Abstract
Daqu is an important saccharifying and fermenting agent. It provides various microorganisms and enzymes for the fermentation of Baijiu and plays a vital role in the formation of Baijiu flavor. However, it is difficult to obtain information on microbial growth and metabolism in time for Daqu production. Therefore, the “Qu Xiang” obtained by smelling is an important index in the traditional production process to evaluate the microbial fermentation in the process of Daqu-making, “Qu Xiang” mainly represents the volatile flavor compounds in Daqu. The microbial diversity and volatile metabolites on 0, 6, 16, and 29 days of the fermentation process were measured using high-throughput sequencing and gas chromatography–mass spectrometry. Significant differences were found in the composition of the microbial community. Pseudomonas, Weissella, Bacillus, and Pelomonas were the main bacterial genera. Alternaria, Rhizopus, and Pichia are the main fungal genera. A total of 32 differential volatile metabolites were detected in samples at four time points using differential metabolic analysis. The correspondence of prevailing microorganisms with differential metabolites distinguished by Spearman correlation and two-way orthogonal partial least square analysis show that Saccharopolyspora exhibited a significant connection for the 12 differential metabolites. A significant positive correlation was observed between Rhizomucor and 13 different metabolites. These findings further understanding of the metabolism of microorganisms in Daqu fermentation and also help to control the microorganisms in the Daqu-making process, to obtain more stable Baijiu products.
Highlights
Fermentation is a traditional strategy that has been used to process and preserve foods all over the world
The Specaccum curves based on Operational taxonomic units (OTUs) number (Supplementary Figure 2), approaching the saturation plateau, suggested that the sequencing depth in this research was adequate to represent the microbial structure of samples
We explored the correlation between the dominant microorganisms and different metabolites in different stages of the fermentation process, and verified the relationship by using functional prediction and pathway enrichment
Summary
Fermentation is a traditional strategy that has been used to process and preserve foods all over the world. Daqu is produced from raw materials in an open environment without sterilizing During this period process, liquefaction enzymes, saccharifying enzymes, proteases, and esterifying enzymes were accumulated through microbial metabolism (Wu et al, 2020). In Baijiu liquor, the starch and protein in the Daqu can be degraded into small molecules by the aforementioned enzymes, serving as nutrients and substrates for microbial growth and flavor metabolism (He et al, 2020). These enzymes are mainly produced by some specific microorganisms, such as Aspergillus, Rhizopus, Bacillus, LactoBacillus, Wickerhamomyces, and Saccharomycoposis (Liu and Sun, 2018; Li et al, 2020). If we master the rule of microbial succession in the production process of Nongxiang Daqu, we can get high-quality Daqu
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