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The goal of this project is to test the microwave and ohmic combination heating technology for heating uniformity of particulate foods. Combining two heating technologies to eliminate the downsides would be beneficial for uniform heating of multiphase foods since it can allow liquid parts of the product to be heated immediately via current and solid particles, independent of their conductivity values, can be heated to the cores very rapidly via microwave. <P>
Specific research objectives addressed in this project are as follows: <OL type="A"> <LI> Design a continuous flow microwave and ohmic combination heater. <LI> Explore the heating patterns of solid-liquid mixtures in a combined heater. The project director (PD)'s lab is equipped with a 5kW custom-designed dual magnetrons microwave heater and a 2kW pulsed ohmic heater. The proposed combination of independent two units will include a series of helical coil heat exchangers and non-conductive ohmic chamber with impedance matching and circuit design as needed. Temperature values of solid particles (i.e. carrot, potato, and meat cubes) and liquid carrier medium via the influence of microwave and ohmic frequencies will be measured and analyzed. <LI> Measure dielectric properties and electrical conductivities of solid-liquid mixtures. <LI> Develop the flow and heat transfer model for combined microwave and ohmic heating system. Maxwell's equations governing the electromagnetic field distribution will be solved using the finite element method (FEM) based commercial code. The solution will be coupled with the flow and thermal modules to solve the momentum and heat equations using the computational fluid dynamics (CFD) codes. <LI> Calculate and compare the efficiency of energy conversion for combined two heating technologies. Microwave heating which is alone poor energy efficient will be complemented by the use of ohmic technology since the energy transfer efficiency of ohmic process is close to 100%. <LI> Estimate metal ions migration and fouling occurrence in foods under the ohmic treatment. Concentration of Fe and Cr migrated into the heating media taken as measures of electrode corrosion will be estimated using inductively coupled plasma - mass spectrometer (ICP-MS). Fouling mass in a dry form after treatment will be collected by rubbing off and weighed from the electrode. </OL> The expected outputs are: <OL> <LI> Uniform heat distribution for multiphase foods will be obtained by the proposed technology.<LI> Microwave and ohmic combination heating patterns of particulate foods will be predictable using numerical modeling based on computational fluid dynamics (CFD). <LI> Combined short-term volumetric heating will effectively minimize undesired electrochemical reactions at the electrode interfaces. <LI> Combined two technologies will increase the overall energy efficiency by 20%. <LI> Combined two technologies will enhance food qualities, compared to the control. <LI> The project outcomes found in laboratory are highly likely to find their applications on a massive scale in numerous segments of food and the US industry.
NON-TECHNICAL SUMMARY: The key objectives of this Research Strengthening Standard Grant proposal are (1) to design a continuous flow microwave and ohmic combination heater for particulate foods and (2) to test for its heating uniformity and food qualities in terms of sensory values and lethal sterility. Uniform heating of particulate foods (i.e. soups containing vegetables and meats) has been a challenge because temperatures of solid cores could lag behind that of liquid. Non-uniform heat treatment of particulate foods would allow for inadequate microbial destruction and therefore, the full recovery of foodborne pathogens, in turn threatening the public health. Instant and volumetric heating of microwave and ohmic technologies are outstanding for food quality preservation; however, microwaves can lead to partial over-heating and cold spots, whereas ohmic heating requires additional blanching pretreatment of a product to obtain desired electrical conductivities for heating uniformity. To eliminate individual technology limitations, it is proposed to combine both microwave and ohmic heating technologies in such that solid particles with lower electrical conductivities will be heated via microwaves with strong penetration depth, and a carrier medium with higher salt concentrations will be treated by ohmic current. As the first attempt to design a microwave and ohmic combination heater in a continuous mode, this approach will effectively address one of the program FY 2009 Priorities, 2. Generating the knowledge base for advanced and innovative processing, engineering, and technologies that enhance food quality attributes and development and application of analytical characterization techniques of physical, chemical, biological, and sensory natures.
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APPROACH: Objective A: Design a continuous flow microwave and ohmic combination heater. Microwave cavities that will be modified from the existing unit in the PD's lab will be combined with an ohmic chamber where solid-liquid mixture will flow under the influence of external electric field strength between the two electrodes. The diameters of coil typed heat exchangers will be determined based on the sizes of particles entering into the unit. Impedance matching between the microwave power source and loaded cavity needs to be carried out to minimize wave reflection. Objective B: Explore the heating patterns of solid-liquid mixtures in a combined heater Temperature values of solid particles (i.e. carrots, potatoes, and meats) and liquid medium via microwave with pulse width modulation (PWM), and ohmic frequencies will be measured and analyzed. Carrot, potato, and beef cubes will cut to approximately 1.5 cm each side. Each sample will be stored in a refrigerator prior to use in the experiments. The PD proposes that 2% NaCl solution and 1% carboxymethylcellulose (CMC) solution with 2% NaCl could be good candidates as carrier media. Objective C: Measure dielectric properties and electrical conductivities of solid-liquid mixtures. Dielectric properties of the solid particles and carrier medium will be measured using an open-ended coaxial probe (Model Agilent 85070E, Agilent Technologies) connected to a network analyzer. Electrical conductivities of solid particles and carrier fluid will be calculated from the resistance of samples and the geometry of the ohmic cell. Objective D: Develop the flow and heat transfer model for combined microwave and ohmic heating system. The flow pattern of multiphase foods in a combined microwave and ohmic heating unit can be solved by means of a general purpose CFD code, i.e. Fluent (Fluent, Lebanon, NH) that is based on the finite volume method (FVM). We also need separate commercial code, COMSOL (COMSOL, Inc., Burlington, MA) to get the power density term as a function of time. The approach used in coupling microwaves with heat transfer is to solve Maxwell equations inside a microwave cavity (COMSOL) and then couple the solution with the thermodynamic and hydrodynamic modules to solve for temperature and flow patterns of solid-liquid mixtures (Fluent) Objective E: Calculate and compare the efficiency of energy conversion for combined two heating technologies. Input power to run for (1) microwave, (2) ohmic and (3) combined microwave and ohmic heating units will be separately measured using a wattmeter. The energy conversion efficiency will be calculated based on the amount of energy accumulated in solid particles and carrier medium. Objective F: Estimate metal ions migration and fouling occurrence in foods under the ohmic treatment. Concentration of Fe and Cr migrated into the heating media taken as measures of electrode corrosion will be estimated using inductively coupled plasma - mass spectrometer (ICP-MS, available in Agricultural Diagnostic Service Center, University of Hawaii).