Thermal resistance of Listeria monocytogenes in natural unsweetened cocoa powder under different water activity

Abstract Listeria monocytogenes can survive in dry conditions for long periods. Despite an increasing research studying 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 the factors impacting their survival in these products. Cocoa powder is an essential and widely incorporated ingredient in many desserts and drinks that do not need thermal processing. This study evaluated the thermal resistance of L. monocytogenes in cocoa powder and investigated the impact of water activity (a w ) on its survival in cocoa powder. Natural unsweetened cocoa powder was inoculated with a 3-strain L. monocytogenes cocktail (∼9.0 Log 10  CFU/g), equilibrated to a w 0.30, 0.45 or 0.60 at 22 °C and subjected to isothermal treatments. Survivors were enumerated to obtain thermal-inactivation parameters. L. monocytogenes population was stable in cocoa powder (a w 0.30) over the first month of storage, then decreased gradually but remained detectable after 12-month storage at 22 °C. Thermal inactivation of L. monocytogenes in cocoa powder at target a w and different temperatures showed a log-linear trend. Heat resistance of L. monocytogenes is a w -dependent with the highest resistance at a w 0.30. The range of D-values (in min) at 70, 75 and 80 °C at a w 0.30. and 0.45, respectively, were: 21.9–5.0 and 7.3–1.8. The range of D-values (in min) at 65, 70 and 75 °C at a w 0.60 was 9.1–2.0. The z-value at a w 0.30, 0.45 , and 0.60 was 15.5, 15.9, and 14.9  ° C, respectively. In summary, L. monocytogenes can survive in cocoa powder stored at 22 °C for an extended time. Thermal resistance of L. monocytogenes adapted to low a w cocoa was conversely related to a w . This study provides valuable information for the food industry to develop thermal inactivation strategies to control L. monocytogenes in cocoa powder.