The Antimicrobial Activity of Phenolic Compounds Against Listeria monocytogenes and Their Effectiveness in a Model Milk System

The minimum inhibitory concentration (MIC) of several phenolic compounds against eight strains of Listeria monocytogenes in tryptose phosphate agar (TPA) was determined. Based upon concentration, the most effective compound was the phenolic antioxidant tertiary butylhydroquinone (TBHQ) which had a MIC of 64 μg/ml. Among the FDA approved food antimicrobials, the most effective was propyl paraben with a MIC of 512 μg/ml. Propyl paraben and TBHQ were then compared to potassium sorbate, a commonly used food antimicrobial, in a model milk system containing 10% nonfat milk solids. In this study, only one strain of the test microorganisms, Scott A, was used and two levels of inoculum, 10 and 1000 CFU/ml, were tested. As expected with the basic pH of the model system, both phenolic compounds were significantly more effective than potassium sorbate against L. monocytogenes at 35°C. Both compounds caused a noticeable increase in lag phase of this microorganism. There was about a three log difference in viable cell counts between propyl paraben and TBHQ and the control. The TBHQ was inconsistent in its activity. The inhibitory action of propyl paraben was not affected by the level of inoculum and had consistent activity throughout testing.