Applications of computer simulation model in developing microwave assisted thermal processing
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Microwave heating is a novel technology that has advantages in reducing heating time and improving food quality. A 915 MHz single-mode microwave assisted thermal sterilization (MATS) system was developed at Washington State University with ultimate goal of industrial applications. This research addresses several engineering issues in future scaling-up of the MATS technology.A major challenge in developing microwave thermal processing has been non-uniform heating patterns which can result in cold and hot spots. The non-uniform heating pattern is caused by the uneven electric field distribution. In this study, computer simulation method was used to analyze the electric field distribution and its components in each direction within a MATS system. The dominant electric field component was in y direction which could be reformed by adjusting the dimensions of Ultem window and horn applicator in y direction or placing metal/Ultem bars within the horn applicators. In developing a microwave assisted thermal process, time-temperature profile at the cold spot is recorded to establish the process schedule to control most heat resistant food pathogens. Compared with fragile fiber-optic sensor, a mobile metallic temperature sensor is more suitable for continuous moving packages. However, interaction might occur between the metallic temperature sensor and the electromagnetic field. Thus another main objective of this work was to study the performance of a mobile metallic sensor within microwave environment. Results showed that a mobile metallic temperature sensor could be used for temperature measurement in the MATS system with suitable probe orientation. Certain design features, such as round probe tip and thinner probe diameter, could improve the sensor accuracy used in microwave environment. Dummy loads provide consistent dielectric properties for power delivery and system stability tests of a microwave heating system in production. The low cost bentonite water paste with high thermal stability was used to develop reusable dummy load for industrial microwave heating systems. Dielectric properties of bentonite pastes of various compositions were measured over 300 to 3000 MHz and 20 to 120 ºC. Vegetable oil and salt were used to adjust the dielectric constant and loss factor of bentonite pastes to match dielectric properties of different food materials.