Inhibition of Listeria monocytogenes by Cocoa in a Broth Medium and Neutralization of this Effect by Casein

Abstract

Dutch-processed cocoa (0.75 to 10.0%, w/v), when added to a broth medium, inhibited/inactivated Listeria monocytogenes strain V7. With agitated incubation at 30°C, samples with 5.0, 7.5, and 10.0% cocoa were free of detectable viable cells (<1/ml) 15 to 24 h after inoculation to contain ca. 1 × 105 L. monocytogenes strain V7/ml. Without agitation at 30°C, presence of 0.75 to 10.0% cocoa lengthened (1.02 to 1.12 h) the generation time of the pathogen when compared to samples without cocoa (0.94 h). However, the pathogen, in samples containing cocoa, eventually reached a higher (9.05 to 9.18 log10 CFU/ml) population than in samples without cocoa (8.81 log10 CFU/ml). The lag phase of L. monocytogenes was longer with (8.15 h) rather than without (4.41 h) agitation, at a lower (ca 1×103 CFU/ml) (6.92 h) rather than a higher (ca. 1 × 105 CFU/ml) (5.65 h) inoculum level, and in the presence of 0.75% (10.52 h) rather than 0% (2.04 h) cocoa. Higher maximum populations (9.18 log10 CFU/ml) developed in samples that were agitated rather than incubated quiescently (8.39 log10 CFU/ml), and in samples with (9.23 log10 CFU/ml) rather than without (8.88 log10 CFU/ml) cocoa. Casein (1.5 or 3.0%) relieved the inhibition when incubation was quiescent and inactivation of L. monocytogenes by cocoa when incubation was with agitation. Without agitation, the lag phase of the pathogen was extended in the presence of cocoa (4.01 h) compared to samples with cocoa and casein (1.77 to 2.74 h). Casein did not significantly (p>0.05) affect the maximum population attained by the pathogen, but presence of cocoa increased the maximum population (8.86 and 9.25 log10 CFU/ml for 0 and 5.0% cocoa). With agitation, presence of 5.0% cocoa completely inactivated the pathogen; however, addition of 2.5% casein to these samples allowed growth of L. monocytogenes to a population of 9.47 log10 CFU/ml (compared to 8.90 log10 CFU/ml in broth).