Microwave foods dielectric properties

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

The dielectric properties of water have been extensively used to determine moisture content in food systems. However, only veiy recently have we used the complex dielectric properties of emulsions in the microwave frequency region to characterize both emulsion type and water content [50-52], We have developed both a cavity resonance dielectrometer capable of operating at 8-11 GHz and an interference dielectrometer operating at 23.45 GHz. 

Mathematical equations, presented by Maxwell in 1864, are able to predict the behavior of microwave radiation s interaction with any type of food in any geometry. In order to do this, a single pair of parameters describing the electrical (or dielectric) properties of the food are required. This pair of parameters is known as the complex permittivity, or as is more commonly called in the United States, the complex dielectric constant. This parameter pair is defined as ... 

Solid food materials have dielectric properties dependent upon their composition. In many instances, particularly when developing microwavable food products, it is necessary to know the effective bulk microwave properties of the product, crushed, as is, or when agglomerated together. Typical examples are peas, beans, com, pasta, flour... 

Early work using microwaves as a diagnostic tool relied upon measuring a secondary effect of the dielectric properties of the material under interrogation, i.e., reflection, absorption and transmission. The two fundamental microwave parameters, e and e" are related to the food or material composition. These two fundamental parameters also determine the reflection, absorption and transmission of the materials exposed to a microwave signal. Thus by measuring the amplitude and phase of the reflected or transmitted wave, or the characteristics of absorption of a wave through the material, one is able to empirically establish a relationship to the constituency of the product. 

Bengtsson, N. and Risman, P. 1971. Dielectric properties of foods at 3 GHz as determined by a cavity perturbation technique. Measurement on food materials. Journal of Microwave Power. 6(2) 107-123. 

Engelder, D. and Buffler, C. 1991. Measuring dielectric properties of food products at microwave frequencies. Microwave World. 12(2) 6-15. 

Mudgett, R. 1985. Dielectric properties of foods. In Microwaves in the Food Processing Industry. R. Decareau (ed.), pp. 15-37. Academic Press. New York, NY. 

Mudgett, R., Goldblith, S., Wang, D., and Westphal, W. 1977. Prediction of dielectric properties in solid food of high moisture content at ultrahigh and microwave frequencies. Journal of Food Processing and Preservation 1 119-151. 

There is little available literature on the interaction of flavor components with food systems during microwave heating. However, numerous authors have reported on the dielectric properties of nonflavor food ingredients during microwave processing (1,2,3,4). 

The dielectric properties of water and oil differ radically. A high water concentration in food systems greatly increases its dielectric properties. Oil, however, contributes relatively little to the dielectric behavior of a food system (1). Consequently, in the 90/10 oil/water mixture, the microwave energy was directed primarily at the 10% aqueous phase. Acids added to this 90/10 mixture will partition into this aqueous phase to the extent of their relative solubility in the two phases. Greatest losses were observed for acetic acid which exhibits the greatest solubility in water and was concentrated in the aqueous phase. Losses of the more nonpolar acids, i.e. caproic, were also much greater in microwave samples. Losses of the relatively... 

Table VI summarizes the effect of heating medium on the loss of acids after 3 minutes of microwave heating. Loss of volatile acids varied widely dependent on the microwave medium. Acetic and caproic acids had losses ranging from 20-80% and 0-73%, respectively, depending on medium composition. The dielectric property, specific heat, or other physical/chemical properties of individual flavor compounds can provide valuable insight into the potential behavior of these compounds during the microwave process. The dielectric property of the total food system and the affinity of the flavor compound for the microwave medium, however, were primarily responsible for the behavior of these flavor compounds during microwave heating.

Resonant cavities are designed to increase the apparent interaction between microwaves and the sample in order to induce measurable attenuation, and are thus particularly useful for measuring the dielectric properties of low loss materials such as fatty foods (Roussy and Pearce, 1995). 

Food materials are poor electric insulators and can store and dissipate energy when exposed to microwaves. Dielectric properties describe an interaction of an electromagnetic field with non- or low-conducting matter. The dielectric properties data are very limited in literature and usually available only for a few foods or food components. Wang et al. (2008), Tanaka et al. (2008), and Liao et al. (2001, 2002, 2003) have conducted experiments to get a better understanding of the dielectric properties of various food products. The dielectric properties of some food materials are shown in Table 3.1 (Regier and Schubert, 2001). 

Herve, A. -G., J. Tang, L. Luedecke, and H. Peng. 1998. Dielectric properties of cottage cheese and surface treatment using microwaves. Journal of Food Engineering 37 389-410. 

Roebuck, B. D., S. A. Goldblith, and W. B. Westphal. 1972. Dielectric properties of carbohydrate-water mixtures at microwave frequencies. Journal of Food Science 37 199-204. 

Tanaka, R, T. Uchino, D. Hamanaka, G. G. Atungulu, and Y.-C. Hung. 2008. Dielectric properties of mirin in the microwave frequency range. Journal of Food Engineering 89 435 0. 

Venkatesh, M. S. and G. S. V. Raghavan. 2004. An overview of microwave processing and dielectric properties of agri-food materials. Biosystems Engineering 88 1-18. 

Wang, Y, T. D. Wig, 1. Tang, and L. M. Hallberg. 2003. Dielectric properties of foods relevant to RF and microwave pasteurization and sterilization. Journal of Food Engineering 57 257-268. 

Tang J (2005) Dielectric properties of foods. In Schubert H, Reiger M (eds) The microwave processing of foods. Woodhead, Cambridge... 

Wang, R Zhang, M., Mujumdar, A. S Sun, J. C Jiang, H., 2011a. Effect of salt and sucrose content on dielectric properties and microwave freeze drying behavior of restructured potato slices. /. Food Eng. 106 290-297. 

The mote general food processing appHcations requite data on dielectric and thermal properties (139). Considerable effort has been expended by food companies in the design of food for the microwave oven. These principles have been reviewed (140). The microwave oven at 2450 MH2, used for reheating, cooking, and thawing foods, may also be used for drying (qv), eg, flowers or food materials (141). Commercial microwave ovens ate used extensively in restaurants and fast-food estabUshments. 

Buffler, C. and Stanford, M. 1991. The effects of dielectric and thermal properties on the microwave heating of foods. Microwave World 12(4) 15. 

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