less than that of mashed potato containing 0.8% salt under thesame test conditions in 27.12 MHz RF cavities. Based on Eq. (2),the relatively low electric field intensity in sauce prevented it fromoverheating among other ingredients in spite of its twice as greatrelative loss factor.The electric field was concentrated near the tray corners, causedmainly by the fringe effect of RF field due to the difference betweenthe dielectric properties of the food sample and circulation wateroutside the food package. Simulated power density was relativelyhigh near tray corners and in noodles as compared to the rest ofthe food (Fig. 10).Similar temperature patterns were predicted in computer simulation(Fig. 11). But the temperature variations (<2 C) among differentfood components in close proximity, in particular betweensauces other food components, is much smaller than one wouldanticipate judging only from the large differences in their loss factors(Tables 2–5). We believe that sufficient heat transfer tookplace between the sauce, noodles, cheese, and beef to help mitigatethe power density variations.The temperature histories of the beef meatballs at four differentlocations as shown in Fig. 2b within a tray were obtained in experimentswith RF and from a conventional thermal process (Fig. 12).This conventional process was conducted with the same RF sterilizationsystem by circulating hot water at 121 C over food packages inthe pressurized vessel without turning on RF power. During the RFheating, the greatest temperature difference among the four differentlocations was less than 5 C. The time for the sample core to arisefrom 60 to 121 C with RF energy was 35 min compared to 110 minwith a conventional thermal process. As shown in Fig. 12, in a conventionalthermal process, the foods close to the container walls(points 1 and 4) were exposed to temperatures above 100 C formore than 80 min, as compared to 40 min in RF processing. Similardifferences in processing times between conventional thermalprocesses and RF processing in the same size containers have beenreported for mashed potatoes (Luechapattanaporn et al., 2004),scrambled eggs (Luechapattanaporn et al., 2005) and Macaroniand cheese (Wang et al., 2003c). The shortened exposure times offood to high temperatures in RF processing significantly reducedthermal degradation in those early studies.The greatest temperature difference among the meatballs(shown in Fig. 12) as calculated by computer simulation duringan RF process was 5 C. The hottest parts of the food were at thecorners of the trays (Fig. 11a). However, according to Fig. 12, theexperiment exhibited center heating (i.e. the highest temperatureoccurred at the center of the food package). Three reasons mighthave contributed to the disagreement between the experimentand simulation. First, the simplification of the boundary conditions
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