## 3 Results and Discussion  
### Change in Volatile Compounds during Yulu Fermentation  
The change in volatile compounds in Yulu during fermentation with _B. subtilis_ is shown in Figure 1 and Table S1. A total of 133 volatile compounds were identified from Yulu, including 25 aldehydes, 19 esters, 17 alcohols, 17 hydrocarbons, 13 ketones, 12 aromatic compounds, 11 acids, 10 nitrogen compounds, 6 furans, and 3 sulfur compounds according to their functional groups. As shown in the PCA results (Figure 1A), three distinct groups were found during fermentation with _B. subtilis_, including early fermentation (BS5 and BS10), middle fermentation (BS15), and late fermentation (BS30 and BS45); the volatile compounds in the BS15 group were more different than the other fermentation groups. As shown in Figure 1B, the total concentration of volatile compounds in each classification increased as the fermentation went on. The total concentration of hydrocarbons, aldehydes, alcohols, ketones, and furans reached their maximum in the BS15 group, while the total concentration of esters, aromatic compounds, acids, nitrogen compounds, and sulfur compounds peaked in the BS45 group. Among the volatile compounds, hydrocarbons, aldehydes, esters, and alcohols had the highest total concentrations. The heatmap of volatile compounds during Yulu fermentation with _B. subtilis_ is presented in Figure 1C. Aldehydes, ketones, and alcohols are formed similarly through lipid oxidation or amino acid catabolism, and most of them contribute significantly to the whole flavor in various fermented foods owing to their low thresholds [4; 18; 19]. In this study, most aldehydes, ketones, and alcohols shared similar trends during fermentation; their highest concentrations were in middle fermentation (BS15 group), followed by the BS45 group. Various types of aldehydes were found in Yulu during fermentation with _B. subtilis_, including straight-chain aldehydes, branched aldehydes, olefine aldehydes and phenyl aldehydes. Hexanal was one of the most abundant straight-chain aldehydes, which reached 0.409 mg/kg after 15 d fermentation and remained 0.100 mg/kg at the end of fermentation. Hexanal is also reported to be abundant in Yulu [7; 20]. Nonanal was another important straight-chain aldehyde which reached 1.322 mg/kg after 15 d fermentation, but disappeared after 30 d. In addition to straight-chain aldehydes, many branched aldehydes, such as 3-methylbutylraldehyde and 2-methylbutylraldehyde, peaked after 45 d fermentation, reaching 0.835 and 0.385 mg/kg, respectively. Plenty of olefine aldehydes were also found in Yulu, especially for illac aldehyde D with the maximum concentration of 1.379 mg/kg after 15 d fermentation and 0.540 mg/kg after 45 d fermentation. The maximum concentrations of (E)-2-octenal, (E,E)-2,4-heptadienal, 2-hexenal, (Z)-13-octadec, and 2-ethyl-2-butenal reached 0.335, 0.239, 0.154, 0.150, and 0.116 mg/kg, respectively, in the BS15 group. Plenty of (E)-2-nonenal (0.408 mg/kg) and (E)-2-decenal (0.271 mg/kg)  
Figure 1: Change in volatile compounds of Yulu during fermentation with _B. subtilis_. (**A**) Similarity analysis of volatile compounds among different fermentation groups via PCA. (**B**) Change in total concentration of volatile compounds in each classification in different fermentation groups. (**C**) Heatmap of volatile compounds in each classification in different fermentation groups. were also found at 15 d, but disappeared after 30 d. Meanwhile, phenyl aldehydes were found in abundance in Yulu, especially for benzaldehyde and 3-ethylbenzaldehyde, whose maximum concentrations were 1.073 and 0.339 mg/kg in the BS15 group, followed by 0.705 and 0.174 mg/kg in the BS45 group, respectively. Benzaldehyde has also been reported to be abundant in other fermented aquatic products [21; 22; 23]. Plenty of saturated alcohols were observed in this study, especially glycerin, which increased with the increasing fermentation time and peaked in the BS45 group (0.668 mg/kg).