Dielectric properties of foods

Table 2. Dielectric Properties of Foods and Other Materials at 2450 MHz and 20°C...

This chapter introduces the reader to dielectric properties of foods and materials and then describes the technology, instrumentation and equipment advances which have applicability to the food industry. 

The temperature dependence of the dielectric properties of foods has been extensively measured and reviewed by Bengtsson and Risman (1971) and Buffler (1993). Mudgett et al. (1977) has pioneered the prediction of dielectric properties of foods as a function of constituency and temperature. Prediction of the temperature behavior of dielectric properties is crucial for accurate mathematical modeling of foods. Many workers today still use constant room temperature values or a look-up table at best. In the author s opinion, dielectric prediction of food properties is still a very fertile and useful research field. 

The most simple model for the dielectric properties of foods is called the distributive model. Here, the dielectric properties of each constituent of the food are added together according to their fractional make-up of the total product. The model assumes that the various constituents of the food are distributed uniformly throughout the product. For example, Figure 3 shows the total dielectric loss factor for a 0.5 molar aqueous solution of water at two temperatures. Note that the total loss factor, e"t is the sum of the ionic and polar contributions, e"c and e" An example of loss factor properties of mustard, ketchup, mayonnaise and water is shown in Figure 8. A comparison of food constituents important in determining dielectric properties is shown in Table 2, (USDA1963). 

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. 

Kent, M. 1987. Electrical and Dielectric Properties of Food Materials. Science and Technology Publishers. Hornchurch, UK. 

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. 

Nelson, S.O. and Bartley, P.G. Jr. FrEquationuency and temperature dependence of the dielectric properties of food material, Trans. ASAE, 45, 1223, 2002. 

Food is a complex mixture of different components and its dielectric properties are highly dependent on its compositions. Moisture content and salt concentration usually play a major role in determining the dielectric properties of foods. Figure 3.1 shows the relationship between the dielectric loss factor components and temperature. 

Nelson, S. O. and P. G. Bartley. 2000. Measuring frequency- and temperatnre-dependent dielectric properties of food materials. Transactions oftheASAE 43 1733-1736. 

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... 

Sosa-Morales, M. E., Valerio-Junco, L., Ldpez-Malo, A., Garda, H. S., 2010. Dielectric properties of foods Reported data in the 21st century and their potential applications. LWT-Food Sci. Technol. 43(8) 1169-1179. 

Fig. 1. Properties of foods near 2.45 GHz as a function of temperature, where A represents distilled water B, cooked carrots C, mashed potatoes D, cooked ham E, raw beef F, cooked beef and G, com oil (a) dielectric constants and (b) load factors, e = etan6 (32).

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 ... 

Figure 1. Food Map of Dielectric Properties of Common Foods (Buffler and Stanford 1991)...

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. 

Finally, an area which is in need of much further research is that of the dielectric properties of two-phase systems such as frozen foods, emulsions, whips and foams. It is well known that the dielectric behavior of particles of one dielectric property imbedded in a substrate of another, behave very differently from a distributive mixture of both. Fricke (1955) developed a model for randomly oriented oblate spheroids suspended in a continuous medium. It is expected that this model may be used successfully to model two-phase food systems, but to date there is very little literature reporting such studies. 

Ohlsson, T., Enriques, M. and Bengtsson, N. 1974. Dielectric properties of model meat emulsions at 900 and 28 MHz in relation to their composition. Journal of Food Science. 39 1153. 

Pace, W., Westphal, W. and Goldblith, S. 1968. Dielectric properties of common cooking oils. Journal of Food Science. 33 30-36. 

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). 

FIGURE 3.1 Temperature dependencies of the dielectric loss factor components. (From Sahin, S. and Snmnn, S.G., Physical Properties of Foods, Springer, New York, 2006. With permission.)... 

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