Progress 05/07/01 to 05/06/08
Outputs
Sterilization of solid foods using microwave power was studied using numerical modeling and specialized experimental verification. Maxwell's equations and the heat conduction equation were coupled using two separate finite-element programs and specially written modules to couple the programs. Spatial distributions of thermal-time, representing sterilization, were calculated from time-temperature history and first order kinetics. Experimentally, concentrations of marker compounds formed during heating were measured and taken as indices of thermal-time. Experimental data on marker formation combined with numerical calculations provide an accurate and comprehensive picture of the sterilization process and represent a major step in establishing the efficacy of microwave sterilization processing. Unlike conventional sterilization, heating patterns can change qualitatively with geometry (shape and size) and properties (composition) of the food material, but optimal heating
is possible by choosing suitable combinations of these factors. Combined with marker yield measurements, the numerical model can give comprehensive descriptions of the spatial time-temperature history, and thus can be used to verify the sterilization process. Role of the shape of the food and its properties in the optimization of the microwave sterilization process was demonstrated.
Impacts
The concepts developed in this project led to myself becoming a partner in the million dollar industry-university-government consortium that is developing microwave sterilization for commercial food processing. Our ideas of how the microwave absorption in the food changes as the product heats up and how modeling to understand and optimize the process is central to this ongoing project.
Publications
- No publications reported this period
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Progress 01/01/02 to 12/31/02
Outputs
Microwave sterilization technology can be used to produce a broad range of high-quality shelf-stable food products. Shelf-stable foods require no energy for storage, less energy to prepare, and are convenient to use when compared to chilled and frozen foods. The control of temperature variations during microwave heating is of utmost importance to the successful development of microwave sterilization for producing high quality products. The heat generated by microwaves is non-uniform and therefore creates temperature gradients. A study of these gradients is required, as the major barrier to widespread application of microwave sterilization has been the lack of heating uniformity. The objective of this study is to obtain the temperature profiles in the food material (macaroni and cheese) as it undergoes microwave sterilization. Based on the temperature profiles the time for sterilization would be predicted and an optimum power cycle would be proposed. It's also an aim
to this project to determine the extent of influence each parameter has on the thermal process. A finite element implementation of the problem was carried out using a commercial CFD software package. The volumetric heat generated data is obtained from an electromagnetic simulation done beforehand, and the temperature contours obtained for different times of heating. Convective heat transfer of food with surrounding water is included in the model. For bacterial kinetics, first-order decay has been assumed and the F0 distribution inside the food is calculated. The temperature profiles show that there is more heating at the corners and the edges. The total time for sterilization is around 8 minutes, though the actual heating time required is less than 2 minutes. The water surrounding the food helps in equilibrating the temperature non-uniformity by cooling the surface. The hot spot moves from the surface of the food to the interior with time. F0 values at end of sterilization vary from
2.52 to 4.31. Sensitivity analysis shows that though the input parameters (like heat transfer coefficient, microwave power intensity and initial conditions) effect the come-up time and power cycling, they have insignificant effect on the time of sterilization. Preheating the food and using lower microwave power intensity helps in achieving greater uniformity in F0 values.
Impacts
The results confirm that non-uniformity of microwave heating is the single most important concern that microwave engineers have to deal with. The time for equilibration by diffusion heat transfer inside the food and convective heat removal from the surface is very slow and hence offsets the rapid heating advantage of microwaves. There is also a need to model the fully coupled electromagnetic and thermal problem to predict runaway heating effects. Development of better oven designs and novel process techniques is needed if we are to make microwave sterilization a mainstream process.
Publications
- No publications reported this period
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Progress 01/01/01 to 12/31/01
Outputs
Overall goal in this project is to develop a commercial microwave sterilization system that can deliver sterile foods of improved quality. My work is part of a million dollar effort (of Washington State University, WSU) funded jointly by the Department of Army and several industries. My specific goal is to see the food temperature and sterilization depends on various process parameters such as microwave deposition, food thickness and properties, surrounding conditions in the processing vessel. We have achieved quite a bit in the last seven months. Data provided by WSU on the rate of heating from electromagnetic calculations was used by us to obtain temperature profiles in the food. We obtained information on cold point, time-temperature histories and sterilization profiles in the food during heating. We considered insulated boundary (air surrounding the food) and convective boundary condition (water surrounding the food). We are now at the stage of optimizing the
heating rate so that the uniformity of heating is improved.
Impacts
The project's expected immediate impact is to provide our soldiers with improved quality rations that are easily stored.
Publications
- No publications reported this period
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