## 3 Results and Discussion
### Volatile Compound Analysis of Black Rice Wine
#### 3.2.2 Characteristic Flavor Substances of Black Rice Wine  
To further differentiate aromatic substances between different black rice wines, samples from the fermentation endpoint (336 h) were selected for orthogonal partial least squares discrimination analysis (OPLS-DA) and discriminant VIP prediction analysis (VIP\({}_{\text{pred}}\) > 1 and \(p\) < 0.05). As depicted in Figure S1, the differential volatile components between Q1 BRW and Q2 BRW comprised isopentyl alcohol (al2), 2.3-Butanediol (al3), phenethyl alcohol (al11), ethyl acetate (et1), isoamyl acetate (et5), hexadecanoic acid ethyl ester (et22); between Q2 and Q3 they included ethyl acetate (et1), isopentyl alcohol (al2), phenethyl alcohol (al11), hexadecanoic acid ethyl ester (et22), 2.3-Butanediol (al3), isoamyl acetate (et5), butanedioic acid diethyl ester (et9); and between Q1 and Q3 they consisted of ethyl acetate (et1), isopentyl alcohol (al2), isoamyl acetate (et5), and phenethyl alcohol (al11). Results revealed that alcohols and esters, as primary differential volatile components, may contribute to the diverse aromas among different BRWs. Isoamyl alcohol, phenethyl  
Figure 1: Change in volatile compounds during black rice wine fermentation. (**a**) Changes in the number of flavor substances, total flavor, alcohols, esters, aldehydes, ketones, terpene alkanes, and phenols during the black rice wine fermentation; (**b**) Heatmap visualization of the 61 volatile compounds during the black rice wine fermentation; (**c**) PCA analysis of volatile compounds among three Xiaoqu black rice wines. alcohol, ethyl acetate, and isoamyl acetate were identified as common differential volatiles with higher content in BRW, and could potentially be key aromatic substances contributing to distinctive flavors. This conclusion has been echoed in other rice wines [17; 18]. However, an abundance of volatile substances does not always mean that they will contribute significantly to the characteristic aroma [19]. Hence, in order to excavate the characteristic aromatic substances in BRW, an OAV was calculated for each aroma substance at the fermentation endpoint through concentration and threshold [19; 20]. It is commonly accepted that compounds with OAV \(\geq\) 1 have a major influence on the overall aroma profile, and are referred to as key characteristic flavor substances. Compounds with 1 \(>\) OAV \(>\) 0.1 have a minor effect on the overall aroma, and are known as auxiliary flavor substances. Conversely, compounds with OAV \(<\) 0.1 are generally disregarded in terms of their impact on the overall aroma. According to Table 1, there were 32 potential aroma components identified that could contribute to the characteristic flavor of three different types of BRW. The key characteristic flavor compounds (OAV \(\geq\) 1: isoamyl acetate (et5), ethyl octanoate (et10), 2-octenal (ad5), ethyl decanoate (et17), 2-decenal (ad9), ethyl heptanoate (et8), ethyl butyrate (et3), nonanal (ad6), and guaiacol (bp2)) and auxiliary presentation flavor compounds (1 \(>\) OAV \(>\) 0.1: ethyl palmit (et22), phenyl ethyl acetate (et13), phenyl ethanol (al11), limonene (hf1), ethyl acetate (et1), 4-ethyl guaiacol (bp3), isoamyl alcohol (al2), butylolactone (et7), and ethyl cinnamate (et18)) have been identified. It is implied that these 18 substances may be the essential components of BRW's characteristic aroma, thereby leading to a discrepancy in overall flavor with other rice wines. Furthermore, the key aroma compounds of these wines were mainly ethyl esters with fruit and floral aromas, which were the metabolic products of microorganisms during fermentation, rather than the raw materials themselves. Hence, to identify the core microbial community that influences the 18 characteristic flavor substances of BRW, it was essential to perform a correlation analysis between the characteristic flavor substances and the microorganisms.