Evaluation and modeling of a microwave-assisted unit for continuous flow pasteurization of liquid foods: Residence time distribution, time–temperature history, and integrated lethality

摘要

Focused microwave heaters have potential to replace heat exchangers in continuous flow pasteurization of liquid foods. The objective of this work was to evaluate a pilot scale unit used for microwave assisted pasteurization of low viscosity liquid foods in order to model the heat transfer and residence time distribution (RTD) to, subsequently, predict the average time-temperature history and estimate the level of heat treatment of the process and contributions from each step. RTD data using water showed that it was possible to assume plug-flow (negligible axial dispersion). Heat transfer experiments using water provided heat transfer coefficients as functions of Reynolds number and microwave power absorption. Mathematical modeling was used to determine the temperature distribution along the product path, and results were validated. Integrated lethality calculated from the time-temperature histories (pasteurization at 70 degrees C) revealed the contribution of each process step. Results showed that focused microwave heating provided the necessary temperature increase in a very short time, with a lethality contribution of only 0.7% as compared to 59-68% when using only the conventional heat exchanger. The methods described can be useful for the evaluation of other continuous flow pasteurization units processing low viscosity liquid foods.Practical applicationsThe mathematical modeling approach and experimental methods presented herein can be used for the analysis of continuous flow pasteurization systems based on tubular heat exchangers and with low axial dispersion (plug flow), providing temperature and lethality distribution along the product path, showing the contribution from each section on inactivation. Specifically for the equipment used in the experiments, the obtained heat transfer and resident time distribution parameters will be useful to study the processing of low viscosity foods such as fruit juices and nectars on ongoing works from the group.