Turning over fermented grains elevating heap temperature and driving microbial community succession during the heap fermentation of sauce-flavor baijiu

Turning over fermented grains is widely used during the heap fermentation of sauce-flavor baijiu to increase heap temperature, however, the effect of turning over fermented grains on heap fermentation is still unclear. The present study investigated the changes of microbial community structure and volatile compounds before and after turning over by using high-throughput sequencing combined with gas chromatography-mass spectrometry (GC-MS). Firstly, turning over fermented grains lead to a redistribution of acidity and moisture content in different layers of fermented grains and an increase in heap temperature. Secondly, a total of 41 volatile compounds were identified, among which the concentrations of ethanol, phenylethyl alcohol, acetic acid, and ethyl-9,12-octadecadienoate in fermented grains were elevated after turning over. Next, the changes in fermentation parameters altered or enhanced their driving effects on microbial community succession, resulting in reducing the relative abundance of Pediococcus, Aspergillus, Thermoascus and Thermomyces, while boosting the relative abundance of Lactobacillus and Issatchenkia and making these two members as the predominant genera. The bio-heat generated by microbial community succession contributed to a rapid and continues increase in heap temperature. These results provided a basis for understanding the intrinsic mechanism of the traditional brewing process of sauce-flavor baijiu.