In this study, the effect of fermentation with Bacillus subtilis B-2, a salt-tolerant strain capable of producing high yield of protease, on the microbial community and the contents of biogenic amines and amino acid nitrogen (AAN) during the fermentation of low-salt fish sauce. The results of high-throughput 16S rRNA gene sequencing showed that fermentation with B-2 reduced the richness and evenness of the microbial community, Bacillus was dominant throughout the fermentation process and the abundance of spoilage microorganisms significantly decreased. B-2 significantly inhibited the formation of histamine, putrescine, cadaverine, and tyramine, and their contents at the end of fermentation decreased by 25.9%, 35.6%, 23.6% and 9.3%, respectively, compared with those in naturally fermented fish sauce. The content of AAN in low-salt fish sauce was significantly improved by inoculated fermentation, reaching 1.23 g/100 mL after 15 days of fermentation, which was significantly higher than that of naturally fermented fish sauce (0.79 g/100 mL). The correlation network map showed that the decrease in the abundance of Brevibacterium, Dietzia, Paracoccus, Aequorivita, and Brachybacterium was the major reason for the decrease in microbial diversity at the late stage of fermentation of naturally fermented fish sauce. The abundance of Stenotrophomonas showed a significantly positive correlation with the contents of many biogenic amines in naturally fermented and B-2 fermented fish sauce, suggesting its important role in the formation of biogenic amines in low-salt fish sauce. Comparative analysis of the microbial community and quality attributes at the end of fermentation showed that the metabolism of B-2 was the main reason for the decrease in the species and abundance of spoilage microorganisms, the increase in AAN content, and the decrease in the contents of key biogenic amines. B. subtilis B-2 is expected to be developed as a special fermentation starter for fish sauce to improve the quality and safety of rapidly fermented low-salt fish sauce.

In order to improve the drying rate and rehydration of tilapia meat, the freeze-drying characteristics and quality changes of blanched or steamed fish meat were analyzed. The effect of heat pretreatment on the quality of freeze-dried fish meat was elucidated by considering the denaturation degree of myofibrillar protein, water distribution and microstructural changes in fish meat during the drying process. The results showed that heat pretreatment could effectively improve the drying rate of tilapia meat, shortening the drying time by nine hours compared with the control group; blanching was more effective than steaming. The rehydration temperature range for tilapia meat was expanded by heat pretreatment, and good rehydration was observed at 50–80 ℃. The rehydration temperature for the control samples was 80 ℃, and the rehydration rate of the 10 min heat treatment group was more than 60%, indicating that the dried product can be rapidly rehydrated. The hardness of the rehydrated fish meat was similar to that of cooked fresh fish meat, and it had good mouthfeel. Heat pretreatment caused significant denaturation of myofibrillar protein and consequently changes in water distribution and a conspicuous increase in the peak area of transverse relaxation time T₂₃, indicating that the increase in free water content after heat treatment is an important reason for the increase in freeze-drying rate. The experimental results can provide technical support for the development of ready-to-eat dried fish products that are suitable for rehydration.

Fermented foods are a potential source to produce novel dipeptidyl peptidase-IV inhibitory peptides (D4IPs). In this study, the fermented mandarin fish (Chouguiyu) was used to screen D4IPs and their formation mechanism was studied by metagenomics and peptidomics. A total of 400 D4IPs with DPP-IV inhibition structure and high hydrophobicity were identified. The correlation network map showed that Lactococcus, Bacillus, Lysobacter, Pelagivirga, Kocuria, Escherichia, Streptococcus, and Peptostreptococcus were significantly correlated with the most D4IPs. Four stable D4IPs, including KAGARALTDAETAT, GEKVDFDDIQK, VVDADEMYLKGK, and GQKDSYVGDEAQ were respectively from the precursor proteins parvalbumin, troponin, myosin, and actin, and were mainly formed by the hydrolysis of subtilisin (EC 3.4.21.62), aspartic proteinase (EC 3.4.23.1), thermolysin (EC 3.4.24.27), oligopeptidase B (EC 3.4.21.83), and proteinase P1 (EC 3.4.21.96) from Bacillus, Kocuria, Lysobacter, Lactococcus, and Peptostreptococcus. The inhibition mainly resulted from the hydrogen bond and salt bridge between D4IPs and DPP-IV enzyme. This study provides important information on the proteases and related microbial strains to directionally prepare D4IPs in Chouguiyu.