## 3 Results and Discussion
### Volatile Flavor Profile during BBP-Meju Fermentation with Different Salt Concentrations
Among them, ethyl acetate was not detected on day 7. By day 35, higher levels of ethyl acetate were detected in both medium- and low-salt meju samples at 4.04 \(\pm\) 0.86 ug/kg (M) and 115.82 \(\pm\) 4.42 ug/kg (L), respectively, while still not detected in high-salt meju (Table 2). These results indicated that the gradual formation and accumulation of ethyl acetate accompanied the fermentation of BBP-meju. However, the high-salt environment inhibited the activity of the relevant microorganisms, thereby suppressing the accumulation of ethyl acetate in high-salt meju. In addition, ethyl phenylacetate, ethyl isovalerate, isobutyl acetate, isoamyl acetate, and ethyl palmitate all had the highest content in medium-salt meju (Table 2), characterizing the promotion of specific salinity on the activity of associated microorganisms. Acids and phenols have been reported to be the dominant flavor compounds in low-salt fermented BBP [4]. During fermentation, microorganisms degrade peptides, amino acids, and sugars in raw materials, converting them to pyruvate. Pyruvate is a critical intermediate in organic acid metabolism, further transforming into a large number of volatile organic acids, thereby significantly impacting the flavor of BBP [39]. By day 35, acetic acid and 3-methylbutanoic acid levels were higher in both low- and medium-salt meju than in high-salt meju (Table 2). Acetic acid and 3-methylbutanoic acid both have a sour taste and an unpleasant pungent odor. Moreover, 3-methylbutanoic acid is a typical sour and sweaty compound in BBP [48], affecting the taste and flavor of the final product. Similarly, the levels of phenol and 2,4-di-t-butylphenol in low- and medium-salt meju were significantly higher than in high-salt meju (Table 2), enhancing the pungent odors of BBP-meju. However, 4-hydroxy-3-methoxyrene and 1-naphthalenol have sweet, floral, and smoky flavors. They were found in the highest contents in low- and medium-salt meju, respectively (Table 2), contributing to the high-quality flavor of low-salt fermented BBP-meju. It is known that aldehydes and ketones have lower odor thresholds, significantly influencing the flavor characteristics of fermented foods [52]. Benzaldehyde, the most predominant aldehyde in BBP-meju, accumulated rapidly during fermentation and reached a concentration of 375.42 \(\pm\) 26.64 ug/kg (L), 901.52 \(\pm\) 142.5 ug/kg (M), and 590.74 \(\pm\) 18.71 ug/kg(H), respectively, on day 35 (Table 2). It was evident that the benzaldehyde in high-salt meju was significantly lower than that in medium-salt meju (\(p<0.05\)) (Table 2), consistent with the studies of [PERSON] et al. (2018). Additionally, phenylacetaldehyde, 2-phenyl-2-butenal, and hexanal had the highest abundance in medium-salt meju, exceeding 100 \(\upmu\)g/kg (Table 2). They could impart sweet, honey, and fruity flavors to BBP-meju, positively contributing to the flavor of the final product. 3-Octanone was the major ketone in BBP-meju, with its content increasing as salt concentration decreased (Table 2), imparting sweet and fruity flavors to low-salt fermented BBP-meju. To identify the significantly different volatile flavor compounds during the fermentation of BBP-meju samples with different salt concentrations, partial least squares discriminant analysis (PLS-DA) was conducted, as shown in Figure S1. The values of R2X, Q2Y, and R2Y all exceeded 0.90, indicating the reliability of the PLS-DA model. According to the results, the different salt concentrations of BBP-meju samples exhibited good repeatability, with significant differences among samples on day 35 but no significant differences among samples on day 7 (Figure S1a). This result was in agreement with the results of the E-nose analysis. Furthermore, based on the threshold (i.e., \(p<0.05\) and variable important in projection (VIP) \(>1\)), 22 volatile flavor compounds were selected, including 9 alcohols, 5 aldehydes, 3 phenols, 2 esters, 1 acid, 1 ketone, and 1 ether (Figure S1b).