The organic acids and microbial community structure of fermented grains in the dominant stacking fermentation rounds for sauce-flavor Baijiu production (i.e., the 2nd and 3rd rounds corresponding to the 3rd and 4th rounds of base liquor) were analyzed using high-performance liquid chromatography (HPLC) and high-throughput sequencing, and the interaction between them was investigated. The results revealed that lactic acid, acetic acid, and succinic acid were the primary organic acids present in fermented grains at mean values of 32.70, 4.87 and 1.93 mg/g, respectively. In total, 8 dominant bacterial genera and 7 dominant fungal genera (average relative abundance > 1%) were identified across the two fermentation rounds. The dominant bacterial genera were Virgibacillus, Kroppenstedtia and Bacillus. The dominant fungal genera were Monascus, Thermomyces and Thermoascus. Principal coordinate analysis (PCoA) revealed that there was a significant difference in the structure of bacteria community between the second and third rounds of stacking fermentation, while the difference in the structure of fungal community was not significant. Co-occurrence network analysis showed that the bacterial-fungal (55.60%), bacterial-bacterial (50.57%), and fungal-fungal (91.67%) networks were mainly positively correlated, and there were more unknown interactions between bacterial communities. Mantel test analysis showed a significantly positive correlation between acidity and bacterial community structure (P < 0.01), and succinic acid was highly correlated with both bacterial and fungal community succession (P < 0.01). Spearman’s correlation analysis revealed that the bacterial community had a more significant correlation with organic acids than the fungal community, while the fungal community showed a negative correlation with most organic acids. Nine organic acids citric acid were positively correlated with Oceanobacillus, Bacillus, and Sphingobacterium but negatively correlated with Thermoactinomyces. The results of this study can provide a theoretical basis for the production of high-quality sauce-flavor Baijiu.

In order to screen for potential Thermoactinomyces from stacked fermented grains of sauce-flavor Baijiu, three strains of Thermoactinomyces valgaris were obtained by enrichment and isolation and identified by morphology and molecular biology. The metabolic characteristics of volatile compounds produced in liquid-state fermentation by each strain were investigated. Additionally, solid-state fermentation experiments were performed at 45 or 50 ℃. The flavor metabolism profiles of each strain at different fermentation stages were analyzed and summarized under liquid-state and solid-state fermentation conditions. The results showed that all three T. vulgaris strains grew rapidly in the liquid medium. They entered the logarithmic growth phase between 10 and 16 h and attained the stationary phase at 34 h, and the accumulation of acidic substances and pyrazines was enhanced. Specifically, strain ts5 displayed the highest concentrations of 2-methyl butyric acid and 2,3,5-trimethylpyrazine, amounting to 4.14 and 0.72 mg/L, respectively. The total quantity of volatile compounds produced in solid-state fermentation gradually ascended over the fermentation period. Notably, solid-state fermentation at 50 ℃ enabled a more substantial accumulation of flavor compounds such as esters, phenols, and pyrazines in comparison with that at 45 ℃. Collectively, the aroma production of T. vulgaris in solid-state fermentation on Hongyingzi sorghum was superior to that in liquid-state fermentation. Moreover, fermentation at 50 ℃ was more conducive to the accumulation of flavor compounds when compared with 45 ℃. The results of this study provide a theoretical basis for understanding the aroma-producing performance of T. vulgaris during high-temperature stacking fermentation.