## 4 Discussion
At the alcohol production stage, _Lactobacillus_ mainly contributed to pyruvate metabolism associated with lactic acid, acetic acid, and acetyl-CoA production, which has been well studied previously [49]. Interestingly, the genes of _La. acettolerans_ strongly contributed to alcohol dehydrogenase (EC 1.1.1.1) production, which is responsible for ethanol production during the fermentation of food products [21]. Meanwhile, _La. acettolerans_ was positively correlated with ethanol (Figure 4a). _Lactobacillus_ was previously found to simultaneously produce lactate, ethanol, and acetate [50]. A high abundance of genes encoding alcohol dehydrogenase could explain why _La. acettolerans_ was predominant at the late stage of fermentation, suggesting that this species was generally tolerant to high ethanol concentrations. Specific cultures of _La. acettolerans_ could be adapted to ethanol concentrations of more than 12% (_v/v_) [51]. Some _Lactobacillus_ species such as _La. odoratidifui_, _La. plantarum_, and _La. breviscontributed to carboxylesterases (EC3.1.1.1), which are responsible for ester formation and are important enzymes enhancing the flavor of fermented food products [52]. Ethyl esters are the most typical flavor compounds of light-flavor _Baijiu_[53]. _La. paralimentarius_ was only abundant on day 1 and then almost disappeared during fermentation (Table S3). This may explain why it produced a negative correlation with most metabolites and did not play a significantly role in gene functional annotations. These results imply that _Lactobacillus_ groups with multiple functions are vital for light-flavor _Baijiu_ production, but further research would be needed, including, for instance, fortified fermentation with _Lactobacillus_ species but avoiding excessive acidification. In addition to _Lactobacillus_, _B. licheniformis_ is also a functional species during fermentation, and it is known for starch and degradation functions due to its prominent ability to produce \(\alpha\)-amylase and cellulase under a wide range of pHs and temperatures [54]. Although fungi only accounted for a small proportion of the total microorganisms from day 7 day to the end of fermentation (Figure 2), they were crucial for starch degradation and alcoholic fermentation, and they provided precursors for the synthesis of flavor compounds. In particular, all the microorganisms associated with the production of glucoamylase (EC 3.2.1.3) were fungi, including _Saccharomyces_ _fibuligera_ and _P. kudriavzevii_ (Figure 6). Furthermore, _P. kudriavzevii_ contributed to acetate, acetaldehyde, and ethanol formation in the pyruvate metabolism. _Saccharomyces_ _fibuligera_ is a major amyloidyeast in the food industry which can assimilate multiple carbohydrate sources such as glucose, sucrose, cellobiose, and soluble starch [55]. _P. kudriavzevii_ has shown multi-stress tolerance, and it is known to produce organic acids during wine fermentation [56]. Furthermore, _P. kudriavzevii_ has been reported to contribute to ester formation during _Baijiu_ production [57]. However, in the current study, _P. kudriavzevii_ mainly participated in the synthesis of flavor compound precursors. In addition to the main producer of ethanol, _Saccharomyces cerevisiae_ played a role in the conversion of acetyl-CoA and acetaldehyde, to which flavor compound formation was attributed. _Li. ramosa_ was predominant in starch degradation by the potential production of \(\alpha\)-amylase, glucoamylase, and cellulase, which has been demonstrated in previous studies [58; 59]. In conclusion, microbiomes in a light-flavor _Baijiu_ fermentation were systematically investigated via the combination of shotgun metagenomics and metabolomics. The microbial structure and characteristic metabolites were examined during fermentation. The microbe-metabolite relationship was established, and the further metabolic potential of core microbial species were constructed. _Lichtheimia ramose_, _Saccharomyces_ _fibuligera_, and _Bacillus licheniformis_ contributed the most to starch saccharification.