DRYING CHARACTERISTICS AND DRYING QUALITY OF KIDNEY BEANS USING A TWO-STAGE MICROWAVE PROCESS

The study focuses on describing the microwave drying characteristics of kidney beans and discussing the effect of drying power and initial mass on drying quality. When the power level increased, the dehydration rate increased while the drying energy consumption and the initial mass decreased. There were two falling rate periods during drying of kidney beans. The first falling rate period was at a moisture content of more than 2.0 (dry basis [d.b.]), and the second falling rate period was at a moisture content of less than 2.0 (d.b.). The same water loss will consume more energy when the moisture content is less than 2.0 (d.b.). The quadratic orthogonal regression experiment was conducted, and models of effect of first-stage drying load-power, second-stage drying load-power and first-stage drying time on sensory quality, rehydration ratio and energy consumption rate (three indices) were established. First-stage drying load-power, second-stage drying load-power and first-stage drying time affected the three indices. For the three indices, the drying factor combination is different. First-stage drying load-power is slightly lower for high sensory quality. Second-stage drying load-power should be lower for high sensory quality and higher for low energy consumption rate. First-stage drying timeshould be long for high sensory quality and moderate for low energy consumption rate and high rehydration ratio. To obtain better results for the three indices, the first-stage power should differ from the second-stage power in drying factor combination. This implies that the alternative microwave power drying on kidney beans should be used. PRACTICAL APPLICATIONS Major disadvantages of hot-air drying of kidney beans are low energy efficiency, lengthy drying time during the falling rate period and inferior quality of the dried kidney bean. The application of microwave as a heat source produces a fast drying rate and higher drying quality. It can be employed because kidney beans are sensitive to microwave. Little detailed information was available on the alternative microwave power drying on food products, including drying of kidney beans by microwave. The results of this study will show the effect of drying power and initial mass of kidney beans on drying quality. For sensory quality, rehydration ratio of kidney beans and energy consumption rate, the drying factor combination is different. To obtain better results in the three indices, the first-stage (early drying) power should differ from the second-stage (later drying) power under microwave drying. This implies that the alternative microwave power drying on kidney beans should be used.