Physicochemical Evolution, Biotransformation Behavior and Metabolomic Profiling of Hawthorn Pulp Co-Fermented by Levilactobacillus brevis and Streptococcus thermophilus

Hawthorn (Crataegus pinnatifida) pulp is characterized by high levels of organic acids and polyphenols, yet its strong acidity limits sensory acceptance. In this study, Streptococcus thermophilus 540, Lactobacillus brevis 647, and their mixed culture (1:1) were applied to ferment hawthorn pulp. Changes in organic acids, individual phenolics, free amino acids, volatile compounds, antioxidant capacity, and global metabolic profiles were systematically investigated using targeted analysis combined with untargeted metabolomics. Fermentation markedly reshaped the metabolic composition of hawthorn pulp, with mixed fermentation exhibiting the most pronounced metabolic reprogramming. Co-fermentation significantly enhanced phenolic derivatives and aromatic amino acid-related metabolites, while promoting a more balanced distribution of organic acids and esters, contributing to improved antioxidant potential and flavor coordination. Multivariate analyses confirmed that mixed fermentation formed a distinct metabolic pattern rather than a simple additive effect of single strains, characterized by expanded metabolic diversity and pathway complementarity. These findings provide mechanistic insight into strain-dependent and synergistic metabolic modulation during fruit fermentation and support mixed lactic acid bacteria fermentation as a promising strategy for improving the functional and sensory quality of hawthorn-based products.