Synergistic Antimicrobial Effect of Agro-Industrial Peel Extracts and Saccharomyces cerevisiae Against Listeria monocytogenes in Fruit Juice Matrices

Agro-industrial by-products are rich in polyphenols with potential applications as natural antimicrobials in food systems. This study evaluated the total polyphenol content (TPC) and antimicrobial activity of orange (Citrus sinensis), onion (Allium cepa), cacao (Theobroma cacao), and tamarillo (Solanum betaceum) peel extracts against Listeria monocytogenes, individually and in combination with Saccharomyces cerevisiae. TPC was quantified using the Folin–Ciocalteu method, and minimum inhibitory concentrations (MICs) were determined using broth microdilution. Statistical analysis (two-way ANOVA, p < 0.05) assessed the effect of extract type and yeast addition on MIC values. The highest TPC was recorded in Theobroma cacao peel extract (85.3 ± 2.1 mg GAE/g DW). All extracts inhibited L. monocytogenes, with MICs ranging from 2.5 to 10 mg/mL. This was reduced to 1.25–5 mg/mL when combined with S. cerevisiae, indicating synergism (F = 11.42, p = 0.003). These results suggest that polyphenol-rich peel extracts enhanced by S. cerevisiae can be incorporated into beverage preservation systems, aligning with clean-label trends. This study integrates quantitative and mechanistic analyses to link extraction methods, polyphenol content, and synergistic inhibition with Saccharomyces cerevisiae, providing a coherent analytical framework for sustainable antimicrobial strategies.