Metagenomic and Physicochemical Analyses Reveal Microbial Community and Functional Differences Between Three Different Grades of Hongxin Low-Temperature Daqu

Hongxin (HX) is an indispensable Daqu in the production of light-flavor Baijiu (LFB). However, the classification method of HX is highly subjective, and the classification and functional differences in microorganisms in different grades of HX are still unclear. In this study, metagenomics and physiochemical analysis were used to compare three grades of HX (top, first, second) and clarify their brewing functions in LFB. The results showed that a total of 1556 genera and 5367 species were detected in all samples. Bacteria and fungi are the main microorganisms in HX, and the relative abundance of bacteria and fungi is above 4.5:1. <i>Kroppenstedtia</i> (11.43%), <i>Leuconostoc</i> (10.52%), <i>Fructilactobacillus</i> (9.00%) were the top three genera in HX. Although the microbial community composition of the three grades of HX is highly similar, each HX has a specific microbial community structure and macrogene functional characteristics, indicating that they have different brewing functions. The dominant microorganisms in top-grade HX and first-grade HX were mainly positively correlated with energy metabolism and lipid metabolism, while the dominant microorganisms in second-grade HX were mainly positively correlated with carbohydrate metabolism and amino acid metabolism. This study revealed the different fermentation effects of different grades of HX in LFB and provided suggestions for the scientific classification and quality control of HX.