Behavior of Listeria monocytogenes in the Presence of Cocoa, Carrageenan, and Sugar in a Milk Medium Incubated With and Without Agitation

Abstract

Enhanced growth of Listeria monocytogenes strain V7 in chocolate milk rather than skim milk was further investigated by testing various concentrations of cocoa powder (two types of Dutch-process, designated A and B), cane sugar, and sodium carrageenan in skim milk at 13 and 30°C with and without agitated incubation. Increasing sugar concentrations (0, 6.5, and 12.0%) were marginally significant (p = 0.06) in shortening generation times (5.17, 5.07, and 5.05 h, respectively) of the pathogen. Maximum populations attained by the pathogen were greater when cocoa (0.75% type A or B) and sugar (6.5 or 12.0%) were present. Sugar concentration affected growth of L. monocytogenes in an approximately linear relationship (8.41, 8.67, 8.82 log10 CFU/ml for 0, 6.5, and 12.0% sugar, respectively) except in samples containing only carrageenan. In this instance, presence of 6.5 and 12.0% sugar resulted in equivalent maximum populations (8.54 and 8.52 log10 CFU/ml). Three factors enhanced growth of the pathogen at 30°C: addition of cocoa, addition of sugar, and agitated rather than quiescent incubation. Without cocoa, generation times of L. monocytogenes were longer (1.04 h) compared to presence of type A (0.87 h) or B (0.90 h) cocoa. L. monocytogenes in agitated samples had shorter (0.82 h) generation times than in quiescent cultures (0.95 h). Highest populations were attained in agitated samples containing sugar and type A (9.21 log10 CFU/ml) or type B (9.22 log10 CFU/ml) cocoa compared to lowest populations in quiescent samples of skim milk (8.56 log10 CFU/ml).