Antimicrobial activity of hop extracts against Listeria monocytogenes in media and in food

Growth of Listeria monocytogenes was inhibited in culture media and in certain foods by four hop extracts (I–IV) containing varying concentrations of α-and β -acids. Extracts (II and III) containing the highest concentrations of β-acids were inhibitory at 0.01 mgl in trypticase soy broth. In food, these hop extracts showed varying magnitudes of inhibition. In coleslaw, hop extract III at 1 mgg enhanced the rate of inactivation of L. monocytogenes Scott A. Hop extract II was inhibitory at 0.1 and 1 mgml in skim and 2% milk, and was inhibitory at 1 mgml in whole milk. Hop extract II was listericidal in cottage cheese at 0.1 to 3 gkg. No inhibition of L. monocytogenes by hop extract III was observed in Camembert cheese. Overall, the antimicrobial activity of hop extracts in food appeared to increase with acidity and lower fat content. Our results indicate that hop extracts could be used to control L. monocytogenes in minimally processed food with low fat content.