Effects of mixed-culture fermentation on bioactive compounds, antioxidant capacity, and flavor metabolomics of red sour soup.

Red sour soup is a fermented food made mainly from tomatoes, chili peppers, and glutinous rice. It is commonly used as a sour and salty seasoning in everyday cooking by the Miao and Dong people in southwestern China, where it helps adapt to the humid climate. However, the quality of traditional naturally fermented red sour soup can be inconsistent, which may lead to safety issues. This study aimed to understand how inoculated fermentation affects the physical and chemical properties, bioactive substances, and flavor of red sour soup. The results showed that when specific strains-Lactiplantibacillus pentosus Y5, Lactiplantibacillus plantarum Y9 and Meyerozyma guilliermondii H1-were added, the lactic acid production was 3.61 times higher than in uninoculated fermentation. The inoculated group also had lower nitrite levels. Moreover, the inoculated fermentation contained more bioactive compounds and showed stronger antioxidant activity. Through single-factor experiments and response surface optimization, the optimal process parameters for red sour soup were determined as follows: fermentation time of 5 d, fermentation temperature of 28 °C, inoculum size of 2.5%, and strain ratio of 1:1:1. Inoculated fermentation resulted in faster acid production compared to uninoculated fermentation, with the pH reaching 3.21 and total acid content reaching 19.59 g/kg on day 5. In total, 901 differential metabolites were detected, predominantly organic acids and their derivatives, fatty acyls and organic oxygen compounds. Their related major metabolic pathways were biosynthesis of amino acids, aminoacyl-tRNA biosynthesis and ABC transporters. This study offers a useful reference for improving the production and research of red sour soup.