Microwave assisted thermal processing of homogeneous and heterogeneous food packed in a polymeric container

Fermin Pangilinan Resurreccion

Consumption of ready-to-eat food has steadily increased over the last decade. This development can be attributed to the fast pace of the modern lifestyles. This trend along with recent recalls caused by outbreaks of microwavable convenience food has forced government regulatory agencies such as the Food and Drugs Administration (FDA) of the United States to pay closer attention and to provide strict regulatory standards to the production of safe ready-to-eat foods. Although major efforts have been made to develop alternative non-thermal processes, heat is still the most efficient and effective means for commercial production of shelf stable foods. In theory, application of heat destroys or inactivates pathogens and microorganisms. Conventional application of thermal energy from sources such as steam or hot water requires a prolonged time of exposure of the prepackaged food which subsequently results in the destruction of heat sensitive nutrients. Microwave sterilization is a new and emerging technology that provides faster heat penetration and can significantly reduce the degradation of heat sensitive nutrients. A four-cavity microwave assisted thermal sterilization (MATS) system was developed at Washington State University (WSU). The system combines traditional hot water heating in pressurized cavities with microwave heating in order to sterilize food packed in polymeric packages. The system is accepted by the FDA for commercial sterilization of homogeneous and heterogeneous foods.Being a novel technology, several aspects that might have an influence on the overall utilization of the technology still remain unresolved as far as research is concerned. They include: (1) sensitivity of the system on dielectric property of both food and circulating water inside the cavity, (2) overall heat transfer coefficient between food and circulating water inside the cavity, (3) the effect of frequency shift as a result of continuous use and aging microwave generators on stability of heating patterns, and (4) reduction of reflected power as a result of impedance mismatch between microwave generator (source) and microwave cavity (load). These aspects are the focus of this dissertation. The dissertation is arranged as follows: Chapter1 and Chapter 2 discuss relevant concept of microwave propagation inside waveguide and cavities and how microwave energy penetrates food materials and is subsequently converted into heat. Fundamentals of Maxwell's equations, power conversion, and electromagnetic-heat transfer solution through finite-difference time-domain (FDTD) are among the highlights of these two chapters. Chapter 3 outlines the steps and procedure for creating a computer simulation model that would theoretically describe the microwave system. Electromagnetic field distribution, power dissipation into heat, and the resulting heating patterns in foods were obtained from the computer simulation model. Results were validated experimentally through the chemical marker method. Chapter 4 centers on discussion of the operating frequency of the generators and how it affects the heating patterns in food. In Chapter 4, the computer simulation model created in Chapter 3 was fully utilized. Operating frequencies of the four generators powering the microwave system were evaluated considering the frequencies of generators manufactured by two different companies, repeatability of measured frequencies over time, and dependency of frequencies with power setting on generators over a period of one year. Heating patterns in foods were then simulated considering the Federal Communication Commissions (FCC) allocated frequency bandwidth for industrial, scientific, and medical (ISM) purposes. Chapter 5 discusses efforts to improve the efficiency of the microwave system through impedance matching using a 3-probe tuner along waveguide. The strategy is Chapter 5 was to reduce power reflection by incorporating variable and controllable inductive elements (3-probe tuner) that would balance the impedance mismatch between the generators and the cavities. Chapter 6 which supports Chapter 7 shows how dielectric properties of precooked salmon were established as affected by marinating condition, precooking temperature and precooking time. Chapter 7 discusses the influence of inherent variation in dielectric properties of salmon to heating pattern and location cold spot and its implication on sterilization value calculation. Finally Chapter 8 summarizes and provides an insightful overview of our overall conclusions and recommendations for future study.In theory, application of heat destroys or inactivates pathogen and other microorganism that can cause food spoilage. Conventional application of heat such as those from steam or hot water requires a prolong time of exposure which subsequently results in the destruction of heat sensitive nutrients. Microwave sterilization is a new and emerging technology that provides faster rate of heat penetration and can significantly reduce the degradation of heat sensitive nutrients. A four-cavity microwave assisted sterilization thermal sterilization (MATS) system was developed at Washington State University (WSU). The system combines both traditional hot water heating in a pressurized cavity and microwave heating to sterilize food packed in polymeric trays. The system is FDA approved for commercial sterilization of homogeneous and heterogeneous food.Being a novel technology, several aspects that might have an influence on the overall utilization of the technology still remain uncharted as far as research is concern. Included are; (1) sensitivity of the system on dielectric property of both food and circulating water inside the cavity, (2) overall heat transfer coefficient between food and circulating water inside the cavity, (3) the effect of frequency shift as a result of continuous used and aging microwave generator, and (4) reduction of reflected power as a result of impedance mismatch between microwave generator (source) and microwave cavity (load). These aspects are the focus of this dissertation.

Microwave assisted thermal processing of homogeneous and heterogeneous food packed in a polymeric container

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