Carbon source-driven modulation of Lactiplantibacillus plantarum S61 postbiotics enhances antifungal activity against Rhodotorula glutinis

Microbial spoilage caused by yeasts such as Rhodotorula glutinis remains a major challenge in food preservation, highlighting the need for natural antimicrobial alternatives. This study investigated the influence of carbon source on the metabolite profile, antifungal efficacy, antioxidant activity, and genotoxic safety of postbiotics produced by Lactiplantibacillus plantarum S61. The strain was cultivated using glucose (CFS-Gl) or arabinose (CFS-Ar) as the sole carbon source, and the resulting cell-free supernatants were characterized by HPLC-DAD, ATR-FTIR spectroscopy, and colorimetric assays. Seven organic acids were identified, with phenyllactic acid (PLA) as the dominant metabolite. PLA reached 15.39 mg/mL in CFS-Ar, approximately 3.5-fold higher than in CFS-Gl, whereas lactic acid predominated in CFS-Gl. CFS-Ar showed stronger inhibition of R. glutinis UMP22 than CFS-Gl, with MIC values of 1.17 and 2.34 mg/mL, respectively. Checkerboard analysis confirmed synergistic antifungal activity between PLA and lactic acid (FICI = 0.365), while molecular docking predicted the strongest binding affinity for PLA against three fungal targets. CFS-Ar also showed enhanced antioxidant activity, and neither supernatant induced detectable DNA strand breaks. These findings support arabinose fermentation as a strategy to enhance postbiotic functionality for food biopreservation.