Evaluation of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella during cocoa powder thermal processing

Salmonella is capable of surviving in a low moisture environment for long periods. Once adapted to the xeric conditions, the thermal resistance of Salmonella is enhanced. Cocoa powder is a low water activity (a(w)) food (LawF) that is an essential component in a wide variety of desserts and ready-to-eat (RTE) foods and drinks. The aim of this study was to evaluate the desiccation and thermal resistance of Salmonella in cocoa powder, as well as to examine the suitability of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella during cocoa powder thermal processing. Natural unsweetened cocoa powder was inoculated with a 3-strain Salmonella cocktail or E. faecium and was equilibrated to a(w) 0.30 and 0.45 at room temperature, then subjected to isothermal treatments at 70-80 degrees C or 12-month storage at RT (room temperature, 22.0 +/- 0.5 degrees C, a(w) 0.30). At 70 and 80 degrees C, D-values of both Salmonella and E. faecium increased with decreasing a(w). D-values of Salmonella at a(w) 0.30 cocoa powder were 46.2 +/- 4.7, 20.5 +/- 1.7, and 11.5 +/- 0.9 min at 70, 75 and 80 degrees C, respectively. Higher heat resistance of E. faecium in a(w) 0.30 cocoa powder was observed with D-values of 59.9 +/- 5.0, 28.9 +/- 1.8, and 16.1 +/- 1.4 min at 70, 75, and 80 degrees C, respectively. However, E. faecium demonstrated less heat resistance than Salmonella when a(w) was increased to 0.45. D-values for Salmonella at a(w) 0.45 were 31.6-7.0 min at 70-80 degrees C compared to 25.8-4.7 min for E. faecium. During 12 months of storage at RT, surviving E. faecium population in a(w) 0.30 cocoa powder was higher than that of the Salmonella cocktail; the population decreased by 1.39 and 3.75 logs, respectively. These findings indicate that the suitability of E. faecium as a surrogate organism for Salmonella is influenced by a(w) of cocoa powder. The a(w) correlates with thermal inactivation rates in both Salmonella and E. faecium, and should be considered as a significant contributor to the thermal resistance of Salmonella in cocoa powder.