Effects of Phenylalanine, Valine, Isoleucine, and Leucine on the Composition and Flavor of Lactic Acid Bacteria-Fermented Celery

Fermented celery puree is valued for its low caloric content and health benefits. However, its characteristic grassy flavor often limits consumer acceptance. This study examined how supplementation with four amino acids (i.e. valine, leucine, isoleucine, and particularly phenylalanine) modulated the fermentation of celery puree by three lactic acid bacteria (LAB) strains, including Lactiplantibacillus plantarum NCU116, L. casei NCU215, and L. acidophilus NCU402. Among all fermentations, phenylalanine supplementation showed the most pronounced enhancement in both metabolic activity and aroma formation. Amino acid addition accelerated acidification and microbial growth in NCU116 and NCU215 fermentations, whereas its influence on NCU402 was minimal. Phenylalanine supplementation markedly enhanced floral and fruity volatiles, elevating phenylacetaldehyde (∼23-fold), phenylethyl alcohol (∼3-fold), linalool (∼2-fold), and 1,4-dimethyl-4-acetyl-1-cyclohexene (∼1.25-fold). In contrast, valine, isoleucine, and leucine showed only minor effects, mainly on energy metabolism and overall volatile balance. Metabolomic pathway enrichment revealed activation of the phenylpropanoid and shikimate pathways by phenylalanine, indicating enhanced aromatic metabolism, whereas the other amino acids mainly modulated glycolysis/TCA-linked energy metabolism. These findings indicate that phenylalanine acts as an important exogenous precursor and key contributor to aromatic metabolism and volatile formation in NCU215-fermented celery puree, providing mechanistic insights and a practical strategy to enhance sensory acceptance and functional quality of plant-based fermented foods.