Effects of Lactiplantibacillus plantarum 16 fermentation on antioxidant capacity and bitterness of enzymatically hydrolyzed skim milk

Lactic acid bacteria fermentation can enhance the probiotic properties of foods and ameliorate their bitterness. This study investigated the possible mechanism by which Lactiplantibacillus plantarum 16 fermentation enhances the antioxidant capacity of enzymatically hydrolyzed skim milk (ESM) while ameliorating its bitterness. The results showed that fermentation with L. plantarum 16 significantly increased total peptide concentration in ESM and exerted a marked regulatory effect on specific peptide abundance (P < 0.05). Among these, 47 peptides were significantly upregulated (P < 0.05), including 17 low-molecular-weight peptides with a molecular weight of less than 1 kDa; 27 peptides were significantly downregulated (P < 0.05), including the hydrophobic bitter peptides LAMAASDISLLDAQSAP and FLIPYMIY (P < 0.05). KEGG pathway annotation and functional analysis of the parental proteins corresponding to these differential peptides identified 2 key antioxidant-related pathways, namely "Peroxisome metabolism" and "Glutathione metabolism," with antioxidant-associated differential proteins outnumbering those assigned to other functions. Furthermore, fermentation by L. plantarum 16 significantly modulated the metabolites abundance in ESM. A total of 613 significantly differential metabolites were identified, among which 295 metabolites were significantly upregulated (P < 0.05), while 318 metabolites were significantly downregulated (P < 0.05). The upregulated metabolites included flavor compounds belonging to alcohols, acids, and ketones (e.g., γ-δ-dioxovaleric acid and acetoin), as well as metabolites with potential antioxidant activity such as tyrosine lactate, phenylalanine-tyrosine and malic acid. The downregulated metabolites included bitterness-related metabolites like Cucurbitacin D, corchoroside B, and Ganoderic acid I. These differential metabolites were significantly enriched in antioxidant pathways such as "Phenylalanine, tyrosine and tryptophan biosynthesis" and "Biosynthesis of various alkaloids," as well as bitterness-alleviating pathways like "D-amino acid metabolism" (P < 0.01). Fermentation with L. plantarum 16 not only enhanced the antioxidant capacity of ESM, but also reduced its bitterness. These findings provide a theoretical basis for the development of functional fermented milk beverages.