Ashim K. Datta

Handbook of microwave technology for food applications

Part 1 Fundamental physical aspects of microwave absorption and heating: electromagnetics - fundamental aspects and numerical modelling electromagnetics of microwave heating - magnitude and uniformity of energy absorption in an oven dielectric properties of food materials and electric field interactions fundamentals of heat and moisture transport for microwavable food product and process development. Part 2 Chemical and biological changes due to heating: generation and release of food aromas under microwave heating bacterial destruction and enzyme inactivation during microwave heating. Part 3 Processing systems and instrumentation: consumer, commercial and industrial microwave ovens and heating systems measurement and instrumentation. Part 4 Processes at industry and home: microwave processes for the food industry basic principles for using a home microwave oven. Part 5 Product and process development: ingredient interactions in microwave heating packaging techniques for microwavable foods. Part 6 Safety: safety in microwave processing.

Moisture transport in intensive microwave heating of biomaterials: a multiphase porous media model

Abstract A multiphase porous media model was developed to predict moisture transport during intensive microwave heating of biomaterials. Internal pressure gradients arise from internal heating and vaporization and significantly enhance the transport. Even a moderate internal pressure gradient in a low moisture material can cause more moisture to move to the surface than can be removed from the surface, leading to a soggy surface. A strong internal pressure gradient in a high moisture material can fully saturate the surface, leading to a very high moisture loss by liquid outflow at the surface. Addition of hot air and⧹or infrared heating of the surface to increase evaporation is suggested as a way to reduce surface moisture build-up.

Infrared and hot-air-assisted microwave heating of foods for control of surface moisture

Temperature and moisture profiles for infrared and hot-air-assisted microwave heating of food were studied using a multiphase porous media transport model for energy and moisture in the food. Microwave-only heating results in surface moisture build-up due to enhanced (pressure-driven) flow of moisture to the surface and the cold ambient airs inability to remove moisture at a high rate. For foods in which infrared energy penetrates significantly, addition of infrared actually increases the surface moisture build-up. When absorbed mostly on the surface, infrared can reduce surface moisture and, beyond a threshold power level, it can reduce the surface moisture to lower than its initial value. Hot air also can reduce surface moisture and increase surface temperature, but not as effectively as infrared heat, perhaps due to a much lower surface heat flux for hot air compared to the infrared energy. Increasing air velocity and therefore the heat and mass transfer coefficients can eliminate the accumulated surface moisture.

Moisture, Oil and Energy Transport During Deep-Fat Frying of Food Materials

A multiphase porous media model has been developed to predict the moisture migration, oil uptake and energy transport in a food material such as a semi-dry potato during deep-fat frying. The model predictions are validated using experimental data from the literature. Spatial moisture profiles show two distinctive regions (dry or crust region near the surface and wet region in most of the core) with a sharp interface which can be referred to as the evaporation front. Spatial temperature profiles show two distinctive regions—higher but constant temperature gradient region near the surface, and lower temperature gradient region in the core. In the crust region, vapour diffusional flux is comparable with vapour convective flux. In the more moist core region, capillary flux of liquid water is comparable to the convective flux of liquid water. Therefore, all three modes of transport—diffusional, capillarity, and pressure driven (Darcy) flow are found to be important. Sensitivity of the final product moisture and temperature to changes in oil temperature, initial moisture content of the sample, thickness of the sample, and the surface heat and mass transfer coefficients are discussed.

Some Considerations in Modeling of Moisture Transport in Heating of Hygroscopic Materials

Abstract Hygroscopic materials are those in which the equilibrium pressure of water vapor changes with moisture content and temperature, such as food, soil or wood, etc. Heat and moisture transports are coupled in heating of hygroscopic materials. One of the major links between temperature and moisture changes is water evaporation. There have been different formulations on modeling of evaporation in the past. A typical approach (Model 1 in this article) is to equate the evaporation rate to the rate of local moisture loss. The first part of this paper illustrates that such an approach is physically incorrect based on fundamental conservation relationships. A conservation-based coupled heat and moisture transfer model (Model 2) is presented here based on previous multiphase transport models. It shows that total evaporation rate over the entire material is included in Model 1 while the local evaporation rate is not. The situations when Model 1 may or may not generate large errors are discussed. The second part of this article completes the modeling of evaporation using Model 2. Two types of formulations are given depending on the phase equilibrium of moisture in the hygroscopic materials. When phase equilibrium between water and vapor is assumed for any location at any time, vapor pressure is provided as known variables. In a nonequilibrium approach, evaporation rate needs to be provided. The latter poses numerical difficulties near the material surface, which arises from the possibility that equilibrium state may have a large change near the surface. Further discussions were made on the physical and numerical considerations in using both approaches.

Heat and Mass Transfer in Microwave Processing

Publisher Summary This chapter focuses on the heat and mass transfer in microwave processing. The heat and mass transfer characteristics of microwave processing systems are of considerable interest for materials development and process engineering. The ability of microwave radiation to penetrate and couple with materials provides an attractive means of obtaining controlled and precise heating. Microwave appliances have three major components—a microwave generator, a waveguide, and an applicator. It is important to appreciate the role of each component to understand how microwave ovens operate. Electrical properties dictate the nature of microwave-material interactions. Metals (electrical conductors) are excellent reflectors and are not, in general, heated significantly by microwaves. Electromagnetic field intensity and distribution are prime factors that determine microwave absorption. The unique character of heating in microwave processing can be understood only by appreciating how electromagnetic radiation propagates and is absorbed by materials. During microwave heating of porous materials, temperature gradients and internal evaporation can promote transport of mobile components (liquids and gases). As compared with conventional surface heating, microwaves can generally provide faster, selective, and more uniform heating. Microwaves offer many potential benefits for processing materials—time and energy savings, improved product quality, better process control, and new process applications.

Coupled Electromagnetic and Termal Modeling of Microwave Oven Heating of Foods

Temperature distributions from heating in a domestic microwave oven were studied by considering the coupling between the electromagrzetics and heat transfer through changes in dielectric properties during heating. Maxwell’s equations for electromagnetics and the thermal energy equations are solved numerically using two separate finite-element softwares. The coupling between the softwares was developed by writing special modules that interfaced these softwares at the system level. Experimentally measured temperature profiles were compared with the numerical predictions. The importance of coupling was evident when the properties changed significantly with temperature for low and high dielectric loss materials and more so for the high loss materials. For moderate loss materials, when the properties do not change as much with temperature, coupled solutions lead to results very close to the results for the uncoupled solution.

Computation of airflow effects on heat and mass transfer in a microwave oven

Abstract The magnitude of surface heat and mass transfer coefficients in microwave ovens is important to control food surface temperature and moisture and are a result of the faint airflow present in the oven cavity and of surface radiation. Magnitude and patterns of airflow inside a microwave oven and the resulting surface heat transfer coefficients were studied using a computational fluid dynamics model of the process. The governing Navier–Stokes and energy equations were solved for both natural, forced and combined convection. The magnitude and distribution of surface heat transfer coefficients on the food surface were computed for a 3-D oven cavity with one inlet and one outlet and a cylindrical food placed inside the oven. Calculated convective heat transfer coefficient values were found to be in the same range as has been used in the literature. A combined convection regime proves beneficial for heat transfer uniformity and the reduction of moisture accumulation inside the oven. Radiation heat transfer coefficients for energy exchange between food surface and oven interior were calculated and shown to be of the same order of magnitude as the convection heat transfer coefficients.

Modeling Transport in Porous Media With Phase Change: Applications to Food Processing

Fundamental, physics-based modeling of complex food processes is still in the developmental stages. This lack of development can be attributed to complexities in both the material and transport processes. Society has a critical need for automating food processes (both in industry and at home) while improving quality and making food safe. Product, process, and equipment designs in food manufacturing require a more detailed understanding of food processes that is possible only through physics-based modeling. The objectives of this paper are (1) to develop a general multicomponent and multiphase modeling framework that can be used for different thermal food processes and can be implemented in commercially available software (for wider use) and (2) to apply the model to the simulation of deep-fat frying and hamburger cooking processes and validate the results. Treating food material as a porous medium, heat and mass transfer inside such material during its thermal processing is described using equations for mass and energy conservation that include binary diffusion, capillary and convective modes of transport, and physicochemical changes in the solid matrix that include phase changes such as melting of fat and water and evaporation/condensation of water. Evaporation/ condensation is considered to be distributed throughout the domain and is described by a novel nonequilibrium formulation whose parameters have been discussed in detail. Two complex food processes, deep-fat frying and contact heating of a hamburger patty, representing a large group of common food thermal processes with similar physics have been implemented using the modeling framework. The predictions are validated with experimental results from the literature. As the food (a porous hygroscopic material) is heated from the surface, a zone of evaporation moves from the surface to the interior. Mass transfer due to the pressure gradient (from evaporation) is significant. As temperature rises, the properties of the solid matrix change and the phases of frozen water and fat become transportable, thus affecting the transport processes significantly. Because the modeling framework is general and formulated in a manner that makes it implementable in commercial software, it can be very useful in computer-aided food manufacturing. Beyond its immediate applicability in food processing, such a comprehensive model can be useful in medicine (for thermal therapies such as laser surgery), soil remediation, nuclear waste treatment, and other fields where heat and mass transfer takes place in porous media with significant evaporation and other phase changes.

Composition-based prediction of dielectric properties of foods.

Prediction of accurate dielectric property data from fundamental principles for systems as complex as foods has not been possible. Simple prediction models based on easily measurable composition data can serve many useful purposes. Literature dielectric data on foods and their composition were statistically correlated. Dielectric data on salt solutions were measured to explain some of the results. When composition data were not available, standard handbook compositions were used. Inclusion of all types of foods (meats, fruits, and vegetables) inhibited any useful correlation with composition. Based on a smaller data set of meats, both dielectric constant and loss increased with water and salt content. Dielectric constant generally decreased with temperature whereas dielectric loss decreased with temperature at lower salt concentrations and increased with temperature at higher salt concentrations.