## 3 Results and Discussion
### Analysis of Microbial Diversity during the Fermentation of Passion Fruit Wine
#### 3.2.1 Fungal Population Structure
This is consistent with the result that _Wickerhamomyces_ is the dominant fungal genus in Beijing light-flavor Daqu [27]. The abundance of _Debaryomyces_ first increased and then decreased, and studies have shown that _Debaryomyces_ are capable of producing a variety of volatile flavor compounds associated with butter, caramel, cheese and fruit, and show high biotransformation activity [28], which can promote the production of volatile compounds during passion fruit wine fermentation and enhance the aroma characteristics of products. The abundance of _Meyerozyma_ showed a trend of increasing and decreasing, which could increase the formation of pyranosides or anthocyanins to enhance the color stability of dry red wine and produce high concentrations of aromatic substances such as ethyl acetate [29; 30; 31]. The abundance of _Wickerhamomyces_ gradually increased; it can secrete a variety of glycosyles such as \(\beta\)-D-glucosidase, \(\beta\)-D-xylosidase, \(\alpha\)-L-rhammosidase, etc., which can promote the formation of aroma and flavor substances and has the ability to produce ethyl acetate and 2-phenylethanol, so it can improve the sensory quality of wine [32; 33; 34]. In CF, the dominant fungi were mainly _Meyerozyma_, _Cidasporium_ and _Saccharomyces_, and the abundance of _Debaryomyces_, _Meyerozyma_ and _Cidasporium_ gradually decreased from 5.93%, 24.23% and 35.23% in the ICF, respectively, which may be because the commercial strains (_Saccharomyces cerevisiae_) caused the demise of _non-Saccharomyces_ yeasts such as _Debaryomyces_ and _Meyerozyma_. This is consistent with the result that the decline and fall of _Wickerhamomyces anomalous_ in the early stages of mixed fermentation may not be due to nutrient deficiencies and toxic metabolites produced by _Saccharomyces cerevisiae_, but due to the presence of a high concentration of _Saccharomyces cerevisiae_ in the system [35]. On the one hand, the microbial diversity increased in the MCF because the proteases secreted by _Saccharomyces cerevisiae_ can effectively decompose  
Figure 2: Species distribution histogram in phylum and genus of fungal structure. _Unclassified_ represents species that are not taxonomically annotated. (**a**) Phylum level; (**b**) genus level. proteins, generate growth factors such as amino acids, and promote the growth of other microorganisms. [PERSON] et al. [36] found that _Saccharomyces cerevisiae_ can secrete various nutritional factors, such as amino acids and vitamins, which promote the growth of _Lactobacillus_ in the process of beer brewing. On the other hand, the presence of _non-Saccharomyces_ yeasts inhibits the growth and reproduction of commercial strains that are added by people, so that commercial strains (_Saccharomyces_) are not the dominant fungi in the ICF and MCF. Before LCF, due to the consumption of substrates and the increase in alcohol concentration, _non-Saccharomyces_ yeasts such as _Debaryomyces_ and _Cladosporium_ have a low tolerance to alcohol, while _Saccharomyces cerevisiae_ has good adaptability to high-concentration alcohol environments, resulting in commercial strains becoming the main dominant fungi at this stage. The results can be seen in the fermentation of grapes; as the alcohol content continues to increase, the _non-Saccharomyces_ yeasts, which have a predominance in the early fermentation period, are gradually replaced by _Saccharomyces_[37; 38]. _Saccharomyces cerevisiae_ can convert sugars into alcohols in fermentation, which has an important impact on the yield, aroma and style of liquor [39]. The structures of fungal communities produced with different fermentation methods were very complex, and the flavor substances produced during the fermentation of passion fruit wine were closely related to the growth and metabolism of microorganisms. In addition, microorganisms can break down proteins into small peptides and form important aroma components, such as aldehydes, alcohols, esters, ketones, etc., which in turn form the fruity characteristics of fruit wine [40].