Beer is one of the most popular beverages in the world. The increased popularity of craft beers has led to the development of unique beers that are alcohol-free, gluten-free, low calorie, or with functional properties through fermentation with probiotic microorganisms. In this study, functional unhopped beers were evaluated by utilizing probiotics (Lacticaseibacillus paracasei Lpc-37 and ibSium Saccharomyces cerevisiae CNCM I-3856) as starter cultures. The metabolites produced by probiotics were investigated using a nontargeted metabolomics approach and identified against metabolomics databases (Kyoto Encyclopedia of Genes and Genomes (KEGG), Human Metabolome Database (HMDB), Yeast Metabolome Database (YMDB), METLIN tandem mass spectrometry (MS/MS)). Derivatives of branched-chain (leucine) and aromatic amino acids (phenylalanine, tryptophan, and tyrosine) were enriched (one-way analysis of variance (ANOVA) p < 0.05) in probiotic-fermented unhopped beers, especially tryptophan metabolites. In addition, the synergistic effects of yeast-lactic acid bacteria (LAB) interactions led to further enrichment of higher acids such as (S)-(-)-2-hydroxyisocaproic acid, phenyllactic acid, hydroxyphenyllactic acid, and indolelactic acid. The potential pathways for the formation of novel bioactive tryptophan metabolites (indole and indoleacrylic acid) by LAB were elucidated. Altogether, probiotic LAB-fermented unhopped beer showed the highest antioxidant capacity and total phenolic content. This work provides the basis for the discovery of bioactive metabolites in probiotic-fermented foods.
Keywords: Lacticaseibacillus paracasei; Saccharomyces cerevisiae; bioactive metabolites; metabolomics; probiotic-fermented unhopped beer.