Multiphysics modeling and validation of a microwave applicator with three-stub tuner for the continuous flow pasteurization of fruit juices with low reflection

Guilherme Russo ¹ Jorge Gut ¹ Dorin Boldor ^2*

• ¹ Food Research Center, University of São Paulo, São Paulo, São Paulo, Brazil
• ² Louisiana State University Agricultural Center, Baton Rouge, United States

Microwave heating technology offers advantages for the continuous flow pasteurization of fluid foods vs. conventional heat exchangers, including fast volumetric heating, high energy efficiency, utilization of renewable electricity, and reduced losses of sensorial and quality attributes. For fruit products, as low pH inhibits the growth of potentially harmful micro-organisms, enzymes are the most thermally resistant targets of the process (peroxidase, polyphenol oxidase, pectin methylesterase). The microwave applicator heats up the stream to the inactivation temperature, followed by a holding period to reach the desired enzyme inactivation level. Modeling and simulation of the microwave applicator is challenging as it requires the coupling of three physics: propagation of electromagnetic radiation, fluid flow, heat generation and heat transfer. Changes in the product dielectric properties with temperature affects radiation penetration, which influences heat generation. Herein a multiphysics model was developed in COMSOL v.5.3 to numerically simulate the heating of orange juice, mango puree and mango nectar (flow rate: 0.2-1.2 L/min; outlet temperature: 70-100°C) in the cavity of a pilot-scale microwave-assisted pasteurizer. Dielectric properties were measured and the model was validated with experimental results. The cavity is an octagonal prism with a vertical applicator tube, coupled to a 6 kW microwave generator at 2.45 GHz. The model includes the waveguide and a 3-stub tuner, which is seldom integrated in this type of model. Simulation results provide the distribution of electric field in the domain and of temperature in the product, as well as the loss of incident power due to reflection. Electromagnetism and Monte Carlo simulation were used to optimize the tuner stubs heights to minimize power reflection (scattering parameter S11). Seventeen processing conditions with different products, temperatures and flow rates were simulated and compared with experimental data; overall prediction error at the outlet for orange juice was 1.6±1.3 °C, while for mango puree and juice were between 1.8 and 5.5°C. The validated model is useful for the analysis, design and optimization of the microwave applicator; as an example, it was used to evaluate the influence of the cavity cross section shape and tube position on power reflection and heating uniformity.