Effects of Dielectric Properties on Temperature Distributions in Food Model during Microwave Heating.

Abstract

To investigate heat transfer phenomena in cylindrical foods of different dielectric properties with microwave heating, the dielectric constant, Ioss factor and the temperature distributions of samples were measured. The temperature distributions changed with their dielectric properties (dielectric constant, Ioss factor and penetration depth). As the penetration depth increased, the region of high temperature moved from the surroundings of the cylinder to the center. To describe these phenomena theoretically, the temperature distributions in the samples were calculated under the same conditions as those in the experiments using the mathematical model. The calculated results agreed closely with the experimental values. Microwave heating has a tendency to heat foods unevenly causing problems with both sensory and microbiological quality; therefore, control of heating uniformity is essential. The prediction of temperature distributions in dielectric materials heated by microwave radiation has been reported. G.W. Padua developed a mathematical model for predicting the temperature profiles of cylindrical samples of agar gels containing sucrose heated by microwaves (Padua, 1993). Nykvist and Decareau (1978) developed a two-dimensional mathematical model to simulate microwave cooking of cylindrical meat roasts and have demonstrated good agreement with limited experimental data. Ohlsson and Risman (1978) studied the temperature distribution with hot and cold spot in spheres and cylinders using an infrared thernrograph technique and using computer simulations in calculations. In those studies, the incident angle of the microwave was considered only towards the center of cylinder, so that the calculated temperature at the center became higher than those in the experiment. In our previous studies (Cheng et al, 1996), the temperature distributions of a flat cylinder were calculated numerically by fundamental equations with consideration of the incident angle of microwave energy. The calculated temperature distributions that varied with the diameter of the cylinder agreed closely with the measured values. However, in spite of the fact that the dielectric properties of food affect the internal heat generation, Iittle study has been reported about temperature distributions in cylindrical samples with different dielectric properties. The object of this study is to examine the formation of temperature distributions related to dielectric properties within cylindrical foods. In this study, cylindrical samples were covered by metallic shields on both ends to simulate one-dimensional heating (Padua, 1993; Prosetya & Datta, 1991; Ho & Yam 1992; Mudgett, 1986), as basic research for the study of two-dimensional heating. Mathematical Model During microwave heating, the microwave energy was considered to penetrate into the sample and to be converted to heat energy throughout the heated body, as shown in Fig. l. It was assumed that the thermal properties of the samples were constant and that the samples were thermally insulated during the heating period. A one-dimensional heat transfer equation with a term for internal heat generation, which can be modeled in terms of dielectric properties, is represented as the following, O~ r ~ R~ a2 T +1L aT +~ c! at~ r ar ,c ar2 where q is the internal heat generation. The details of the method to calculate the heat generation, q, were described in the previous paper (Cheng et al, 1996). The dielectric properties, particularly the penetration depth, notably affect the value of q. Initial condition and boundary conditions are represented by Eqs. (2)-(5). I.C.: B. C. : Ri and Ro are container, respectively. difference approximation, were calculated along the radial sarnple. t=0; O~r~R~; T=T (2) r=0' aT =0 (3) ' ar r=R ' aT O (4) " ar Ri