## 3 Results and Discussion
### Relative Abundance of Fungal Communities  
To investigate the dynamic changes in the fungal community during the fermentation of Laobaigan _Baijiu_, effective sequencing reads were classified into different taxon levels by an RDP classifier. The distribution of the OTUs was categorized at the phylum and genus levels (Figure 3). In detail, there were four phyla detected in Laobaigan _Baijiu_. Ascomycota (90.57%) accounted for the highest proportion, followed by Zygomycota (7.12%), Basidiomycota (1.65%), and unidentified fungi (0.025%). Ascomycota was identified as the crucial fungal phylum throughout the whole _Baijiu_ fermentation process, which agreed with the previous results for Laobaigan _Baijiu_[4]. Ascomycota showed an increasing trend at stage D-2 but experienced a small decline at stage D-3 (Figure 3a). In contrast, Zygomycota decreased dramatically at stage D-2 and then increased slightly at stage D-3. Similar changes occurred during the _Ercha_ fermentation stages. Figure 2: Principal coordinates analysis of fungal formation during the fermentation based on the weighted UniFrac algorithm. The dots in the figure represent individual samples, with different colors indicating that the samples belong to different subgroups. At the genus level, the relative abundances of the top twenty fungal OTUs were calculated (microorganisms with a relative abundance of less than 0.07% were grouped together under the umbrella term \"others\"). As shown in Figure 3b, Saccharomyces, Saccharomyces_unidentified_1, and Candida were identified as the crucial fungal genera throughout the whole Laobaign _Baijiu_ fermentation process. Saccharomyces accounted for the highest proportion (50.80%), followed by Saccharomyces_unidentified_1 (19.3%), Candida (11.94%), Mucor (4.88%), Aspergillus (3.04%), and Rhizopus (1.97%). In stage D-1, Saccharomyces_unidentified_1, Candida, and Mucor were the initially dominant genera. Mucor, which mainly grew in the lower temperature period of Laobaign _Baijiu_ fermentation, was mostly harmful microorganisms, but small amounts of Mucor favored protein breakdown. Candida possessed a robust capacity for ester compound production, coupled with a certain alcoholic fermentation ability, thereby establishing the groundwork for imparting unique flavor profiles to Laobaign _Baijiu_. These genera exhibited optimal growth and proliferation at lower fermentation temperatures. This observation was in line with the reported higher presence of Mucor and Candida at stage D-1 [21]. The relative abundances of these three genera decreased rapidly in stage D-2, while the relative abundance of Saccharomyces increased rapidly up to a maximum (94.13%) and became the dominant genera. Alcohol compounds were mainly generated in the early stages of _Baijiu_ brewing, and Saccharomyces had a strong alcohol production capacity. This accounted for the significant increase in Saccharomyces at stage D-2. As fermentation processed, these alcohol compounds were gradually converted into esters and other compounds under suitable conditions [4]. In stage D-3, the relative abundances of Saccharomyces and Saccharomyces_unidentified_1 decreased gradually to 75.33% and 1.60%, respectively. In contrast, the relative abundance of Candida experienced a dramatic  
Figure 3: Taxonomic classification of sequences from fungal community of Laobaign _Baijiu_ samples. D-1-E-6 represent the _Baijiu_ samples collected at six specific fermentation stages. (**a**,**b**) represent the classification at the phylum and genus level, respectively. upward trend from stage D-2 to D-3, in which the value increased from 0.68% to 12.30%. At this stage, Candida used organic acids and inorganic salts in raw materials to produce esterases. These enzymes catalyzed the esterification reaction between organic acids and alcohols, resulting in the formation of the corresponding esters [22]. The values of Aspergillus, Rhizopus, Pichia, Pencillium, Wickerhamomyces, et al. showed a tendency to initially decrease and then increase during the _Dacha_ fermentation.