Salmonella Inactivation Kinetics on Apple Slices over a Wide Range of Water Activity

Understanding the complex moisture-temperature interactions affecting pathogen inactivation during apple drying is essential to ensuring sufficient pathogen control, as mandated by the U.S. Food Safety Modernization Act. The objective was to model such effects based on isothermal/iso-moisture thermal treatment of apple slices, across a wide range of moisture levels, from a water activity (a_w) of 0.99, for fresh apple slices, to 0.10 a_w for very dry slices. For treatments at 0.99 a_w, fresh apple slices were inoculated with six-Salmonella-strain cocktail, whereas for conditions at a_w < 0.99, partially-dried slices were inoculated and equilibrated in controlled humidity chambers (3-5 days). Isothermal treatments were conducted at 60, 70, 80, and 90°C, using a non-factorial experimental design with three levels of a_w tested for each temperature, using 3 biological replications. Salmonella survivors were enumerated, and a_w, both at ambient and high temperatures, and moisture content (MC) were measured. Salmonella thermal inactivation on the apple slices was modeled with a primary log-linear/modified-Bigelow model, in which the moisture effect was represented by a_w or MC. The a_w change due to temperature (0.004±0.029) was not significant (P > 0.05); therefore, only room-temperature a_w or MC was used for modeling. Models based on temperature and both moisture metrics successfully described Salmonella inactivation across a wide a_w range (0.10-0.99), with a root mean squared error (RMSE) of 0.94 and 0.97 log CFU/g for the MC- and a_w-based models, respectively. The estimated z_T was 15.4 and 17.9°C for the a_w and MC approaches, respectively. The z_aw was 0.586, and z_MC was 41.2% (wet basis). Overall, this study modeled the impact of temperature and product moisture (a_w or MC) on Salmonella inactivation on apples and was the first known to successfully model such effects across a full range of moisture that is representative of typical changes during apple drying processes.