Fate of Listeria monocytogenes in Cocoa Powder during Isothermal Inactivation

Introduction:  Listeria monocytogenes can survive in dry condition for a long period of time. Despite an increasing number of studies addressing Salmonella inactivation in low-moisture foods, there is a general lack of knowledge related to L. monocytogenes inactivation in low moisture foods during thermal processing and to the factors impacting their survival in low moisture food. Cocoa powder is an essential ingredient and widely incorporated in different disserts and drink and thus a possible source of L. monocytogenes contamination.

Purpose:  To evaluate the thermal resistance of L. monocytogenes in cocoa powder and further investigate the impact of water activity (a_w) on its survival in cocoa powder.

Methods:  Natural unsweetened cocoa powder (Hershey’s) was inoculated with three-strain L. monocytogenes cocktail (~9.0 log CFU/g), equilibrated to water activity (a_{w, 25°C}) to 0.3 or 0.45, then subjected to isothermal treatments using aluminum TDT test cell containing 0.4 g of inoculated and equilibrated sample. The survivors were enumerated on TSAYE plates.

Results:  Inactivation data resulted from different temperatures at both a_w showed log-linear trend which was used to obtained thermal inactivation parameters. Thermal resistance of L. monocytogenes at 0.3 a_w was significantly higher than that at 0.45 a_w across all three temperatures. Listeria monocytogenes at 0.3 a_w and 0.45 a_w had D_70°C of 7.44±1.17 and 4.77±0.28 min, D_75°C of 3.62±0.03 and 2.84±0.09 min, and D_80°C of 1.35±0.04 and 1.06±0.05 min, respectively, z-value at 0.3 a_w and 0.45 a_w was 13.48 and 15.31°C, respectively.

Significance:  Listeria monocytogenes demonstrated much higher thermal resistance in cocoa powder compared to the high moisture foods, which is impacted by a_w. Data provide valuable information for industry to validate thermal processing for control of L. monocytogenes in low moisture foods.