## 3 Results and Discussion
### Improvement in Volatile Flavor of Yulu after B. subtilis Addition  
The characteristic volatile flavor compounds in Yulu produced with _B. subtilis_ addition at the end of fermentation (BS45) were further compared with those without microbial starter addition (C45). As shown in Figure 3A, 11 volatile aldehydes were identified as the characteristic flavor aldehydes in Yulu fermented with _B. subtilis_ according to the concentrations and thresholds. Interestingly, except (E)-2-octenal, benzaldehyde, and 3-ethylbenzaldehyde, all the other characteristic aldehydes were improved by _B. subtilis_, especially hexanal, 2-methylbutylraldehyde, 3-methylbutylraldehyde, lilac aldehyde D, (E,E)-2,4-heptalien, (Z)-13-octadec, and 2-ethyl-2-butenal, contributing to the increase in pleasant grass, fat, and flower flavors in Yulu. A total of six volatile alcohols were identified as the characteristic flavor alcohols in the low-salt Yulu fermented with _B. subtilis_, and were all found to be improved after comparison with the C45 group. The long-chain saturated alcohols 1-octanol, 1-nonanol, and 1-heptanol all possess pleasant fat, flower, and fruit flavors with low thresholds (<0.15 mg/kg) [4]. In this study, the concentrations of these alcohols all exceeded their thresholds and were significantly enhanced by _B. subtilis_, contributing to the improvement of the overall flavor in Yulu. As an unsaturated alcohol, 1-octen-3-ol is one characteristic flavor alcohol found in plenty of fermented aquatic products [14; 15; 33; 34] because of its low flavor threshold (0.001 mg/kg) and high concentration. In this study, the concentration of this alcohol remarkedly exceeded its threshold and was significantly improved by _B. subtilis_, contributing to the improvement of mushroom flavor in Yulu. In addition, the significant increase in 1-penten-3-ol after the addition of _B. subtilis_ resulted in the increase in pleasant roasted onion flavor [5] in Yulu. A total of four volatile ketones were identified as the characteristic flavor ketones in the low-salt Yulu and were all improved by _B. subtilis_. Among these ketones, 2-undecanone and 2-nonanone have the sweet, fruit, and grass flavors with low thresholds (<0.01 mg/kg) [4; 15]. In this study, both ketones in the BS45 group exceeded their flavor thresholds and were obviously higher than those in the C45 group, contributing to the improvement of the overall flavor in Yulu. Figure 3: Comparison of (**A**) characteristic volatile flavor compounds, (**B**) \(\alpha\) diversity indexes, and (**C**) microbial genera in low-salt Yulu fermented with _B. subtilis_ and without microbial starter at the end of fermentation. Comparison of the difference between the C45 and BS45 groups was performed using student’s _t_-test (* \(p\) < 0.05 and ** \(p\) < 0.01). A total of six volatile acids were identified as the characteristic flavor acids in Yulu and they were all improved by _B. subtilis_, except acetic acid. Similarly, a total of six volatile esters were identified as the characteristic flavor esters in Yulu and were all improved by _B. subtilis_. The higher concentrations of methyl tetradecanoate, ethyl acetate, methyl hexadecanoate, methyl palmitoleate, methyl dodecanoate, and methyl pentadecanoate were helpful in forming the fruity and sweet flavors in Yulu. Four volatile furans were identified as the characteristic flavor furans in Yulu. Except for the significant decrease in trans-2-(2-pentenyl)furan, most of the furans showed no significant change after the addition of _B. subtilis_. Trimethylamine was the characteristic volatile flavor nitrogen compound in low-salt Yulu fermented with _B. subtilis_. Trimethylamine is produced through reduction or demethylation of trimethylamine N-oxide, especially under anaerobic conditions [35]. It is one of the main compounds in fish and fish products [5; 34]. _B. subtilis_ could significantly inhibit trimethylamine formation in low-salt Yulu, leading to the decrease in the unpleasant fishy flavor. Similarly, the addition of _T.