Thermal Modeling of Vacuum Assisted Steam Pasteurization for Improved Product Safety of Low Water Activity Foods

Abstract

Vacuum assisted steam pasteurization is a common method used to kill harmful pathogens on low water activity foods (LWAFs). During this treatment process it is generally believed that the steam will quickly penetrate through the small gaps within a package of product and condense on the surfaces leading to efficient heat transfer. Through experimentation with a full bag (127 x 178 mm) of product it was found that the steam actually penetrated extremely slowly with unpredictable behavior. To improve the effectiveness of the steam condensation process, a system to force flow through the package was incorporated. Single product experimentation was performed in order to develop a conduction model which predicts the product surface temperature using the measured chamber ambient steam temperature. A number of full bag experiments were conducted with 5 products (whole macadamia nuts, macadamia nut pieces, pumpkin seeds, mustard seeds, and Brazil nuts). During these experiments, the local steam temperatures at the top, middle, and bottom of the package were measured. Computational fluid dynamic (CFD) models were developed for each product to model how the steam penetrates and the transient temperature response throughout the package. The model was validated at a total of three temperatures, 60, 70, and 80 °C. Using the CFD model results as the input for the conduction modeled allowed for the surface temperature of the product within the package to be predicted accurately. A dynamic inactivation model was developed to predict the time needed for reduction of the bacteria Enterococcus faecium (E. faecium) on whole macadamia nuts and was used to predict the inactivation of E. faecium on macadamia nut pieces and pumpkin seeds using their respective dynamic temperature profiles predicted from the thermal models.