Computer modelling for the control of particulate sterilization under dynamic flow conditions

A finite difference model was used for predicting the centre temperature of particulates during ultra-high temperature processing under continuous flow conditions. This model compared favourably with the analytical solutions. A sensitivity analysis was carried out on the input parameters to the numerical model, which indicated that particulate size and thermal diffusivity were the most critical parameters influencing process lethality and cook value. For example, a spherical particulate (thermal properties defined) of 20 mm diameter required a holding time greater than 5 min at 135°C to achieve a minimum safe cook (F₀ of 3.0 min for Clostridium botulinum), compared to a 10 mm spherical particulate which would have received an F₀ value of 68 min after 5 min processing at 135°C. The centre temperatures predicted using the model were sensitive to changes in the convective surface heat transfer coefficient in the range 100–300 W m⁻² K⁻¹, but became less sensitive as the values increased up to 500 W m⁻² K⁻¹. An initial product temperature change from 20 to 60°C resulted in an 8 min increase in process lethality for a 15 mm spherical particulate processed for 5 min at 135°C. The determination of experimental results in particulates of less than 20 mm (the critical dimension) was found to be significantly affected by conduction errors through the thermocouple wires, and therefore the model presented will be a useful prediction tool in situations of experimental uncertainty.