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 inactivate microorganisms on dried, ready to eat foods, including nuts, seeds and spices. Thermal experiments were performed on five types of dried seeds and nuts that varied in size and shape, both individual products or contained within mesh bags of product. Based on heat transfer and temperature measurements it was found that the steam penetrated extremely slowly with unpredictable behavior in bags when the system first pulled a vacuum followed by steam injection. Consequently, a small fan system was added to increase the steam velocity and flush out non-condensable gasses (NCGs) trapped in the open spaces between surfaces. A conduction heat transfer model was developed based on single product experimentation. Computational fluid dynamic (CFD) models were developed for full bags of each product based on the steam penetration and the transient temperature response throughout the package. The model was validated at a total of three treatment temperatures, 60, 70, and 80 °C. The measured surface temperature of the product within the package was successfully predicted using the CFD model result as input to the conduction heat transfer model.