Comparison of microbial community and metabolites in spontaneous fermentation of two types Daqu starter for traditional Chinese vinegar production.

Daqu starter, an important saccharifying and fermenting agent for the brewing process of traditional vinegar, is manufactured by spontaneous solid-state fermentation which routinely undergoes low or medium incubation temperature. Previous studies have demonstrated that the temperature plays a pivotal role in Daqu quality. Hence, to explore the feasibility of high temperature fermentation applied in the vinegar Daqu brewing and provide guidelines of controlling environmental parameters in traditional vinegar industries, the microbial community and metabolites of vinegar Daqu during medium-temperature and high-temperature fermentation processes (namely, MTFP and HTFP) were compared. The results indicated that the glucoamylase activity, amylase activity and microbial community showed no significant difference in the end of two batches (P > 0.05). Enterobacteriales, Lactobacillales, Bacillales, Saccharomycetales and Mucorales were the dominant orders during MTFP and HTFP. Redundancy analysis revealed that incubation temperature showed positive correlation with the microbial composition from days 3-14 of the fermentation process and was positively associated with the predominant phylotypes of Bacillales, Mucorales, Xanthomonadales and Rickettsiales. The acidity and moisture showed major correlations with microbial composition on day 1 of MTFP and were positively related with the predominant phylotypes of Mucorales and Lactobacillales at the order level. Moreover, higher relative contents of all volatiles were shown in the end of HTFP (13.91 mg/100 g Daqu) compared to MTFP (10.01 mg/100 g Daqu). This work illustrates high temperature (approximately 60°C) fermentation is promising to improve the vinegar Daqu flavor and shall likely contribute to preferably make traditional Daqu by modulating steerable environmental parameters.