Mechanism of the release and transformation of polyphenols during germination and fermentation in millets: profile and metabolomics based analysis

Abstract

Germination and fermentation are effective ways to improve polyphenol contents in whole grains; however, their mechanisms await clear explanation. This study aimed to elucidate the mechanisms underlying the release and transformation of polyphenols during germination and fermentation with L. plantarum. Results showed that the phenolic content significantly increased after germination and fermentation. Dynamic changes of carbohydrate-hydrolyzing enzymes and structural characteristics detected by FTIR, X-ray diffraction, and CLSM indicated that the destruction of hemicellulose and the secretion of xylanase were crucial for releasing polyphenols. Targeted metabolomics revealed that a total of 73 polyphenols and their catabolites were identified. Differential metabolites between millets (M) and germinated millets (GM), as well as between GM and germinated-fermented millets (FGM), were revealed. Quantitative analysis of differential metabolites combined with KEGG pathways showed that the contents of cinnamic acid, caffeic acid, ferulic acid, and sinapic acid significantly increased after germination, likely associated with phenylpropanoid biosynthesis. However, these polyphenols markedly decreased after fermentation. The contents of 3,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, and 4-hydroxyphenylethanol significantly increased after fermentation, suggesting associations with phenylalanine, tyrosine, and tryptophan biosynthesis and tyrosine metabolism. Our results demonstrated the enhancement of polyphenol contents via the combination of germination and fermentation and the related mechanisms during these processes, which provides valuable insights for the utilization of cereals in the food industry.