A detailed theoretical analysis has been carried out to study efficient microwave processing of 1D emulsion samples placed on ceramic plates (alumina, SiC). The effective dielectric property of an emulsion is a strong function of the continuous medium of the emulsion (o/w or w/o). A preliminary study has been carried out via average power within an emulsion vs emulsion thickness diagram for various cases. The maxima in average power, also termed as 'resonances', are observed for specific emulsion thicknesses and the two consecutive resonances are termed as R1 and R2 modes. The heating scenarios have been analyzed at the dominant resonance mode. Based on spatial distributions of power and temperature for various cases, alumina support at the left side may be recommended as the optimal heating strategy due to greater heating rates with controlled thermal runaway for both o/w and w/o emulsion samples whereas SiC support may be favored for o/w emulsion samples due to lesser thermal runaway. A comparison of the heating characteristics has been illustrated for 50% o/w and 50% w/o emulsion samples to analyze the role of continuous medium of an emulsion on heating effects. The distribution of microwave incidences also plays an important role to minimize thermal runaway for specific o/w and w/o emulsions. © 2008 Elsevier Ltd. All rights reserved.

Tanmay Basak

Department Of Chemical Engineering

Emulsions

Heating rate

Microwave heating

Resonance

Temperature distribution

Ceramic plates

MICROWAVE PROCESSING

Water-in-oil emulsion

Ceramic materials

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Food Research International

Microwave heating is generally performed by positioning the sample within a container. The container can reflect, absorb or transmit microwaves based on the dielectric properties and that can influence the microwave heating characteristics of the sample. This work is an attempt to theoretically analyze the alteration of the microwave heating characteristics of materials due to the use of either a low-lossy alumina container or a high lossy SiC container. The heating characteristics have been simulated for the high-lossy beef and low-lossy bread samples of a fixed dimension by solving the coupled energy balance equation and detailed Helmholtz wave propagation equations within the sample-container assembly. It has been shown that the microwave heating characteristics can be significantly altered in the presence of the container based on the relative dielectric properties of the materials. The alumina container has been found to be efficient to enhance the microwave heating efficiency of the high lossy material such as beef, while the rapid microwave heating of the SiC container has been found to be beneficial to enhance the heating of the low lossy material such as bread in some cases. © 2017 Elsevier Ltd

International Communications in Heat and Mass Transfer

A theoretical analysis on the effect of containers on the microwave heating of materials

Numerical studies have been carried out for microwave heating of food samples. It has been shown that the food materials can be divided into four groups based on the values of fp and fw and the microwave heating characteristics remain same for the materials in each group. The heating characteristics of each group of materials have been presented based on the heating front, heating rate and heating nonuniformity (thermal runaway). These characteristics are shown to be strongly dependent on sample size, which can be classified as thin (uniform heating), intermediate (localized heating fronts driven be resonances of microwave power absorption) and thick (exponential attenuation of heating rate from the exposed surface). The analysis shows that the localized heating in intermediate and thick samples can lead to significant thermal runaway, which can be efficiently avoided by selecting radial irradiation over lateral irradiation but at the expense of higher processing time. © 2015 Elsevier Ltd. All rights reserved.

Innovative Food Science and Emerging Technologies

A generalized approach on microwave processing for the lateral and radial irradiations of various Groups of food materials

Microwave heating has vast applications in the field of food processing such as cooking, drying, pasteurization and preservation of food materials. In this article, various applications of microwave food processing such as microwave cooking, microwave pasteurization and microwave assisted drying were extensively reviewed. The advantages and the factors affecting the microwave cooking of food materials have been reviewed. Microwave pasteurization of fresh juices, milk and various food products has been elaborately discussed. Microwave pasteurization has the ability to achieve destruction of microorganisms at temperatures lesser than that of conventional pasteurization due to significant enhancement or magnification of thermal effects. Applications of microwave drying include microwave assisted hot air drying, microwave vacuum drying and microwave freeze drying. Microwave drying combined with other conventional methods of drying enhances the drying characteristics of the sole effect of microwave drying. Modeling of microwave heating of food materials based on Maxwell's equations and Lambert's law equations have been reviewed along with their applications. Microwave modeling can be used to predict the temperature and moisture distributions during microwave heating of food materials. © 2013 Elsevier Ltd.

Microwave food processing-A review

Microwave heating is caused by the ability of the materials to absorb microwave energy and convert it to heat. This article represents a review on fundamentals of microwave heating and their interaction with materials for various applications in a comprehensive manner. Experimental studies of single, multimode, and variable frequency microwave processing were reviewed along with their applications. Modeling of microwave heating based on Lambert's law and Maxwell's electromagnetic field equations have also been reviewed along with their applications. Modeling approaches were used to predict the effect of resonances on microwave power absorption, the role of supports for microwave heating, and to determine the nonuniformity on heating rates. Various industrial applications on thermal processing have been reviewed. There is tremendous scope for theoretical and experimental studies on the athermal effects of microwaves. Some of the unresolved problems are identified and directions for further research are also suggested. © 2011 American Institute of Chemical Engineers (AIChE).

AIChE Journal

Microwave material processing-a review

A detailed analysis has been carried out to study efficient heating due to microwaves for one-dimensional samples placed on ceramic supports (Al2 O3, SiC). The greater effects on microwave heating of samples have been illustrated via average power within a sample versus sample thickness diagram for various cases. The maxima in power, also termed as "resonances," is observed for specific sample thicknesses and the two consecutive maxima in average power are termed as R1 and R2 modes. The greater heating effects leading to hot spots would occur in water samples during both-sides incidence when the sample is kept on Al2 O3 support. SiC support may be recommended for water samples due to uniform heating throughout the sample. In contrast, SiC support could cause local hot spots or thermal runaway for oil samples. The localized hot spots are more pronounced for the samples exposed to microwaves on both faces. The choice of support may not be trivial due to the complex dielectric response of sample-support assembly. Current analysis has been illustrated for low- and high-dielectric materials (water and oil) and a representative case study has also been shown for beef samples. Based on such observations, a generalized heating strategy for materials due to uniform plane waves has been derived. © 2005 American Institute of Physics.

Fulltext

Journal of Applied Physics

Role of ceramic supports on microwave heating of materials

A detailed analysis on enhanced heating effects attributed to resonance of microwaves has been carried out to study the efficient heating methodologies for oil-water emulsions. Studies on heating have been carried out for samples incident with microwaves at one side and at both sides. The maxima in average power corresponding to resonances occur at various sample thicknesses for all emulsions and we consider two dominant resonance modes R1 and R 2, where the average power at R1 is larger than that at R2. During one side incidence, it is observed that processing rates are greater at the R2 mode for both oil-in-water (o/w) and water-in-oil (w/o) emulsions, whereas for both-sides incidence, the R 1 mode is favored for o/w emulsion and the R2 mode is advantageous for w/o emulsion. The greater rates in thermal processing are observed when the emulsions (o/w and w/o) are incident at both sides. Current analysis recommends on the efficient way to use microwaves in a customized plane-wave oven for greater heating effects and determines the optimal sample thicknesses vs. various oil-water contents for thermal processing. The emulsion system is a combination of materials with very low dielectric loss (oil) and high dielectric loss (water), and the effective dielectric response is highly nontrivial to predict the efficient way to heat an emulsion, either o/w or w/o. © 2004 American Institute of Chemical Engineers.

Role of resonances on microwave heating of oil-water emulsions

Resonances or maxima in power absorption due to microwaves incidence within a slab are analyzed via transmitted and reflected waves. A generalized mathematical formulation for uniform plane waves has been established to analyze traveling waves, stationary waves and microwave power absorption within a multiphase sample. Preliminary studies based on the generalized mathematical analysis in ice and water slabs illustrate that greater amplitudes of traveling and stationary waves occur within ice samples, whereas, greater intensity of spatial resonances in microwave power occurs for water samples due to a greater dielectric loss of water. Microwave thawing is studied for specific sample thicknesses which are selected based on greater power distribution within water samples. The enthalpy method is employed for modeling of thawing of ice samples where a superficial mushy region is assumed around the melting point. Depending on the sample thickness, thawing may occur from the unexposed face as well as both the faces when the sample is exposed to microwaves at one face only, whereas thawing may originate from both the center as well as the faces when the sample is exposed to microwaves at both faces. Our analysis based on the generalized mathematical formulation validates the local maxima in spatial power distribution during intermediate thawing stages obtained with the finite element based enthalpy formulation. The generalized mathematical analysis on multiple thawed regimes further illustrates the role of traveling waves on resonances in microwave power. The influence of resonance is attributed by a non-monotonic variation of thawing time with sample thicknesses either for one side incidence or both side incidence due to microwaves. Optimal thawing strategies are recommended based on greater power savings. © 2003 Elsevier Ltd. All rights reserved.

Journal	Food Research International
ISSN	09639969
Open Access	No