Metagenomic and metabolomic profiling analyses to unravel the formation mechanism of n-propanol during the first and second round of Jiangxiangxing Baijiu fermentation

N-propanol is one of the higher alcohols, a moderate amount of n-propanol is beneficial for the harmony of the liquor body, whereas excessive or repeated intake will lead to discomfort and pose significant harm to human health. In actual production process of Jiangxiangxing Baijiu, the n-propanol content of the base baijiu in first round (FR) is far higher than that of second round (SR). Nevertheless, the formation mechanism and the key n-propanol producing microbials remain unclear and this limits the quality control of baijiu fermentation. Here, we combined metagenomics and metabolomics to verify the biosynthesis pathway of n-propanol and to identify characteristic microorganisms in FR and SR. The results showed that the preliminary period of pit fermentation was critical for the accumulation of n-propanol. FR was enriched in Lactobacillus plantarum, Lactobacillus ponits, Lactobacillus brevis and Lactobacillus panis, while it was harbored greater abundances of Pichia kudriazevii, Saccharomyces cerevisiae and Lactobacillus acetotolerans in SR. Function analysis combined with KEGG providing comprehensive evidence for the main synthetic pathways of n-propanol in Jiangxiangxing baijiu, and L. panis was key microbial. In addition, the experiments of inoculating L. panis and L. acetotolerans in situ indicated L. panis was mainly responsible for n-propanol production while L. acetotolerans not conducive to the production of n-propanol. Besides, the bioturbation effect on microbiota and flavor compounds were also analyzed. These results are useful for elucidating the mechanism of flavor formation in baijiu fermentation and promoting the further application of bioturbation technology in the traditional fermentation industry.