Synergistic, bacteriostatic effect of propolis and glycerol against Listeria monocytogenes in chocolate milk under refrigerated storage.

ABSTRACT

Propolis ethanolic extracts, with or without glycerol, were added into pasteurized, non-fat chocolate milk, which was artificially contaminated with Listeria monocytogenes. The addition of propolis ethanolic extracts dissolved into glycerol led to a definite anti-listerial effect in milk stored at 4 ℃, with both propolis concentrations tested (2 or 4 mg of dry propolis ethanolic extract per milliliter of chocolate milk) leading to inhibition of L. monocytogenes growth throughout 20 days of storage. The combined addition of propolis ethanolic extracts with glycerol was also effective in significantly reducing the rate of growth of L. monocytogenes in chocolate milk stored under improper (10 ℃) refrigeration storage conditions (more than five-fold increase in the generation time of L. monocytogenes compared to control trials). Finally, the combined addition of a deodorized propolis ethanolic extract with glycerol resulted in a significant anti-listerial effect upon storage of contaminated milk at 4 ℃ (more than three-fold increase in the generation time of L. monocytogenes compared to controls) and in a smaller anti-listerial effect upon milk storage at 10 ℃ (two-fold increase in the generation time of the pathogen compared to controls). Of note, chocolate milk containing deodorized propolis ethanolic extract and glycerol received a positive consumer acceptability score on the nine-point hedonic scale (median acceptability score of "7"). Hence, propolis may possess a promising role as a natural anti-listerial preservative in dairy drinks.