Effect of Mixed-Mold Fermentation on the Bacterial Community Structure and Quality of Fast Fermented Fish Sauce Made from Mandarin Fish Viscera

In order to investigate the effect of mixed-mold fermentation on the bacterial community structure and quality of fish sauce made from mandarin fish viscera, changes in the bacterial community structure of mandarin fish viscera inoculated with a mixture of Aspergillus oryzae and Monascus (fish sauce A) or Aspergillus niger (fish sauce B) at a ratio of 3:1 during the fermentation process were analyzed using high-throughput sequencing. Additionally, changes in various quality parameters including pH, total acid content, amino nitrogen content, total soluble nitrogen content, total volatile basic nitrogen (TVB-N) content, volatile component contents, and free amino acid contents were evaluated. The results showed that mixed-mold fermentation significantly influenced the bacterial community. The diversity of bacterial community decreased during the fermentation process. Staphylococcus was found to be the dominant genus. The pH of fish sauce A and B decreased significantly, and consequently the content of total acids increased significantly throughout the fermentation. At the end of fermentation, the contents of amino nitrogen were 1.23 and 1.02 g/100 mL in fish sauce A and B, respectively, and the contents of total soluble nitrogen were 1.62 and 1.53 g/100 mL, respectively, both of which meet the standard for first-grade fish sauce. The TVB-N contents of fish sauce A and B were 152.81 and 154.12 mg/100 mL, respectively, complying with the national limit requirements. Gas chromatography-mass spectrometry (GC-MS) analysis identified 48 and 54 volatile components in fish sauce A and B, respectively, with the major one being 2,5-dimethylpyrazine at concentrations of 192.79 and 169.04 µg/mL, respectively. The contents of umami taste amino acids and sweet amino acids were significantly higher in fish sauce A and B than in unfermented fish sauce (P < 0.05). The correlation analysis results indicated that Staphylococcus made the greatest contribution to the quality of fish sauce and played a significant role in the formation of key characteristic flavor compounds in the late stage of fermentation. Overall, mixed-mold fermentation could effectively improve the quality of fish sauce. Fish sauce A had higher contents of amino nitrogen and total soluble nitrogen compared with fish sauce B, and consequently better overall quality.