The dielectric constant (ε′) of various edible oils and fatty acids has similar fre-quency dependence. The changes in ε′ value are minimal up to 500 kHz and then it starts decreasing. At lower frequencies, there exists an equilibrium between the oil molecules and the electromagnetic field, and hence the changes in ε′ are minimal. When the frequency increases beyond 500 kHz, the ε′ value gradually decreases.
The ε″ of the oils has inverted bell shape relationship with the frequency. The ε″
value decreases from 100 Hz to 13.2 kHz; after that, an increase in the frequency results in an increase in ε″. Lizhi et al. (2008) reported that the ε′ values of oils were mainly affected by C18 unsaturated fatty acids. An increase in the degree of unsaturation of oils resulted in higher ε′. Hence, it could be concluded that if a fatty acid has higher number of double bonds, it would have higher ε′ values. Pace et al.
(1968) and Rudan-Tasič and Klofutar (1999) reported similar results in dielectric properties of oils.
3.5.7 beef
Beef is one of the important meats in many different cuisines and is consumed all over the world. It is a rich source of selenium, zinc, iron, vitamin B, and carnitine.
The dielectric constant (ε′) and the loss factor (ε″) of beef both increase at higher temperatures. For example, ε′ values of lean beef at 27.12 MHz were 36.0 and 68.8 at −5°C and −1°C, respectively. Temperatures in the range of −1°C and 10°C did not have a significant effect on ε′ and ε″ at any microwave frequency. Higher ture had more effect on ε″ for all meats at 27.12 MHz and ε″ increased with tempera-ture (Ryynänen, 1995; Shukla and Anantheswaran, 2001).
Moisture content plays a major role in determining the dielectric properties of meat. Dipolar nature of water helps in microwave energy absorption. On the other hand, fat content of meat does not play a major influencing role on the microwave ablity of meat as fats have lower dielectric properties. However, higher fat con-tent is usually associated with lower moisture concon-tent and hence indirectly affects the dielectric properties of meat. Protein has a relatively significant effect on the microwave thermal processing of meat than fats, but nevertheless has significantly lower effect than moisture content. Temperature increase significantly reduces the microwave penetration depth. For instance, dp of lean meat was reported to decrease from 54.2 to 17.7 cm at 27.12 MHz and at −10°C and 1°C, respectively (Mudgett and Westphal, 1989; Ryynänen, 1995; Shukla and Anantheswaran, 2001; Mudgett; Farag et al., 2008). This is similar to the results obtained by Ohlsson et al. (1974) on the effect of temperature on microwave penetration depth.
3.5.8 SalmOn
Wang et al. (2008) studied the effect of temperature on the dielectric properties of salmon fillets. Their results showed that between 20°C and 120°C, dielectric constant (ε′) of pink salmon fillets steadily increased with temperature at low frequencies
of 27 and 40 MHz. However, the ε′ decreased slightly at higher frequencies up to 1800 MHz. The decrease in ε′ could be attributed to higher intermolecular interac-tion at higher temperatures. Herve et al. (1998) and Tang et al. (2002) reported simi-lar variations in ε′. The moisture content of salmon plays a major role in affecting the ε′ and ε″. Ash content and dissolved ion content also have a significant effect on the dielectric properties and subsequent microwave thermal processing of salmon (Ohlsson et al., 1974; Nelson and Bartley, 2002; Wang et al., 2003).
3.5.9 mirin
Mirin is a condiment with almost 40%–50% sugar and is widely used in Japanese cuisine. Dielectric loss factor of mirin is affected by both the dipolar loss component and the ionic conductivity. Ionic conductivity is lower at higher microwave frequen-cies. The combined effect of temperature, microwave frequency, and sugar content is complex and hard to describe. Nevertheless, the ε″ increases with frequency and tem-perature. The penetration depth decreases as the processing temperature increases.
The effect of temperature on the dp is significant at lower processing frequencies;
at higher frequencies, temperature has only moderate effect on the dp. Tanaka et al.
(2005) reported similar results for soy sauce. It was noted by Liao et al. (2003) that this trend is distinctive for thick or complex solutions.
3.6 ConCludIng reMarks
Microwaves could potentially provide highly effective and versatile thermal pro-cessing methods. However, to successfully utilize the microwaves for thermal processing, it is necessary to understand the changes in food products processed with microwaves. The dielectric properties of food materials play a major role in determining the microwave ablity of the food materials. The complex nature of the food materials, combined with external factors involved in microwave thermal pro-cessing, makes it a challenge to successfully develop newer microwave processing methods. Microwaves could also be used in conjunction with other conventional thermal processing methods. More details on dielectric properties and microwave processing of the examples quoted in this chapter could be obtained from the quoted references.
aCknoWledgMents
The authors extend their sincere gratitude for the financial support provided by the Natural Science and Engineering Research Council of Canada (NSERC) and Fonds Québécois de la recherché sur la nature et les technologies (FQRNT).
noMenClature C0 Speed of light dp Penetration depth E Electric field, V m−1
Ea Activation energy F Frequency, Hz J Complex number K Constant
Pv Energy developed per unit volume, W m−3 R Universal gas constant
T Absolute temperature V Volume, m3
V Viscosity v0 Initial viscosity
E Complex relative dielectric constant ε′ Dielectric constant
ε″ Dielectric loss factor
ε0 Absolute permittivity of vacuum (8.854 pF m−1) λ0 Free space microwave wavelength
μ0 Magnetic constant (1.26 μH m−1) Τ Relaxation time, ns
referenCes
Ahmed, J., H. S. Ramaswamy, and V. G. S. Raghavan. 2007. Dielectric properties of but-ter in the MW frequency range as affected by salt and temperature. Journal of Food Engineering 82:351–358.
Ahmed, J., H. S. Ramaswamy, and G. S. V. Raghavan. 2008. Dielectric properties of soybean protein isolate dispersions as a function of concentration, temperature and pH. LWT—
Food Science and Technology 41:71–81.
Bircan, C. and S. A. Barringer. 1998. Salt–starch interactions as evidenced by viscosity and dielectric property measurements. Journal of Food Science 63:983–986.
Bircan, C. and S. A. Barringer. 2002. Determination of protein denaturation of muscle foods using the dielectric properties. Journal of Food Science 67:202–205.
Buffler, C. R. 1992. Microwave Cooking and Processing. New York: Van Nostrand Reinhold.
Erle, U., M. Regier, C. Persch, and H. Schubert. 2000. Dielectric properties of emulsions and suspensions: Mixture equations and measurement comparisons. The Journal of Microwave Power and Electromagnetic Energy 35:185–190.
Everard, C. D., C. C. Fagan, C. P. O’Donnell, D. J. O’Callaghan, and J. G. Lyng. 2006.
Dielectric properties of process cheese from 0.3 to 3 GHz. Journal of Food Engineering 75:415–422.
Farag, K. W., J. G. Lyng, D. J. Morgan, and D. A. Cronin. 2008. Dielectric and thermophysical properties of different beef meat blends over a temperature range of −18 to +10°C. Meat Science 79:740–747.
Funebo, T. and Ohlsson, T. 1998. Microwave-assisted dehydration of apple and mushroom.
Journal of Food Engineering 39:353–367.
Goldblith, S. A. 1966. The wholesomeness of irradiated foods: Past history, present status, international aspects, and future outlook. Food Technology 20:93–98.
Herve, A. -G., J. Tang, L. Luedecke, and H. Feng. 1998. Dielectric properties of cottage cheese and surface treatment using microwaves. Journal of Food Engineering 37:389–410.
Lewandowicz, G., J. Fornal, and A. Walkowski.1997. Effect of microwave radiation on physico-chemical properties and structure of potato and tapioca starches. Carbohydrate Polymers 34:213–220.
Lewandowicz, G., T. Jankowski, and J. Fornal 2000. Effect of microwave radiation on physico-chemical properties and structure of cereal starches. Carbohydrate Polymers 42(2):193-199.
Liao, X., G. S. V. Raghavan, and V. A. Yaylayan. 2001. Dielectric properties of alcohols (C1-C5) at 2450 MHz and 915 MHz. Journal of Molecular Liquids 94:51–60.
Liao, X., G. S. V. Raghavan, and V. A. Yaylayan. 2002. Dielectric properties of aqueous solu-tions of α-D-glucose at 915 MHz. Journal of Molecular Liquids 100:199–205.
Liao, X., G. S. V. Raghavan, G. Wu, and V. A. Yaylayan. 2003. Dielectric properties of lysine aqueous solutions at 2450 MHz. Journal of Molecular Liquids 107:15–19.
Lizhi, H., K. Toyoda, and I. Ihara. 2008. Dielectric properties of edible oils and fatty acids as a function of frequency, temperature, moisture and composition. Journal of Food Engineering 88:151–158.
Macosko, C.W. 1994. Rheology: Principles, Measurements and Applications. Berlin, Germany: Wiley-VCH.
Mashimo, S., Kuwabara, S., Yagihara, S., and Higasi, K. 1997. Dielectric relaxation time and structure of bound water in biological materials. Journal of Physical Chemistry 91:6337–6338.
Mudgett, R. E. 1995. Electrical properties of foods. In M. A. Rao, and S. S. H. Rizvi (Eds.), Engineering Properties of Foods, pp. 389–455. New York: Marcel Dekker.
Mudgett, R. and W. Westphal. 1989. Dielectric behavior of an aqueous cation exchanger.
Journal of Microwave Power and Electromagnetic Energy 24:33–37.
Nelson, S. O. and P. G. Bartley. 2000. Measuring frequency- and temperature-dependent dielectric properties of food materials. Transactions of the ASAE 43:1733–1736.
Nelson, S. O. and P. G. Bartley. 2002. Frequency and temperature dependence of the dielectric properties of food materials. Transactions of the ASAE 45(4):1223–1227.
Nyfors, E. and P. Vainikainen. 1989. Industrial Microwave Sensors. Norwood, MA: Artech House.
Ohlsson, T. and N. E. Bengtsson. 1975. Dielectric food data for microwave sterilization pro-cessing. The Journal of Microwave Power 10:93–108.
Ohlsson, T., N. E. Bengtsson, and P. O. Risman. 1974. The frequency and temperature depen-dence of dielectric food data as determined by a cavity perturbation technique. The Journal of Microwave Power 9:129–145.
Pace, W. E., W. B. Westphal, and S. A. Goldblith. 1968. Dielectric properties of commercial cooking oils. Journal of Food Science 33:30–36.
Regier, M. and Schubert, H. 2001. Microwave processing. In P. Richardson (Ed.), Thermal Technologies in Food Processing, pp. 178–207. Cambridge, U.K.: Woodhead Publishing Ltd.
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.
Rudan-Tasic, D. and C. Klofutar. 1999. Characteristics of vegetable oils of some Slovene manufacturers. Acta Chimica Slovenica 46:511–521.
Ryynänen, S. 1995. The electromagnetic properties of food materials: A review of the basic principles. Journal of Food Engineering 26:409–429.
Sahin, S. and S. G. Sumnu. 2006. Physical Properties of Foods. New York: Springer.
Shukla, T. P. and R. C. Anantheswaran. 2001. Ingredient interactions and product develop-ment for microwave heating. In A. K. Datta and R. C. Anantheswaran (Eds.), Handbook of Microwave Technology for Food Applications, pp. 355–395. Boca Raton, FL:
CRC Press.
Sipahioglu, O. and S. A. Barringer. 2003. Dielectric properties of vegetables and fruits as a function of temperature, ash, and moisture content. Journal of Food Science 68:234–239.
Sunjka, P. S., T. J. Rennie, C. Beaudry, and G. S. V. Raghavan. 2004. Microwave-convective and microwave-vacuum drying of cranberries: A comparative study. Drying Technology 22:1217–1231.
Tanaka, F., K. Morita, P. Mallikarjunan, Y. C. Hung, and G. O. I. Ezeike. 2005. Analysis of dielectric properties of soy sauce. Journal of Food Engineering 71:92–97.
Tanaka, F., 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 89:435–440.
Tang, J., F. Hao, and M. Lau. 2002. Microwave heating in food processing. In X. H. Yang and J. Tang (Eds.), Advances in Bioprocessing Engineering, pp. 1–44. Singapore: World Scienctific Publishing.
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.
Von Hippel, A. 1954. Dielectrics and Waves. New York: Wiley.
Wang, Y., T. D. Wig, J. 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.
Wang, S., M. Monzon, Y. Gazit, J. Tang, E. J. Mitcham, and J. W. Armstrong. 2005.
Temperature-dependent dielectric properties of selected subtropical and tropical fruits and associated insect pests. Transactions of the ASAE 48:1873.
Wang, Y., J. Tang, B. Rasco, F. Kong, and S. Wang. 2008. Dielectric properties of salmon fillets as a function of temperature and composition. Journal of Food Engineering 87:236–246.
Zhang, M., J. Tang, A. S. Mujumdar, and S. Wang. 2006. Trends in microwave-related drying of fruits and vegetables. Trends in Food Science & Technology 17:524–534.
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