MRI phase mapping of temperature distributions induced in food by microwave heating.

A range of temperature-sensitive MRI parameters of water (T2, T1, diffusion coefficient, and chemical shift) were evaluated to map in three dimensions the non-uniform temperature distributions induced by microwave heating in both model and real food systems. Phase mapping was found to be the most robust method, and evaluations of possible experimental errors were based on semi-quantitative studies of homogeneous and heterogeneous systems. The MRI protocol provides complementary phase and magnitude data, which are related to the sample temperature and structural heterogeneity, respectively. Used together, they relate the temperature changes to the differential thermal properties of the various components within a heterogeneous sample. The potential applications of this technique to microwave and other forms of heating is discussed.

Variable frequency microwave heating of food.

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.