Addition of Cocoa Powder, Cane Sugar, and Carrageenan to Milk Enhances Growth of Listeria monocytogenes

Abstract

Previously we found that under similar conditions Listeria monocytogenes achieved populations in chocolate milk that were 10 times greater than those in other fluid milk products. The current studies were undertaken to determine why the bacterium grew so well in chocolate milk. Autoclaved samples of milk with 2% milkfat, 2% milk + sugar, 2% milk + cocoa, and 2% milk + sugar + cocoa were inoculated with one of two strains of L. monocytogenes and incubated at 13°C. Carrageenan was also added to one-half of all samples containing cocoa. Growth curves were derived and generation times and maximum populations were calculated for each combination of product and strain of the bacterium. Strain V7 grew faster than strain CA in all products, with most rapid growth occurring in samples containing cocoa (with or without added sugar). Addition of carrageenan further reduced the generation time of this strain. Overall, growth rates ranged from 3 h 55 min (V7 in 2% milk + sugar + cocoa + carrageenan) to 4 h 53 min (CA in 2% milk + cocoa). Product type was primarily responsible for differences in maximum populations achieved by L. monocytogenes. In each instance, final numbers reached were at least 10⁸ cells/ml with highest levels in samples containing all ingredients. The data suggest that sugar, cocoa and carrageenan when added to milk contributed to enhancing growth of L. monocytogenes.