Temperature distribution in vertical and horizontal high pressure vessels and its impact on food safety

The temperature distribution during High Hydrostatic Pressure Processing (HHPP) is of particular concern when high pressure process is carried out at higher temperatures, such as those associated with pressure assisted thermal pasteurization (PATP) and pressure assisted thermal sterilization (PATS). The pressure assisted thermal processes combine high pressure and heat to achieve pasteurization/sterilization temperature in order to eliminate the bacteria while maintaining better quality. During these processes, inactivation by temperature is the primary mechanism. Therefore for such processes, it is important to examine the temperature variation within the vessel to evaluate its impact on product safety. The objective of this research was to predict the temperature distribution in vertical and horizontal high pressure vessels during pressure assisted thermal processing and to quantify the temperature non-uniformity and its impact on microbial inactivation. The CFD software ANSYS®-Fluent was used to numerically predict the flow, temperature and C. botulinum inactivation distributions. Coefficient of variation (COV) was used to quantify and compare the process uniformity for various cases. Further, alkaline phosphatase (ALP), an enzyme present in raw milk was used as an indicator for experimentally verifying process uniformity. The results obtained from the numerical simulations showed that the temperature non-uniformity arises in both vertical and horizontal high pressure vessels during HHPP due to the water added for compression, adiabatic compression heating, and conduction heat loss at vessel wall and ensuing natural convection cooling near the vessel wall. Based on the COV approach, (i) the temperature of water added for compression was found to have a major impact on the temperature distribution and inactivation of C. botulinum, (ii) adding an insulation to the vessel diminished the non-uniformity in temperature and C. botulinum, and (iii) process non-uniformity was found to be higher for a vertical vessel as compared to a horizontal vessel of same dimensions. Also, the experiments with alkaline phosphatase (ALP) in the vertical vessel showed location dependent inactivation, further confirming the existence of process non-uniformity. The results from this research will help in process validation and process filing with regulatory agencies (USDA-FSIS and US-FDA) to develop guidelines for producing safe PATP and PATS processed foods.Ph. D.Includes bibliographical referencesby Meenakshi Khuran