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Call number: LD2668 .T4 1951 H35 / Master of Science

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The current study was conducted for evaluating the effects of two different strategies viz., phase control and cycle control, on the microwave/air drying process. A phase-controlled electrical power regulator was developed and connected in series with the original cycle-controlled power regulator of an existing domestic microwave oven. The microwave oven was further modified such that combined microwave and convectional drying can be accommodated. / The system performance was evaluated. It was observed that phase-controlled power regulator could be successfully used for quasi-continuous (fast-switching) power regulation with the maximization of power efficiency. The degradation of output microwave power was recorded and the nonuniform distribution of microwave field in the cavity was also verified. / The effects of phase control and cycle control were evaluated through combined microwave and convective drying of potato samples. Results showed that different power control methods had different impacts on drying kinetics and product quality. In both drying modes, the drying time increased with the decrease of microwave power density and the increase of air velocity. The drying rates of cycle-controlled drying are significantly higher than those of phase-controlled drying. In terms of rehydration capacity the phase-controlled drying mode produced better results. The product colour and sensory attributes were independent of the power control methods.

The influence of freezing rate, rate of heat input and drying chamber pressure on freeze-drying rate was studied to determine the thermal and physical properties of MacIntosh apple and Netted Gem potato tissue. The samples were frozen either by immersion in dry ice and ethanol (fast frozen) or by placement in a refrigerated cabinet maintained at a temperature between -10° and +5° F (slow frozen). The samples were suspended in a chamber maintained at a pressure of 550 or 1400 microns of mercury and surrounded by a constant temperature water bath which provided a radiant heat source of 86° or 104°F. The weight, and the surface and centre temperature of the sample were recorded continuously during freeze-drying. Vapor diffusion was the rate limiting factor for fast frozen samples while heat transfer was rate limiting for slow frozen samples. Chamber pressure had little influence on the freeze-drying rate of slow frozen samples. Potato tissue thermal conductivity varied from 0.66x10⁻² BTU/Hr.°F Ft. at a pressure of 550 microns to 0.78x10⁻² at 1400 microns. The thermal conductivity of apple tissue was 1.0 x 10⁻² BTU/Hr.°F Ft. at both pressures. The eutectic temperature of apple and potato tissue was found to be -10°F and -1°0F. respectively. / Land and Food Systems, Faculty of / Graduate

LD2668 .T4 1954 S32 / Master of Science

MSCAGR / Department of Animal Science