Simulation of Gastrointestinal Conditions and Antagonistic Potential of Probiotic-Fermented Tomato–Watermelon Juice

This study evaluated the gastrointestinal tolerance and antagonistic potential of probiotic-fermented tomato–watermelon juice formulated with Lactiplantibacillus plantarum LP01 and Bifidobacterium lactis BB12. A 75:25 (tomato: watermelon) formulation inoculated with 1.0% probiotics was selected based on physicochemical stability and viability (≥8 log 10 CFU/mL). Under simulated gastrointestinal conditions, probiotic counts remained above 6 log 10 CFU/mL, with intestinal counts reaching 9.71 log 10 CFU/mL, and only minor reductions under gastric and bile stress. Antagonistic assay demonstrates that during 72 h storage, fermented samples effectively inhibited Bacillus cereus, Bacillus subtilis, Staphylococcus aureus, Salmonella Enteritidis, Salmonella Typhimurium, and Enterococcus faecalis in fermented samples at both 4°C and 25°C, whereas nonfermented controls supported pathogens’ growth up to 8.56 log 10 CFU/mL at 25°C. Probiotic populations remained ≥8 log 10 CFU/mL during storage and reached approximately 8.4 log 10 CFU/mL at 25°C, while fermentation reduced pH to 3.0–3.4, contributing to antimicrobial activity. These findings confirm strong gastrointestinal tolerance, sustained probiotic viability, and effective biopreservative capacity of the fermented formulation.