This report describes the results of experiments to determine how the runaway and edge effects of microwaves on foods can be reduced. A technology using microwave is very useful for heating or sterilizing food, especially for high viscosity foods or those foods containing solid foodstuffs. However, this technique has some problems such as runaway and edge effects. The former results from the difference in dielectric constants of used foodstuffs and the latter from the concentration of energy at the corners of packages or at sharp edges. For the sake of resolving these problems, we examined whether the shapes of a package, the scattering of microwaves under stirring conditions and the use of microwaves under a steamed atmosphere would be beneficial. As experimental foods, Okara, chikuzenni and cream cheese were used. A round package was less scorched at edges than a square one. The circulation of warm air into the chamber did not reduce the edge effects. Irradiation with microwaves under a steamed atmosphere substantially reduced both the runaway and edge effects. The foods were heated uniformly and the uniform electric fields were observed at all places of package. We proposed that the cause of this phenomenon is the phase translation of steam. The results of this study indicate that irradiating foods under an atmosphere of steam with microwaves increase the uniformity of the heating process.
An effect of pulsed electric field treatment on Escherichia coli K-12 suspended in sterilized 1.1g/L NaCI solution was investigated using two types of electrode system, ring-to-cylinder (with and without insulating plate) and spiral electrodes. In ring-to-cylinder electrode system, the microbial inactivation was efficient when the cylinder diameter was small and when insulating plate with a hole was inserted between ring and cylinder electrode. In spiral electrode system, an efficient microbial inactivation was achieved when the wire diameter was 0.90mm.
Since pre-fermentation of Saccharomyces cerevisiae cells in fresh medium for 1 hour is considered as a good model of the first-fermentation in bread making, the freeze-tolerance of S. cerevisiae cells after the pre-fermentation was studied. We have found that S. cerevisiae cells after the pre-fermentation showed lower freeze-tolerance than the cells grown to stationary phase. Moreover, among heat-resistant mutants, we isolated a mutant that maintained freeze-tolerance after the pre-fermentation, because a positive correlation between heat shock tolerance and freeze-tolerance was found in S. cerevisiae cells after the pre-fermentation. When the freeze-tolerant mutant was used in frozen dough, the frozen dough expanded better than the one using the parent strain. The effects of cultivation conditions and freezing temperature of S. cerevisiae cells were also investigated to increase freeze-tolerance of the mutant cells. The pre-fermentation in the medium containing the no carbon source is the most effective to enhance survival ratio of the cells during the storage at -35°C. By use of the freeze-tolerant cells, the size of the bread baked from frozen dough, which stored for 7 days at -35°C, was almost the same as that of the one baked from unfrozen dough.
Temperature difference of microwave heating was studied, comparing with 3 kinds of microwave oven. Samples were globe-shaped 100g and 180g potatoes. The local temperature of potato during microwave heating was measured with optical fiber thermometer. Photographs of potato section in the process of microwave heating were input into the computer and analyzed by image processing. In the 3 kinds of oven, the center of globe-shaped potato was cooked the most rapidly and around the potato was cooked slowly. The potato position in the microwave oven affected the rate of temperature rise and the temperature difference. The potato in the edge of a oven plate was cooked faster and taken less temperature difference than that in the center of a oven plate. The rate of temperature rise of 180g potato was about double as slow as 100g potato, but pattern of the temperature difference of 180g potato was in the same manner as 100g potato.
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