## 3 Results and Discussion
### Change in Volatile Compounds during Fermentation  
A total of 58 aroma compounds were identified in the samples, including 17 alcohols, 25 esters, 7 acids, 5 aldehydes and others. The odor descriptions of volatile compounds (Supplementary Table S3) referred to previous literature reports [2; 29; 30]. The esters represented the largest group in terms of the number in almost all wines, followed by alcohols and fatty acids. Most ester compounds, which could provide the fruity aroma characters desired in wine, are formed during fermentation, especially acetate and ethyl esters [31; 32]. Acetate is formed with the higher alcohol production by the degradation of amino acids or sugars and acetyl coenzyme A [33]. In this experiment, 9 kinds of acetates were detected in the fermentation broth of different yeasts. Furthermore, 9 kinds of fatty acid ethyl esters were detected in this work. Fatty acid ethyl esters are produced by the reaction of ethanol with the fatty acid coenzyme A produced by the yeast fatty acid metabolism during alcohol fermentation [34]. It is also related to the content of fatty acids in the raw materials. The concentrations of Ethyl caprylate and Ethyl laurate initially increased and then decreased. This was consistent with the dynamics of octanoic acid and lauric acid during fermentation. Ethyl nonanoate could only be detected in the middle and late stages of fermentation inoculated with CEC01, which can give wine a strong grape flavor and rose aroma; nevertheless, the generated content was small and therefore not obvious. At the end of fermentation (Supplementary Table S3), Ethyl caprylate (banana, pear, flower), Ethyl caprate (pear, brandy), Ethyl caproate (apple, pineapple, banana), isoamyl acetate (banana) and Ethyl 9-decenoate (fruity) will bring pleasant floral and fruity aromas to the wine. This accords with previous reports [35].  
Higher alcohols are produced by yeast-degrading amino acids through the Ehrlich pathway, and their contribution to wine aroma depends on the concentration [36]. In general, a pleasant aroma can be produced when its mass concentration is lower than 300 mg/L [37]. When the alcoholic fermentation finished, the total higher alcohol concentration of wine fermented by CECA was the highest, followed by that of F15. In particular, the content of phenethyl alcohol in Ccd was significantly higher than the other samples in the same period, which may bring the aroma of roses and honey to the wine [38].  
Volatile fatty acids in wine are mainly produced by yeast and lactic acid bacteria in the fatty acid metabolism, usually resulting in fruit, cheese, fat and sour tastes [39]. In this work, five fatty acids were detected in the wine samples after alcohol fermentation. Compared with the wine samples of the same period, the unique fatty acids in F6d were acetic acid and stearic acid, while trans-2-hexenoic acid was unique to S6d.  
The similarity of the four wines was visualized by PCA (Figure 3), using the concentrations of the volatile compounds detected in the samples of the four starters on day 6 as analytical variables. The percentage of the cumulative contribution of the variance of the first two PCs was 81.9%. PC1 and PC2 represented 51.4 and 30.5% variabilities of the volatile compounds, respectively. The score scatter plot shows that the four samples were situated in four quadrants. The wine fermented by the yeasts 796 and F15 formed a single cluster, suggesting that these two yeasts produced wine with a similar aroma profile. The wine samples of CECA and CEC01 were distributed in the second and third quadrant, respectively, and these two samples were relatively far away from each other in the plot, indicating that a difference in volatile aroma between them existed.