Microbiome analysis and random forest algorithm-aided identification of the diacetyl-producing microorganisms in the stacking fermentation stage of Maotai-flavor liquor production

The spatio-temporal diversity and succession of microbial community and its environment driving factors during stacking fermentation of Maotai-flavor baijiu

Stacking fermentation is an important stage of microbial expansion and enrichment in the brewing process of Maotai-flavor baijiu and has an important impact on quality. However, the structure and succession of microbial communities at different spatial points of fermented grains, as well as the key environmental factors driving community assembly, remain unclear. Here, we analyzed spatio-temporal similarities and differences in the microbial community structure and succession during 1-6 rounds of stacking fermentation of Maotai-flavor baijiu. The microbial diversity and richness in the pile center were higher than those at the pile surface. The dominant bacterial genus changed from Lactobacillus to Acetobacter, while the dominant fungal genus Pichia was gradually replaced by Candida, however, some microorganisms (Acetobacter, Thermoascus) could not occupy community dominance in both the pile surface and the pile center of fermented grains. Most of the biomarkers (Kroppenstedtia, Thermomyces, etc.) of the pile surface showed thermostable or thermophilic characteristics, while most biomarkers (Aspergillus, Hyphopicia, etc.) of the pile center were functional microorganisms. Furthermore, pH and moisture were the main environmental driving factors of community construction at the pile surface and the pile center, respectively, with starch and reducing sugars having a greater impact on the microbial community assembly of the pile center than that of the pile surface. The main differences in the metabolic pathways of the dominant bacterial genera of the pile surface and the pile center were concentrated around cell growth and death, amino acids, leading to enrichment and growth of microbial communities at the pile surface and nitrogen utilization at the pile center, respectively. This study reveals the spatio-temporal differences in microbial community structure, succession and corresponding environmental driving factors during stacking fermentation, which will provide guidance for regulating the microbial community diversity to produce high-quality Maotai-flavor baijiu.

Spatiotemporal accumulation differences of volatile compounds and bacteria metabolizing pickle like odor compounds during stacking fermentation of Maotai-flavor baijiu

Pickle like odor (PLO) is undesirable in Maotai-flavor baijiu; however, its formation mechanism is unclear. Furthermore, there is a lack of understanding of the spatiotemporal accumulation of volatile compounds (including PLO compounds, PLOC) and of the microorganisms responsible for the production of PLOC during stacking fermentation. In this study, we analyzed the spatiotemporal distribution differences of 132 volatile compounds in piled fermented grains. PLOC (n = 5) were higher in pile surface than in pile center, reaching their highest levels at 6th and 5th rounds, respectively. The microorganisms in pile center were more conducive to the formation of alcohols, while those in the pile surface more promoted the synthesis of esters. Rhodococcus and Zygosaccharomyces promoted the formation of PLOC. Acetobacter was negatively correlated with the content of sulfur compounds by promoting their conversion into non-volatile sulfur compounds, thereby reducing the content of PLOC. This study provides information on the spatiotemporal differences of volatile compounds (especially PLOC) in piled fermented grains and identified the microorganisms that produce PLOC.