16 Radio Frequency and Microwave Processing of Food

Transcription

1 16 Radio Frequency and Microwave Processing of Food Microwaves have been used as a heat source since the 1940s (Spencer 1949, 1952). Industrial applications of microwave heating include: polymer and ceramics industries (Ayappa, et al. 1991), medicine (Pchelnikov 2003) and food processing (Van Remmen, et al. 1996, Oliveira and Franca 2002, Shaheen, et al. 2012). The food industry is the single largest consumer of microwave energy, where it can be employed for cooking, thawing, tempering, drying, freeze-drying, pasteurization, sterilization, baking, heating and re-heating (Ayappa, et al. 1991). Most homes in western countries have microwave ovens for cooking and reheating food. Water, which is the major constituent of most food products, is the main source for microwave interactions due to its dipolar nature. Heat is generated throughout the material, leading to faster heating rates and shorter processing times compared to conventional heating. Other advantages include space savings and energy efficiency, since most of the electromagnetic energy is converted into heat (Oliveira and Franca 2002). Factors affecting microwave heating of food include: the dielectric, thermal and physical properties of the food stuff; the moisture content and moisture permeability of the food material; the geometry of the food stuff; and the geometry of the microwave applicator used to irradiate the food Dielectric Properties of Foods The dielectric properties of food stuffs vary with frequency, temperature and moisture content. Foodstuffs are complex materials and they exhibit extremely complicated dielectric spectra (Daschner, et al. 2000). The complex dielectric properties of foods vary with frequency, temperature and possibly time, depending on the dipolar relaxation frequencies of the polar molecules involved (Daschner, et al. 2000). Most foodstuffs contain both bound and free water, which itself has a complex dielectric behaviour (Ellison, et al. 1996, Tikhonov 1997, Buchner, et al. 1999, Boyarskii, et al. 2002, Chaplin 2007). Table 16.1 lists the measured dielectric properties of several food stuffs at a frequency of 2.8 GHz. Fat and water content of the food change the dielectric properties. Their heating behaviours are also quite different. Fat will be heated more quickly than water because of its relatively low heat capacity. Fat can be heated to greater than 200 C whereas the temperature of water will be no greater than 100 C until it is completely evaporated (Food and Environmental Hygiene Department 2005 ); therefore mixed foods tend to heat at different rates due to their different thermal and dielectric properties. One of the common concerns of microwave heating is uneven heating. This is partly due to the geometry of the object being heated and partly due to the geometry of the microwave fields inside the applicator used to apply the energy to the material.

2 190 Radio Frequency and Microwave Processing of Food The influence of material geometry over microwave heating has been discussed in Chapter 13 and the influence of applicator geometry will be presented in the next chapter; however differences in dielectric properties are the other major contributor to uneven heating in foods. Most food products have several products in the same sample, with each material having different dielectric properties (Table 16.1), which leads to differential heating. Table 16.1: Dielectric properties of common foods at 2.8 GHz (Source: Ohlsson and Bengtsson 1975.). Material Temperature (K) κ' κ" Potato, freeze dried Potato, Raw Aqueous non-fat dry milk Pizza stuffing Pizza dough Pizza dough Pineapple syrup Pineapple syrup Fish, Cooked Fish, Cooked Fish, Cooked Gravy Gravy Gravy Water Water Water Potato, mashed Potato, mashed Potato, mashed M NaCl M NaCl M NaCl Beef, raw Beef, raw Liver pate Liver pate Liver pate Peas, cooked Peas, cooked

3 Dielectric Properties of Foods 191 Table 16.1: Dielectric properties of common foods at 2.8 GHz (Source: Ohlsson and Continued Bengtsson 1975.). Material Temperature (K) κ' κ" Peas, cooked Carrots, Cooked Carrots, Cooked Carrots, Cooked Peanut Butter Peanut Butter Frankfurters Ground beef patties Ground beef patties Concentrated orange juice Beef steak, cooked Beef steak, cooked Ham Pork chop Pork chop Fillet of turbot One of the consequences of uneven heating is that some parts of the heated material will be over heated while other parts will remain under heated. Under heating can be a concern for food safety (Food and Environmental Hygiene Department 2005 ) because pathogenic organisms can survive and cause illness. Concerns have arisen regarding whether microwave cooking can kill food-borne pathogens as effective as conventional cooking methods since microwave cooking generally requires shorter times and may sometimes result in lower temperatures at the food surface; however many studies have concluded that microwave cooking kills microorganisms and spores provided that appropriate temperature and time are reached (Food and Environmental Hygiene Department 2005). There are concerns that microwave heating of food is unhealthy because it could affect its chemical or physical properties, other than simple cooking. For chemical alteration to happen, the microwaves would need to create chemical radicals involving energies of the order of an electron-volt (ev) (Vollmer 2004). Microwave photons have energies of the order of 10 5 ev; therefore simple estimates easily show that the number of microwave photons within commercial microwave oven is orders of magnitude too small to establish multi-photon dissociation or ionization (Vollmer 2004); therefore microwave cooking cannot chemically alter food. Cooking processes, especially the high temperature ones (e.g. grilling, baking, etc.) are known to induce the production of potential carcinogens. There have been concerns that microwave cooking may also increase the production of carcinogens or

4 192 Radio Frequency and Microwave Processing of Food mutagens in foods. Currently there is no scientific evidence that the production of any carcinogenic substances would increase upon the application of microwave heating. A study examined mutagen production in cooked lamb and beef found no evidence of mutagenicity in microwave-cooked lamb chops, sirloin steak, leg of lamb, or rolled beef loaf (Food and Environmental Hygiene Department 2005 ) Comparative Efficiency of Convective and Microwave Heating Thermal transfer in conventional systems depends on the temperature differential between the heated object and its surroundings. As this temperature differential reduces the heat transfer reduces also. Thermal conductivity of the material and the complex interactions between heat transfer and moisture transport also influence the thermal transfer in the system. The coupling between heat and moisture transport in a conventional system usually results in slower heating because the water vapour is moving out of the material opposing the transfer of heat into the material. This creates an evaporation surface inside the material (Shaeri, et al. 2013) that effectively cools the material until the water source is exhausted. Microwave heating is a volumetric phenomenon. Although the interactions are complex, the combination of geometry, material properties, microwave field intensities and changes in these parameters induces very rapid heating. There is also a coupling between microwave heating and moisture transport that enhances both heat and moisture transport (Brodie 2007), rather than diminishing each other as happens in conventional heating systems. Consequently, microwave heating is many times faster than convective heating. As an easy to understand example of the differences in efficiency between microwave heating and other systems, Table 16.2 compares the energy required for cooking a casserole using different heating systems. Table 16.2: Comparative energy requirements for cooking a casserole (Source: Jennifer Thorne, et al. 2012). Appliance Temperature Time Energy Electric Oven hour 2.0 kwh Electric Convection Oven minutes 1.39 kwh Gas Oven hour 3.3 kwh Electric Frying Pan hour 0.9 kwh Toaster Oven minutes 0.95 kwh Electric Crockpot hours 0.7 kwh Domestic Microwave Oven High 15 minutes 0.36 kwh

5 References 193 References Ayappa, K. G., Davis, H. T., Crapiste, G., Davis, E. J. and Gordon, J Microwave heating: An evaluation of power formulations. Chemical Engineering Science. 46(4): Boyarskii, D. A., Tikhonov, V. V. and Komarova, N. Y Modeling of dielectric constant of bound water in soil for applications of microwave remote sensing. Progress In Electromagnetics Research. 35: Brodie, G Simultaneous heat and moisture diffusion during microwave heating of moist wood. Applied Engineering in Agriculture. 23(2): Buchner, R., Barthel, J. and Stauber, J The dielectric relaxation of water between 0 C and 35 C. Chemical Physics Letters. 306(1-2): Chaplin, M Water Structure and Science. 7th June, Daschner, F., Knoechel, R. and Kent, M Rapid monitoring of selected food properties using microwave dielectric spectra. Proc. 2nd International Conference on Microwave and Millimeter Wave Technology. Ellison, W. J., Lamkaouchi, K. and Moreau, J. M Water: a dielectric reference. Journal of Molecular Liquids. 68(2 3): Food and Environmental Hygiene Department Microwave Cooking and Food Safety The Government of the Hong Kong Special Administrative Region Jennifer Thorne, A., Katie, A. and Alex, W. 2012, Consumer Guide to Home Energy Savings, [N.p.]: New Society Publishers. Ohlsson, T. and Bengtsson, N. E Dielectric food data for microwave sterilization processing. J Microw Power. 10(1): Oliveira, M. E. C. and Franca, A. S Microwave heating of foodstuffs. Journal of Food Engineering. 53(4): Pchelnikov, Y. N Features of slow waves and potentials for their nontraditional application. Applications of Radiotechnology and Electronics in Bioligy and Medicine. 48(4): Shaeri, M. R., Beyhaghi, S. and Pillai, K. M On applying an external-flow driven mass transfer boundary condition to simulate drying from a pore-network model. International Journal of Heat and Mass Transfer. 57(1): Shaheen, M. S., El-Massry, K. F., El-Ghorab, A. H. and Anjum, F. M Microwave Applications in Thermal Food Processing. In The Development and Application of Microwave Heating. Shaheen ed. Spencer, P. L Prepared food article and method of preparing. Patent No Spencer, P. L Electronic cooking. Patent No Tikhonov, V. V Dielectric model of bound water in wet soils for microwave remote sensing. Proc. IEEE International Geoscience and Remote Sensing Symposium, : Singapore International Convention and Exhibition Centre, Singapore Van Remmen, H. H. J., Ponne, C. T., Nijhuis, H. H., Bartels, P. V. and Herkhof, P. J. A. M Microwave Heating Distribution in Slabs, Spheres and Cylinders with Relation to Food Processing. Journal of Food Science. 61(6): Vollmer, M Physics of the microwave oven. Physics Education. 39(1):