Microwave Drying of Food and Agricultural Materials: Basics and Heat and Mass Transfer Modeling

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Microwave drying is based on a unique volumetric heating mode facilitated by electromagnetic radiation at 915 or 2,450 MHz. The responses of a lossy food product to dielectric heating result in rapid energy coupling into the moisture and lead to fast heating and drying. A significant reduction in drying time in microwave drying is often accompanied by an improvement in product quality, making it a promising food dehydration technology. The need for improvement in engineering design and process optimization for microwave drying has stimulated the development of computer simulation techniques to predict temperature and moisture history and distribution in the product to be dried. In this article, we present the basics of dielectric heating and drying, examine the heat and mass transfer models developed for the simulation of microwave drying processes, and discuss dielectric properties of selected food products as influenced by moisture, temperature, and porosity. In addition, we analyze nonuniform heating caused by the geometry and composition of the product, as well as by the nonuniform distribution of electromagnetic field in a microwave cavity, followed by a discussion on how to improve the microwave heating uniformity. We focus the discussion on heat and mass transfer models developed over the years to simulate microwave drying, including simplified ones, those based on diffusion theory, and coupled heat and mass transfer analysis with the Philip and de Vries theory, Luikov scheme, Whitaker method, and two-region model. In the end, the determination of the heat source term in the energy equation, numerical schemes used to solve the partial differential equations, and the model validation are also discussed.