Theoretical reasons for rapid heating of vegetable oils by microwaves
Xu Zhou, Yonas Gezahegn, Shuang Zhang, Zhongwei Tang, Pawan S. Takhar, Patrick Pedrow, Shyam S. Sablani, Juming Tang
Abstract
Water and high-moisture foods are readily heated in microwaves due to their relatively high dielectric loss factors. Vegetable oil, on the other hand, has a much smaller loss factor (about 1/100th that of water), and is generally believed to be unsuitable for microwave heating. In this study, we conducted experiments to compare heating rates between vegetable oil and water in a 2450 MHz microwave oven. We found that the vegetable oil samples were heated rapidly in microwaves, and even faster (1.4–2.0 times) than the water sample. To provide a theoretical explanation, we developed a 3-D computer simulation model. The simulation revealed an approximately 10-fold stronger electric field generated in oil compared to water, resulting in a similar amount of microwave power being absorbed by the oil and water samples. As the absorbed microwave power was converted into thermal energy, the oil samples were heated faster due to their smaller specific heat (1/2 that of water). In addition, we established a criterion for microwave heating of vegetable oil: when the dimensions of oil are smaller than half microwave wavelength, oil is heated slower than water due to the absence of hot spot areas, whereas larger dimensions lead to faster heating. This study provides a theoretical explanation for rapid microwave heating of vegetable oils and demonstrates opportunities for utilizing microwave energy to electrify industrial heating of vegetable oils.