Is Enterococcus faecium an Appropriate Surrogate for Salmonella in Thermal Process Validation of Cocoa Powder?

Introduction:  Salmonella is a growing concern in low moisture foods including chocolate. Cocoa powder, an essential ingredient of chocolate, has been observed for Salmonella contamination since the late 1960. High temperature processing is a primary method to inactivate microbial contamination. Although Enterococcus faecium is an accepted surrogate of Salmonella for validating almond thermal processes, no published information is available to support its use in cocoa powder.

Purpose:  To compare thermal resistance of E. faecium and a Salmonella cocktail in cocoa powder (0.30 water activity (a_w)) at 70, 75, and 80°C.

Methods:  Natural unsweetened cocoa powder was inoculated with E. faecium or a three-strain Salmonella cocktail (Salmonella Enteritidis PT30, Salmonella Tennessee K4643, and Salmonella Agona) at 1×10⁹ CFU/gram, conditioned to 0.3 a_w (25°C), a typical a_w of cocoa powder. The inoculated cocoa powder was then subjected to isothermal inactivation (70, 75, 80°C) in aluminum test cells (0.4 g sample). Survivors at selected treatment time points were enumerated to determine D- and z-values.

Results:  Both E. faecium and Salmonella exhibited log-linear kinetics at all temperatures. The D-values of E. faecium and Salmonella cocktail were 105.4±4.4 and 46.0±2.3 min at 70°C, 40.7±3.3 and 22.1±0.7 min at 75°C, 18.3±0.1 and 13.3±0.1 min at 80°C, respectively. D-values for E. faecium were significantly higher (P<0.05) than those for Salmonella at all tested temperatures; E. faecium had a closer D-value to that of Salmonella at 80°C, which warrants further comparison of thermal inactivation at higher temperatures. Z-values for E. faecium NRL B-2354 and Salmonella cocktail were 13.1 and 18.5°C, respectively.

Significance:  Enterococcus faecium appears to be an appropriate surrogate for assessing industrial thermal processes of cocoa powder in inactivation of Salmonella.