HEATING BEHAVIOR STUDY OF LOW MOISTURE FOODS IN RADIO FREQUENCY TREATMENTS
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Radio frequency (RF) heating is considered as a promising technology for pathogen control in low moisture foods because of its advantages of volumetric and fast heating. However, the non-uniform heating problem hinders its application in food industry. It is important to thoroughly understand the RF heating mechanisms and propose solutions to improve heating uniformity of food in RF pasteurization. Peanut butter was selected as a major example of a low moisture food in this study.Firstly, the physical, thermal, dielectric properties of peanut butter samples were determined at different frequencies (10-1800 MHz) and temperatures (20-90 ºC). Results showed RF energy was applicable for peanut butter pasteurization since its penetration depths at 27.12 MHz were >10 cm. The relationship between dielectric properties of foods and the heating rate in RF systems was expressed in a mathematical model and validated by experiments. Results indicated that the maximum heating rate in foods can be reached when the values of dielectric constant and loss factor were close to each other. As a next step, a computer model was developed with COMSOL Multiphysics®. The model described peanut butter in a cylindrical jar subjected into a 6 kW 27.12 MHz RF system with appropriate initial and boundary conditions to predict the heating pattern in foods.The validated model was used to explore heating uniformity improvement methods for low moisture foods. Surrounding the peanut butter sample with polyetherimide (PEI) sheets, which has a closer dielectric constant to peanut butter but a smaller loss factor, was found to be an effective method in heating uniformity improvement. The method was also validated on wheat flour. Another heating uniformity improvement strategy was tested by placing a pair of PEI blocks at the cold spots of peanut butter samples to aggregate electromagnetic energy and improve heating uniformity. Computer simulations were conducted to find the optimum size of PEI blocks. A combination of the two methods was found to be more efficient than any single one in heating uniformity improvement.This research contributes knowledge to improve low moisture foods heating uniformity in RF treatments for designing RF pasteurization process.