Daqu regulates the balance of saccharification and alcoholic fermentation to promote Chinese baijiu fermentation

Saccharifying activity of starter culture plays a key role on the simultaneous saccharification and fermentation of Chinese baijiu fermentation. However, the regulation of alcoholic fermentation by Daqu starter or saccharifying enzymes still remains unclear in the baijiu fermentation. In this study, we identified 13 proteins, affiliated to 4 saccharifying enzymes (including alpha-amylase, beta-amylase, glucoamylase and beta-glucosidase) as the key saccharifying enzymes in Daqu by metaproteomic and culture-dependent analysis. These enzymes were metabolized from plant substrate (13.4%) and microbial metabolism (86.6%), and especially alpha-amylase and glucoamylase from Aspergillus and Rhizopus were dominant in Daqu bio-system. Furthermore, we observed the dynamic of saccharifying activity of Daqu (from 241 to 395 U) was driven by the combinations of isolated saccharifying microbes responsible for key enzymes releasing, indicating AR7 group (with a ration of Rhizopus oryzae: Aspergillus oryzae in 10:1) presented the highest saccharifying activity (395 U) in a simulative Daqu production. However, the highest ethanol production (80.5 mg/g) was obtained from the fermentation driven by AR8 group (with a ration of Rhizopus oryzae: Aspergillus oryzae in 50:1) with the saccharifying activity of 389 U. These results indicated that a proper combination of saccharifying microbes could enhance the saccharifying activity of Daqu and then promote ethanol production in baijiu fermentation, as it did not always exhibit a positive correlativity between saccharifying activity and alcoholic fermentation. Our findings should provide a potential approach to regulate the alcoholic fermentation by optimizing the saccharifying microbes or enzymes of starter cultures for baijiu and other fermented food.