## 3 Results and Discussion
### Profiles of VOCs during Different Paocai Fermentations  
The compositions of VOCs in the paocai samples are shown in Table S2. A total of 62 and 59 volatile substances were identified in the red radish and cabbage paocais. As shown in Figure 4A, the main VOCs detected were alcohols, hydrocarbons, esters, sulfide and ketones during the whole fermentation process. Alcohols were the most abundant compounds among those detected and ranged from 74.1% to 79.3%, with a total of 17 different components. The second largest group was heterocycles which ranged from 9.8% to 13.1% and included 23 component types. Figure 4B shows 22 major VOCs that accounted for more than one of the two paocai volatile profiles. In general, the types of VOCs identified were similar, both types included compounds like linalool, \(\alpha\)-terpineol, eucalypt, d-carvone and other substances. Among these, linalool registered the highest proportions reaching up to 35% and 41.6% in the red radish and cabbage paocais, respectively, similar to previous studies [8]. Linalool contributes to the unique flavor of paocai and is described as having a strong scent, similar to bergamot oil or French lavender, with a mixture of woody, floral aromas with a touch of spiciness [35]. Other alcohols were also evaluated to have a positive effect on paocai's flavor with \(\alpha\)-terpineol contributing a floral typically lilac odor; eucalypt with a eucalyptus, herbal and camphor odor; and nerol with notes of roses [36; 37]. Red radish and cabbage belong to the Cruciferae family of plants, and typically have a strong aroma, bitter taste and pungent taste similar to that of mustard. The compounds responsible for these special flavors were found to be mainly the secondary defense metabolite, glucosinolate and its hydrolysate isothiamate [38]. The composition and content of isothiances is variable based on the type of vegetables subsequently imparting characteristic flavor components and compositions in the fermented paocais. Both tested paocais were found to contain butyl isothianate and amyl isothianate, with significantly higher values in the red radish paocai. Among the sulfides, dimethyl disulfide and dimethyl trisulfide were uniquely found in the red radish paocai, whereas 3-butenylisothiocyanate was found to be the unique sulfide in the cabbage paocai. Figure 4: (**A**) Changes of VOCs in red radish paocai and cabbage paocai during fermentation. (**B**) Major VOCs with relative abundance of more than 1% of the red radish paocai and cabbage paocai. Among the various potential detected substances, those with a low content (lower than the threshold values) were excluded in the statistical analysis to narrow down the range of key volatiles. In detail, 36 potential volatile substances were screened out, which included 14 alcohols, 3 ketones, 5 esters, 10 hydrocarbons, 3 sulfides, and 1 phenol compounds. To further screen the different VOCs in two kinds of paocai, a PLS-DA model was established, as shown in Figure 5A,B. Variable projection importance (VIP) was used to evaluate the contribution of variables. Substances with VIP value \(\geq\)1.0 were selected and combined with the conditions with a significant difference between the two groups (\(p<0.05\)). The point distance between the VOCs of paocai fermented with two different raw materials was obvious, which indicated that the VOCs of paocai fermented with different raw materials were different. The total contribution rate of the first two principal components on variable X was 52.7% (R\({}_{2}\)X = 0.527), and the contribution rate on variable Y was 95.7% (R\({}_{2}\)Y = 0.957). At the same time, the prediction ability based on cross-validation was 94.0% (Q\({}_{2}\) = 0.94), indicating that the model had a good explanatory degree. The stability and reliability of this model were verified by the cross-validation model with 200 permutation validations.