Mohammad M. Farid

An investigation of deactivation of bacteria in a canned liquid food during sterilization using computational fluid dynamics (CFD)

Abstract Thermal processing of a liquid food always results in important biochemical changes such as bacteria deactivation and nutrient concentration changes. To estimate these changes the liquid food needs to be tagged and followed, which is a difficult task for most flow conditions. In this study, the computational fluid dynamics (CFD) code PHOENICS is used to predict temperature distribution and concentration of the live bacteria in a can filled with liquid food. The governing equations for continuity, momentum and energy are solved numerically together with bacteria concentration, using a finite volume method. Arrhenius equation was used to describe bacteria deactivation kinetics, and it was introduced to the existing software package using a FORTRAN code. The diffusion of bacteria was modelled using the modified Brownian diffusion equation. Natural convection that occurs during thermal sterilization of viscous liquid (the aqueous solution of sodium carboxy-methyl cellulose (CMC)) in a cylindrical can heated from all sides, has been studied. Saturated steam at 121°C was used as the heating medium, and the model liquid was assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). The simulations have provided transient flow pattern, live bacteria concentration and temperature profiles, which highlight the slowest heating zone (SHZ) resulted from different periods of heating. The results show that the action of natural convection forces the SHZ to migrate towards the bottom of the can, and eventually stay in a region that is about 10–15% of the can height from the bottom. The secondary flow formation and its effect on the shape of the SHZ were evident. The simulations also show how the concentration of the live bacteria depends on both temperature distribution and flow pattern. The effect of diffusion on the rate of sterilization has been found to be negligible in the cases simulated in this study.