Multiphysics modeling of polyphenol oxidase and peroxidase inactivation in continuous-flow microwave thermal processing of coconut water

Abstract

Microwave-assisted thermal treatment has been used in food processing as an alternative to conventional heating due to the better preservation of food quality and high energy efficiency. This work modeled a continuous flow microwave cavity followed by a holding tube for the thermal processing of liquid foods using COMSOL Multiphysics (v.6.1). The model couples the electromagnetic wave propagation to the physics of laminar flow and heat transfer, allowing the prediction of the fluid temperature along the heater and holding tube. Dielectric properties at 2.45 GHz and electric conductivity were measured and correlations were used in the model. Polyphenol oxidase (PPO) and peroxidase (POD) enzymatic thermal inactivation could be predicted by adding the physics of transport of diluted species to the model. Experimental tests were carried out with a microwave-assisted thermal processing unit (MicroThermics, 2.45 GHz, 6 kW). Outlet temperatures were measured and compared with model predictions. For the model validation, fresh coconut water was processed at different flow rates (0.5, 0.7, 0.9 and 1.1 L/min) and target temperatures (80, 90, 100 and 110 °C). Comparison between numerical and experimental results showed that the model is reliable for predicting outlet temperature of the heater, with a mean absolute error of 3.3 °C. Reliable predictions for residual activities of PPO and POD after the holding tube were also provided, with most deviations under 10%. This computational model can adequately simulate the microwave cavity and the holding tube for the continuous flow processing of coconut, showing it can be further used for process analysis and design. © 2023, The Author(s) under exclusive licence to Associação Brasileira de Engenharia Química.