Unraveling the metabolite profiles and microbial roles during dry-pickled radish fermentation

Bacterial community shapes the quality of fermented vegetables through metabolism; however, no information is available on the dynamics of the microbiota and metabolites in dry-pickled radish. Herein, the study aim to reveal the changes in bacterial communities, volatile metabolites, and nonvolatile metabolites, and to speculate how the core bacterial flora affects metabolites by high-throughput sequencing, metabolomics, and flavoromics. During the initial fermentation (d1–d14), the bacterial flora and metabolites of radish considerably changed. The relative abundance of Weissella, Lactiplantibacillus, and Staphylococcus increased, whereas that of Pseudomonas and Lactococcus considerably decreased (p < 0.05). Kyoto encyclopedia of genes and genomes (KEGG) function prediction indicated that functions related to bacterial division and proliferation were high at the start of fermentation, after which functions related to the metabolism and biosynthesis of amino acids, carbohydrates, and nucleic acids increased. Among metabolites, the contents of ethanol, dimethyl disulfide, dimethyl trisulfide, carboxylic acids and derivatives (CAAD, mainly amino acids), and fatty acyls (FAC) increased, whereas those of isothiocyanates and 1-methyl-2-pyrrolidinethione decreased significantly (p < 0.05). During the middle and final stage of fermentation (d14–d42), the bacterial flora and metabolites gradually stabilized. Weissella, Lactiplantibacillus, and Staphylococcus populations correlated positively with the contents of alcohols, sulfides, CAAD, and FAC, and negatively with the content of isothiocyanates. Therefore, Weissella, Lactiplantibacillus, and Staphylococcus may regulate the synthesis and metabolism of amino acids and organic acids by producing enzymes, such as phenylalanine hydroxylase (EC 1.14.16.1), isocitrate dehydrogenase (EC 1.1.1.41), and glutamate mutase (EC 5.4.99.1). This study provides a theoretical basis for improving and standardizing the quality of dry-pickled radish.