Particle size behavior, dielectric properties, and electromagnetic modeling of aqueous starch suspensions during microwave heating

The general objective of this work was to study the behavior of starch in dilute aqueous suspension during microwave heating, while examining the effect of starch type and temperature on the dielectric properties of the suspensions.; In the first part of the study, the particle size behavior of starch in dilute aqueous suspension was measured in-line in a microwave-heated, well-mixed system. The heating rate was controlled to match the rate used in a previous study conducted with conventional heating in order to allow comparison between the results of both studies. For the starches used (common corn, waxy maize, and cross-linked waxy maize), no significant difference between the studies was found in the maximum diameter and the temperature of maximum swelling. The plots of diameter vs. temperature showed the same features for both microwave heating and conventional heating.; In the second part of this study, a method to measure the dielectric properties of dilute starch suspensions in-line during microwave heating was developed. The suspensions were heated from 30 to 80°C and the dielectric properties (epsilonr' and epsilonr") were measured at frequencies between 0.3 and 3 GHz. Additional experiments were conducted at frequencies between 45 MHz and 25.6 GHz. The results show that the dielectric properties of the dilute starch suspensions in general follow the properties of water, but starch lowered the dielectric permittivity (epsilonr') of water and increased its dipole rotation relaxation time (tau2). The dielectric properties of waxy maize suspensions with added sodium chloride suggest that starch decreases the effect of conductivity in this system. The results of these measurements were used to fit a model consisting of a Debye-Hasted model with an additional relaxation peak.; In the third and last part of this study, the dielectric properties measured in the second part were used in conjunction with a numerical method (finite difference time domain, FDTD) to model the electromagnetic fields and the SAR (specific absorption rate, a quantity directly related to the heat generation), inside a domestic microwave oven with a load of either water or starch suspension in a bowl.