Antimicrobial activity of a Tunisian ethanolic extract of propolis in ricotta cheese: In vitro assessment and in situ application

Abstract

This study evaluated the antimicrobial potential of a Tunisian ethanolic extract of propolis in sweetened ricotta cheese, a perishable dairy product. The extract was chemically characterized by HPLC-PDA-MS and its antimicrobial activity was assessed through in vitro assays and in situ application. Chemical analysis revealed a high total phenolic content (7541.7 ± 117.2 mg/L) and 23 phenolic compounds, with isorhamnetin, pinobanksin-propionate, and 4-O-methyl-epigallocatechin as the most abundant. In vitro activity was evaluated against 15 bacterial strains using agar disc diffusion and broth microdilution assays to determine minimum inhibitory (MIC) and bactericidal (MBC) concentrations. The extract showed antibacterial activity against Gram-positive bacteria, while no inhibitory effects were observed against Gram-negative strains. MIC values ranged from 0.25 to 1.25 mg/mL and MBC values from 0.625 to 12.5 mg/mL, indicating predominantly bactericidal activity. In situ efficacy was assessed through a 14-day shelf-life study and a Listeria monocytogenes challenge test on industrial and artisanal sweetened ricotta supplemented with 0.1% and 1% extract. Shelf-life extension was limited, whereas a dose-dependent inhibition of L. monocytogenes was observed. Exploratory sensory evaluation revealed moderate changes. Overall, Tunisian propolis extract appears suitable for targeted control of L. monocytogenes rather than broad-spectrum preservation in ricotta cheese.