Antimicrobial Activity of Vanillin and Mixtures with Cinnamon and Clove Essential Oils in Controlling Listeria monocytogenes and Escherichia coli O157:H7 in Milk

The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of vanillin against Listeria monocytogenes Scott A and Escherichia coli O157:H7 was determined in tripticase soy broth (TSB), pH 7 and 6, incubated at 35 degrees C/24 h and in semi-skim milk incubated at 35 degrees C/24 h and 7 degrees C/14 days. The influence of the fat content of milk on the antimicrobial activity of vanillin was tested in whole and skim milk incubated at 7 degrees C/14 days. Mixtures of clove and cinnamon with vanillin were also evaluated in semi skim milk incubated at 7 degrees C. The MICs for L. monocytogenes were 3,000 ppm in TSB (pH 7) and 2,800 ppm in TSB (pH 6). The MICs for E. coli O157:H7 were 2,800 ppm in TSB (pH 7) and 2,400 ppm in TSB (pH 6). The MBCs in TSB were 8,000 ppm for L. monocytogenes and 6,000 ppm for E. coli O157:H7. The pH values assayed did not influence significantly the MIC or MBC in TSB. The MICs in semi-skim milk for L. monocytogenes and E. coli O157:H7 were 4,000 and 3,000 ppm at 35 degrees C/24 h, and 2,500 and 1,000 ppm at 7 degrees C/7 days, respectively. The MBCs were 20,000 ppm for L. monocytogenes and 11,000 ppm for E. coli O157:H7. High incubation temperatures did not affect the MBC but increased the MIC of the vanillin in milk. This effect could be attributed to the increased membrane fluidity and to the membrane perturbing activity of vanillin at low temperatures. The fat in milk reduced significantly the antimicrobial activity of vanillin, probably due to effect protective of the fat molecules. Mixtures of clove and cinnamon leaves inhibited the growth of L. monocytogenes in a similar way that vanillin alone but had a synergistic effect on the E. coli O157:H7. Mixtures of cinnamon bark and vanillin had always a synergistic effect and some of the combination assayed showed bactericidal activity on the population of L. monocytogenes and E. coli O 157:H7.