microbial roles of Nongxiangxing daqu and may contribute to optimize the process during daqu manufacture.