Analysis On The Drying Process And Quality Of Ginger Slices In Microwave Fluidized Drying

The microwave fluidized drying （MFD） introduced in this paper is a new quick drying method for fruit and vegetables, in which process the materials are kept in rolling and suspended state when the microwave power is supplied by several even-distribution magnetrons. Some common problems in microwave drying such as uneven drying and browning caused by local overheating were improved. Based on the MFD equipment developed by our team, the processes and quality of ginger slices dried by MFD and hot-air drying （AD） was studied; the drying models of consecutive MFD （CMFD） about moisture ratio （MR） in some typical processes were researched; the composition of free water, immobilized water and bond water in CMFD was analyzed; the drying processes and drying quality in intermittent MFD （IMFD） and MFD with variable microwave power （VMFD） was researched; the rehydration ability and physical properties of dried ginger slices from MFD were studied. At last, some main conclusions have been got in the paper.（1） MFD has a high drying speed, and the content of Vitamin C in the final products is higher than that from AD. When the dry basis moisture content was decreased to no more than200%, the materials’ quality changed rapidly, and some local areas turned brown, and this time should be considered as the key period to control the products’ quality. The microstructure of the gingers was easy to be damaged in MFD, which would influence the product’s rehydration capacity. Combined with AD and MFD, the rehydration capacity of the product from that2/3of the moisture was removed by AD, and then removed by MFD was strong.（2） The moisture ratios of the ginger slices in MFD were fitted by exponential model and other4drying models, when it was found that Wang and Singh model was the most suitable one to describe the drying process.（3） The moisture’s moving process in ginger slices in MFD was researched though nuclear magnetic resonance （NMR）, when it was found that free water is quickly converted to the immobilized water at the early stage; the free water and immobilized water was the main moisture being dehydrated; at last, when the immobilized water content decreased to a certain level, bound water increased.（4） In IMFD, the temperature of the ginger slices decreased; the distribution of the temperature and moisture became much more balanced and the microstructure could be kept better than that in CMFD. With different ratios of MFD time and intermittent time of5/0,5/1.25,5/2.5,5/3.75,5:5（CMFD, IMFD₅/1.25, IMFD₅/2.5, IMFD₅/3.75, IMFD₅/5）, the intermittent time was longer, the temperature of the materials was lower. Therefore, if the drying efficiency could be controlled, extending intermittent time could make the drying quality improved.（5） In VMFD, the phenomenon of local overheating and burning in materials being dried was alleviated. In addition, the temperature could be controlled efficiently. Compared with MFD with low microwave power and AD, the drying efficient was improved obviously. In VMFD, free water, immobilized water, bond water could be converted to other form. The component of [6]-gingerol,[8]-gingerol and [10]-gingerol in gingers increased in drying processes of all MFD, VMFD and AD. Especially for [10]-gingerol, the content of the product could increase to2-4times of that from the fresh samples. Some new flavor components were achieved in drying process, which have a positive role to protect the taste of the gingers.（6） The rehydration capacity of the dried ginger slices from MFD is worse than that from AD. With increasing of the temperature of the water in water-bath, the rehydration capacity increased, and the difference of the rehydration ratios among the products became small. The creep properties of rehydration ginger slices were determined by dynamic mechanical analyzer （DMA）. It was found that the hardness of the rehydration ginger slices is stronger from MFD than that from AD, and the difference is not obvious among those from MFD with different microwave power. The damage of the microstructure from MFD is much more serious than that from AD. The water’s absorbing capacity would be affected by microstructure’s change. The drying process improved by AD_MFD combined drying could improve the products’rehydration capacity, and the drying efficiency could be controlled.