Heat resistance of Salmonella typhimurium and Salmonella senftenberg 775W in milk chocolate.

The problem of Salmonella contamination in dried foods has underscored our lack of knowledge concerning the heat resistance of enteric microorganisms in dry and semidry environments. Confectionery manufacturing is one of the food industries concerned with dry-heat destruction of Salmonella. The discovery of these organisms in several chocolate-containing products in the recent past has raised the question of whether they can be killed by heating the molten chocolate prior to casting. The results of experiments performed to determine the heat resistance of Salmonella in miLk and dark chocolate are presented in this paper. S. typhimurium and S. senftenberg 775W were inoculated into milk chocolate in the following manner. The cells resulting from 24-hr growth in 600 ml of Trypticase Soy Broth (BBL) incubated statically at 37 C were harvested centrifugally, resuspended in 20 ml of sterile skimmed milk, and lyophilized. Commercially formulated milk chocolate was dispensed in Sorvall Omnimixer cups in 100-g quantities and warmed to the test temperature in an oil bath. The lyophilized cells were added to the molten chocolate and blended for 1 min at top speed on the Omnimixer. The cups were returned to the oil bath and immersed so that the level of the chocolate was /4 (1.9 cm) inch below the surface of the oil. Agitation of the inoculated chocolate throughout the remainder of the experiment was accomplished by a mechanical stirrer. Periodically, 1-ml samples were taken by using wide-bore pipettes and a vacuum-pressure pump. Dilutions were made in sterile 30% sucrose, and viable salmonellae were enumerated by the threetube most probable number technique. Nutrient broth was used as the recovery medium, and tubes showing turbidity after 48 hr at 37 C were streaked on MacConkey Agar (Difco) and incubated at 37 C. Isolates were confirmed biochemically and serologically. The D values obtained in replicate trials at 70, 80, and 90 C are presented in Table 1. The averages of the D values are plotted against