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SUMMARY— The finished drying of potato chips represents the first large‐scale use of microwaves in the food industry on a continuous basis. Hence it was deemed desirable to study the dielectric properties of various types of commercial cooking oils. The dielectric constant (ɛ), and loss tangent (tans) of 11 commercial fats and oils have been determined at three different temperatures and at three different frequencies in the microwave range. The differences in dielectric properties among these fats and oils appear to be attributable to the phase (solid vs. liquid) of the material and generally correspond to the degree of unsaturation as evidenced by iodine values. The differences in loss factors among these fats and oils at any given temperature and frequency (within the range at which the measurements were made) are too small to be of any practical importance in selecting any one of them for use in heating processes using microwaves or in choosing an optimal frequency (300, 1000 or 3000 megahertz, MHz). Data for one of the oils (number 9) were obtained over a wider range of frequencies and indicate that the loss factor peak(s) is/are found in the frequency range of 100 to 1000 MHz. Furthermore, from the data obtained for the other oils, it should be safe to assume that this oil is representative of dielectric properties of the entire group. The dielectric properties of bacon fat rendered by microwaves are almost identical to those of bacon fat rendered by conventional means.

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Recent studies have shown that dielectric properties of raw potato can be predicted over the range of 300–3000 Mhz and 5–65°C by a noninteractive Distributive model derived from lumped circuit analysis by a two‐phase approximation which treats the potato as a binary system consisting of an inert solid phase and an active liquid phase. Dielectric behavior was seen to result primarily from water and ion activities of aqueous regions but subject to appreciable modification by a mechanism of volume exclusion due to effects of colloidal solids. Cell‐free and whole potato extract measurements showed cation binding and complexing effects, resulting in considerably lower effective salts concentrations than implied by ash content. In addition, intracellular cation and biochemical constituent levels, were significantly higher than extracellular levels. However, dielectric behavior of aqueous regions of the potato appeared to be based on bulk average fluid properties subject to displacement by colloidal solids. Low‐frequency measurements of raw potato showed other regions of relaxation and conductivity effects rather than free water and bulk conductivity at low frequencies. But these appeared not to contribute to high‐frequency dielectric response of the potato since observed relaxations were of small magnitude or occurred at frequencies well below the ultrahigh and microwave regions, suggesting that surface properties of solid foods may not be of much significance at high frequencies. Preliminary analysis of solid food measurements by other workers suggests the feasibility of modelling solid food behavior by two‐phase approximations of Distributive, Maxwell or Rayleigh model behavior based on physical‐chemical properties. For example, raw beef measurements were predicted closely by a two‐phase model in Rayleigh form, suggesting modelling characteristics similar to the potato but with specific model behavior due to differencesin biological structure of beef and potato. A general physical‐chemical model is proposed for high‐frequency dielectric behavior of solid foods based on observed mechanisms of interaction between water and the biochemical constituents of foods.