MICROWAVE-VACUUM DRYING OF CRANBERRIES: PART I. ENERGY USE AND EFFICIENCY

Microwave-vacuum drying was investigated as a potential method for drying cranberries. Cranberries were pretreated with either 30°B or 60°B high fructose corn syrup solution for 24 h. They were dried using a laboratory-scale microwave-vacuum oven operating either in continuous or pulsed mode until the final moisture content reached 15% (wet basis). In the continuous mode, two levels of microwave power (250, 500 W) and absolute pressure (5.33, 10.67 kPa) were applied. In the pulsed mode, microwave power of 250 W and two levels of pressure (5.33, 10.67 kPa) were used with two levels of power-on time (30, 60s) and three levels of power-off time (60, 90, 150s). Pulsed application of microwave energy was more efficient than continuous application. In both cases, drying efficiency improved when lower pressure (5.33 kPa) was applied. Shorter power-on time and longer power-off time provided more favorable drying efficiency in pulsed mode. Power-on time of 30 s and power-off time of 150 s was the most suitable setting for maximum drying efficiency.