Impact of salt concentration on bacterial diversity and changes in biogenic amines during fermentation of farmhouse soybean paste in Northeast China

Farmhouse soybean paste in Northeast China is a traditional fermented product made from soybean, and more than 11% (w/w) salt is usually added during production to control the fermentation process. In this study, the variations in bacterial diversity, biogenic amines(BAs) and physicochemical properties during the natural fermentation of soybean paste with different salt concentrations (8%, 9%, 10%, 11%, and 12%) were studied. The results show that at 0 days (0 d) of fermentation in soybean paste, the dominant genera included Staphylococcus, unidentified Clostridiales, and Sporolactobacillus. During fermentation from 30 d to 90 d, the dominant genera were Tetragenococcus and Staphylococcus. However, the proportions of the dominant genera were different depending on the salt concentration. Putrescine(Put), tryptamine(Try), β-phenethylamine(Phe), cadaverine(Cad), histamine(His), and tyramine(Tyr) showed negative correlations with salt concentration. The amino type nitrogen(ANN), titratable acidity(TTA) and total number of colonies were also negatively correlated with salt concentration. Analysis of the correlation between genera and BAs showed that 12 genera were positively correlated with BAs, and 4 genera were negatively correlated with BAs. The results of this study indicated that salt has a significant impact on bacterial diversity during the fermentation of soybean paste, which in turn affects the changes in bacterial metabolites. From the perspective of food safety, the amount of salt added in the soybean paste can be reduced to 10% under the existing fermentation conditions.