Study On Microwave-vacuum Drying At Controlled Freezing-point Of Carrot

Controlled freezing-point dried technology is a new drying method that uses thetheory of controlled freezing-point storage for reference which is applied for food andbio-medicine drying. Commonly, the methods of food and bio-medicine drying aremainly hot-air drying and vacuum freeze-drying. Hot-air drying is of higher efficiencybut lower quality for drying product and unsuitable for heat-sensitive materials; Thevacuum freeze-drying can get product with better quality but it takes longer time andhigher cost. A controlled freezing-point and microwave vacuum drying device isdesigned, With carrot as the study object, the performance of the device and controlledfreezing point drying method were studied.A controlled freezing-point dried experiment device is designed to ensure dryingquality and improve drying efficiency. The experiment device uses the way of efficientmicrowave heating. Take away the vacuum to make the material cool to the controlledfreezing-point temperature. The material is controlled to dry under vacuum conditionswithin its controlled freezing-point temperature. The devices are divided into thevacuum system, microwave heating system, water-catching system, temperaturemeasurement system, data acquisition systems and control system.The carrot was microwave dried by using the controlled freezing-point drieddevice. Also various factors that affect the drying process were studied. Microwavepower and vacuum pressure have significant impacts on the temperature of materialduring the process of drying. The higher the microwave power, the faster the dryingprocess. The lower the microwave power and the pressure, the lower the temperature ofcarrot slices in the drying process. After we get a fixed pressure by studying the controlof controlled freezing-point dried process, the microwave power can be adjusted tocontrol the temperature of material within the controlled freezing-point ranges. Thelower the pressure, the higher the efficiency of controlled freezing-point dried. But thelower the pressures, with the temperatures fluctuate a lot by microwave heating, themore difficult to control the controlled freezing-point dried.By power controlled microwave heating to dry the carrot of which temperaturewithin the controlled freezing-point temperature range. The vacuum pressure is set to150Pa; the thickness of the carrot is3mm. We can get dehydrated carrot with moisturecontent of4.8%after4hours’ drying. By comparing the experiment results of the controlled freezing-point dried; hot-air drying and vacuum freeze-drying, the driedcarrot slice rehydration is81.5%, VC retention is68.8%and carotene retention is93.7%by controlled freezing-point dried method. The sensory quality is close to the vacuumfreeze-dried carrots. The drying time is significantly improved and the quality of driedcarrot is better than hot-air drying.