Study On Combined Microwave Vacuum And Freeze Drying Of Fruits And Vegetables

The combination of microwave vacuum drying (MVD) and freeze drying (FD) has been investigated to replace freeze drying alone in dehydration of fruits and vegetables. The study showed that this combined drying (CD) can produce high-quality end-products, which not only have original color and the nutrition, but also keep the desirable shape and the texture. Furthermore, the energy consumption of this combined drying method decreased greatly.In order to optimize the parameters of CD for dehydrated fruits and vegetables, the mathematical models of shrinkage are developed on the base of MVD experiments, and the technical parameters for drying fruits and vegetables are suggested. The MVD experiments showed that, at a fixed power density, water loss is proportional to the drying time, and the shrinkage coefficient is inversely proportional to the loss of water and power density, while it is proportional to the thickness of samples approximately.The parameters of MVD and FD are then matched and optimized, and the best parameters of CD are obtained by orthogonal experiments. The technical parameters for carrot is: vacuum pressure,3.0—4.5 K Pa;microwave power density , 0.82 W/g and the amount of water,dehydrated by MVD in advance about 40%. As a result, the shrinkage coefficient of carrot will be 65%. While during FD, the setting of heating plate procedure are: keeping the temperature at -15 o C for 12 h at first, then increasing to a temperature of 5 o C for 3 h, then at 15 o C for 1h ,finally keeping 30 o C till the end point of drying for carrots. The technical parameters for apple is: vacuum pressure,3.0—4.5 K Pa; microwave power density, 0.82 W/g; the loss of water by MVD, about 50%. The shrinkage coefficient of apple slices is about 55%. During FD, the setting of heating plate procedure are: keeping the temperature at -12 o C for 11 h, then increasing to a temperature of 0 o C for 3 h, then lasting 1h at 15 o C, finally keeping 30 o C till the end point of drying for apples.The quality,cost and the energy consumption of different drying methods are compared with FD, the quality of carrot and apple slices dehydrated by CD is near to that of FD, the cost of CD is reduced by 56% and its interest is increased by 18% in producing dehydrated carrot slices, and the cost of CD is reduced by 20% and its interest is increased by 10% in producing dehydrated apple slices.The combination of MVD and FD may be used as a alternative method to dehydrate fruits and vegetables in both good quality and low cost.