## 3 Results and Discussion  
### Physicochemical Properties during Fermentation of BBP-Meju with Different Salt Concentrations  
Physicochemical parameters are vital indicators affecting the quality of fermented soy foods. In this study, moisture content, pH, salinity, total acid, AAN, and reducing sugar during the fermentation of BBP-meju with different salt concentrations were determined (Figure 1). The salinity of the 4%, 8%, and 12% salt addition samples was stable and reached 2.48 \(\pm\) 0.12 g/100 g, 5.91 \(\pm\) 0.10 g/100 g, and 8.10 \(\pm\) 0.22 g/100 g, respectively, on day 49 (Figure 1c). The moisture content of the BBP-meju with different salt concentrations decreased during fermentation, with higher moisture content observed with lower salt concentrations (Figure 1a), consistent with [PERSON] et al. [36]. The research indicated that the moisture content is a crucial indicator during the fermentation of BBP, which provides a transport medium for microbial growth and participates in microbial metabolism and reproduction. Therefore, the moisture content affects microbial activity and the occurrence of Maillard reactions in the fermentation system [37]. The higher moisture content in low-salt samples reflected more vigorous microbial growth and metabolism internally. Additionally, the pH of BBP-meju decreased first, then increased, and decreased again, stabilizing at 4.89 \(\pm\) 0.03 (L), 5.14 \(\pm\) 0.04 (M), and 5.19 \(\pm\) 0.06 (H), respectively (Figure 1b), showing an opposite trend to the total acid. Overall, pH increased as salinity decreased, while total acid levels decreased. The total acidity levels of low- and medium-salt samples reached 2.67 \(\pm\) 0.01 g/100 g and 2.06 \(\pm\) 0.09 g/100 g, respectively (Figure 1d). They both exceeded the requirement of total acid (in terms of lactic acid) \(\leq\) 2.0 g/100 g as specified in the Chinese national standard [8]. It is known that salt-reduced fermentation promotes the growth of microorganisms such as LAB, _Staphylococci_, and _Bacillus_. Further breakdown of carbohydrates, proteins, and fats in the raw materials leads to the accumulation of OAs. This ultimately results in an increase in total acid levels and a decrease in pH [30]. Meanwhile, pH is considered a critical factor in controlling the growth of pathogens in fermented foods and a vital hygiene indicator for BBP [38]. Therefore, lower salt concentrations can be used to some extent to achieve inhibition of pathogenic microorganisms through low pH. AAN is another important parameter for measuring the quality of BBP, reflecting the degree of hydrolysis of broad bean protein during fermentation [39]. The results showed that in the early stage of fermentation, the AAN levels in BBP-meju samples showed an increasing trend and reached the highest value on day 7, which was 1.25 \(\pm\) 0.02 g/100 g (L), 1.12 \(\pm\) 0.02 g/100 g (M), and 1.03 \(\pm\) 0.07 g/100 g (H), respectively (Figure 1e). In the middle and late stages of fermentation, the AAN levels decreased gradually and reached 0.77 \(\pm\) 0.01 g/100 g (L), 0.69 \(\pm\) 0.01 g/100 g (M), and 0.60 \(\pm\) 0.01 g/100 g (H) on day 49 (Figure 1e). In the early stage of fermentation, broad bean protein underwent extensive hydrolysis by microorganisms, leading to a rapid increase in AAN. However, in the middle and late stages of fermentation, the growth of microorganisms and the Maillard reactions consumed a large amount of AAN. This resulted in a decrease in the AAN content in BBP-me, which in turn led to a change in the flavor of BBP [40]. Similarly, microorganisms such as _Aspergillus oryzae_ secreted large amounts of amylase to degrade starch in broad beans, causing a rapid increase in the level of reducing sugars in the early stages of fermentation. In the middle and late stages of fermentation, reducing sugars were consumed as a substrate for microbial growth and the Maillard reaction, leading to a decrease in reducing sugars [41].