## 3 Results and Discussion
### Change in Volatile Compounds during Yulu Fermentation
However, glycerin is odorless, having no effect on the overall flavor of Yulu. In addition, high concentrations of 1-octanol, 1-nonanol, and 1-heptanol were observed with the maximum concentrations of 0.384, 0.307, and 0.270 mg/kg, respectively, in the BS15 group. In addition to saturated alcohols, various unsaturated alcohols were also found during Yulu fermentation, and the highest alcohols were concentrated in 1-octen-3-ol, 2-octen-1-ol, and 1-penten-3-ol with the maximum concentrations of 0.780, 0.238, and 0.205 mg/kg, respectively, in the BS15 group. Interestingly, 1-octen-3-ol and 1-penten-3-ol also widely exist in other kinds of Yulu [5; 14]. In this study, 2-undecanone, 3,5-octaledi-2-one, 2-nonanone, and 1,4-cyclohexanedione were abundant during fermentation of Yulu with the maximum concentrations of 1.228, 0.667, 0.302, and 0.124 mg/kg, respectively. Both 2-undecanone and 2-nonanone are also rich in the fermented tilapia sausage [15]. Acetic acid and L-lactic acid were mainly produced during carbohydrate fermentation in lactic acid bacteria [24]. In this study, high concentrations of L-lactic acid and acetic acid were found in the late fermentation of Yulu with maximum concentrations of 1.155 and 0.128 mg/kg, respectively, suggesting the high activity of lactic acid bacteria during that period. Long (C14-C18) and medium (C6-C12) chain acids are mainly formed through the hydrolysis of triglycerides and phospholipids, while hexanoic acid, octanoic acid and nonanoic acid are formed through lipid oxidation [24]. In this study, hexanoic acid and octanoic acid were mostly observed in the BS45 group, reaching 0.276 and 0.243 mg/kg, respectively, while nonanoic acid and decanoic acid peaked in the BS15 group, reaching 0.143 and 0.147 mg/kg, respectively. These acids are also abundant in the low-salt Yulu produced with the addition of _T. muriaticus_. However, most of the volatile acids have a high flavor threshold [25], causing little effect on the overall flavor of Yulu. Volatile esters in foods are generally formed through the enzymatic condensation of acids and alcohols [26; 27]. Different from aldehydes, ketones, and alcohols, most of esters in the low-salt Yulu possessed their highest concentrations in the BS45 group. Methyl tetradecanoate had the highest concentration of 1.021 mg/kg in the BS45 group. This ester is also the most abundant in the low-salt Yulu fermentation with _T. muriaticus_[5]. Although ethyl acetate reached the maximum at the beginning of fermentation (0.941 mg/kg), it remained 0.502 mg/kg at the end of fermentation. High concentrations of ethyl acetate are also found in other fermented Yulu [14; 28], suggesting its key role in the characteristic flavor of Yulu. Methyl hexadecanoate, methyl palmitoleate, methyl dodecanoate, and methyl pentadecanoate also were in high abundance in Yulu, with the concentrations > 0.10 mg/kg at the end of fermentation. A total of four furans were abundant during the fermentation with the concentrations >0.10 mg/kg at 45 d, including 2-ethylfuran, cis-2-(2-pentenyl)furan, trans-2-(2-pentenyl)furan, and 2-pentylfuran. Similarly, 2-ethylfuran and 2-pentylfuran are also abundant in other fermented aquatic products [14; 29]. Most of the nitrogen compounds possessed low concentrations, except trimethylamine which reached 0.145 mg/kg at 45 d. For sulfur compounds, only the concentration of 4-methyl-5-thiazol was over 0.10 mg/kg at 45 d. Although plenty of hydrocarbons and aromatic compounds were present during fermentation with _B. subtilis_, they might contribute little to the overall characteristic flavor of Yulu owing to their high flavor thresholds [4; 30]. D-Limonene was one of the highest concentrated hydrocarbons with the maximum concentration of 1.519 mg/kg in the BS15 group and 1.296 mg/kg at the end of fermentation.