Role of bacterial community succession in flavor formation during Sichuan sun vinegar grain (Cupei) fermentation

Sichuan sun vinegar (SSV) is a traditional Chinese vinegar with a unique flavor and it is fermented with bran as the main raw material. In the present study, we explored the bacterial community succession in fermented grains (Cupei) during SSV production. High-throughput sequencing results showed that bacterial community richness and diversity peaked on day 7 of fermentation. Lactobacillus and Acetobacter were the dominant bacteria throughout the fermentation process. However, Acetobacter, Cupriavidus, Sphingomonas, Pelomonas, and Lactobacillus were the most abundant genera in the late phase of fermentation on day 17. The boundaries of trilateral co-fermentation were determined through cluster analysis. Days 1-3 were considered the early fermentation stage (starch saccharification), days 5-11 were the middle fermentation stage (alcoholic fermentation), and days 13-17 represented the late fermentation stage (acetic acid fermentation). Changes in flavor compounds during Cupei fermentation were subsequently analyzed and a total of 86 volatile compounds, 9 organic acids, and 17 amino acids were detected. Although acetic acid, lactic acid, alcohols, and esters were the main metabolites, butyrate was also detected. Correlation analysis indicated that 20, 21, and 28 microorganisms were positively correlated with the abundance of amino acids, organic acids, and volatile flavor compounds, respectively. We further explored the microbial and metabolic mechanisms associated with the dominant volatile flavor compounds during SSV fermentation. Collectively, the findings of the current study provide detailed insights regarding the fermentation mechanisms of SSV, which may prove relevant for producing high-quality fermented products.