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Magnetic Resonance Imaging (MRI) has been used to map in three dimensions, noninvasively, the complex heating patterns induced by microwave heating. Comparisons with infra-red thermal imaging of the surface heating patterns are demonstrated. The effects of the differential heating properties have been characterized for a model food sample. The potential applications of MRI temperature mapping to optimizing the domestic and industrial use of microwaves for heating of foods is discussed.

Industrial microwave food processing is universally based on single frequency microwave sources. With the emergence of variable frequency microwave ovens, it is possible to exploit the frequency dependence of a food's permittivity and/or choice of heating frequency, for example as a new route to achieving targeted heating. Variable frequency heating procedures are developed to overcome the geometry of a roughly spherical foodstuff dominating the heating pattern when heated in fixed frequency applicators. Target mean temperatures of 55, 75 and 90 degrees C within 2 minutes and without physical damage were set; means of 54.5 +/- 4.1, 75.1 +/- 4.7 and 87.6 +/- 3.5 degrees C respectively were achieved within the time constraint and with no major physical damage, based on combining 8 discrete frequencies between 2.4 and 6.2 GHz.

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Food processors are increasingly looking towards new microwave-based technologies to deliver competitive advantage in the market place, reduce operational costs, allow greater product innovation and increase flexibility without the need for large capital investments. The technical barriers to the widespread exploitation of microwave heating, and opportunities for microwave heating in food processing, are discussed in the context of the need to understand the microwave field/material/process interaction. It is shown how a new application of interference techniques, called phase control microwave heating, may offer opportunities for enhanced heat transfer in food processing, especially for direct control of the spatial power deposition within a foodstuff. Simulations of phase control heating using a 3D FETD microwave model, validation of experimental findings and examples in other applications are presented.