Evaluation of the desirability of a drying method of indicators, including the recovery of dehydrated fruits and vegetables, dried fruits and vegetables to retain nutrients, the dehydration of the surface before and after changes in the vicinity of such organizations. Rehabilitation of physical and chemical indicators of two types, which mainly refers to the physical indicators of rehydration rate [1]; the retention of nutrients in order to vitamin C to reflect, because of high nutritional value of vitamin C, the stability of poor soluble in water, susceptible to oxidation, synthesis of the human body can not in itself, only by the food supply [2]; different drying methods to deal with differences in morphology of the materials can also be a more intuitive approach to reveal the advantages and disadvantages of drying. Therefore, this article will be the use of fresh potatoes with low-temperature adsorption drying, hot air drying, after freeze-drying rate of potato rehydration, vitamin C content, changes in morphology of the comparative analysis method for the choice of potato dry to provide basic research.
Experimental part 1
1.1 Main materials
     Look to buy from the farmers market in good condition, good fresh potato, peeled and washed, the cut into 5mm × 5mm × 5mm cubic particles, in boiled deionized water blanching 1min [4], using filter paper to the surface of the water absorption , READY.
1.2 The main instrument
     In this study, the main instruments and equipment: BS210S-type electronic balance, balance Beijing Cypriot Victoria Limited; WHX-1-based survey instrument temperature and humidity, radio-Science Research Institute of Jiangsu Province; DGF30/7-IA blast type electric oven , Nanjing Experimental Instrument Factory; BC 160A-type refrigerators, refrigerator plant in Suzhou, China; MICRO MODUL YO-230-type freeze-drying machine, Thermo Savant; HH-2-based digital temperature water bath pot, Changzhou Guohua Electric Appliance Co., Ltd.; QUANTA200 type scanning electron microscope SEM, FEI; ZK-82B-type vacuum drying oven, Jintan City, Jiangsu Province, World Scientific Instrument Factory.
1.3 Experimental Methods
     Will be a good pre-treatment tablets Add potatoes desiccator, dry conditions in some dry to constant weight [5]. Re-pot in the water bath, soaking in the 90 ℃ constant temperature 30min, removed with filter paper to the surface of the water absorption, weight. Add in a vacuum drying oven and then dry. Vitamin C content was determined by GB6195-86 fruits, vegetables, vitamin C content determination [6].
2 Results and Analysis
2.1 Drying Methods on the potato rehydration rate
     After low-temperature adsorption drying, hot air drying, freeze-dried potato rehydration tablets rate of the results shown in Table 1.
  
  
     Can be seen from Table 1, low-temperature adsorption drying potatoes after rehydration rate of 93.78%, significantly higher than that of hot air drying, freeze-drying was slightly lower than (99.98%). This is because the potato is to contain a higher sugar and soluble solid material, in the hot air drying process (especially the warming phase), the internal delay in the transfer of water to the surface of materials, material surface temperature higher, so that a layer of dry surface hard layer, the occurrence of "hardening", "crust", so that crude protein, pectin substances, such as severe degeneration and reduced rehydration after dehydration performance. The use of low-temperature adsorption drying of materials, the whole drying process were carried out at lower temperatures, no surface hardening phenomenon occurred degeneration colloid composition is also small, will help to raise the rate of rehydration. Freeze-drying the water first and then freeze into a solid-state direct sublimation, you can maintain good material than the original cavity cells, no significant reduction in the size of cells will not lose moisture and shrink, to keep the basic framework, rehabilitation of water when the water can quickly After drying of the cells into the cavity, so that the rapid restoration of full cells.
2.2 Drying Methods on the potato content of vitamin C
     The purpose of this study were determined by low-temperature adsorption drying, hot air drying, freeze-dried potatoes before and after vitamin C content, as shown in table 2.
    
     From Table 2, we can see that low-temperature adsorption drying of vitamin C on potato damage small, only 14.39 percent loss ratio, hot-air drying, freeze-drying the loss rates were 36.67% and 35.97%, may be the low temperature can reduce the vitamin C of the degree of oxidation. In this experiment, the freeze-drying, the freeze-drying time, the preservation of vitamin C than the adsorption rate of low-temperature drying.
2.3 Drying Methods on the surface of potato tissue
     Using scanning electron microscopy of fresh, low-temperature adsorption drying, hot air drying, freeze-dried material after the organization near the surface were observed to be scanned, the image results as shown in Figure 1 to 4.
              
                 

     From Figure 1 to 4 can be seen, fresh potatoes were full of hexagonal cells, both of which occurred after drying changes to varying degrees, from the internal surface morphology of materials, the greatest degree of freeze-drying materials to keep the original cells cavity, followed by low-temperature adsorption drying, but also maintained some of the new channel materials, hot air drying on the surface of cells, the greatest damage, because the surface temperature of materials as tissue dehydration fast dry channel is damaged, water can not be internal continued high-speed removal, and increased after drying rehydration difficult materials. Therefore, the freeze-drying of the complex water adsorption rate should be higher than low-temperature drying, higher in hot-air drying, which is actually measured in Table 1 the same conclusion.
     The use of hot air drying of potato, a long time at high temperature, the cells will be significant shrinkage, and with "crust", "hardening" phenomenon. Pre-freeze-drying freeze in the cold box of materials, the moisture of materials into a solid, and then in the oven so that the direct sublimation of water, thus maintaining the skeleton structure of materials. Adsorption as a result of low-temperature drying of materials with low temperature heat, the degree of tissue shrinkage than hot air drying light. From the appearance of color, the material after freeze-drying color than white, basically no significant deformation; hot-air drying of the materials after some slight browning and black phenomenon, the more serious deformation; adsorption materials after low-temperature drying was a more bright color The light yellow, surface contraction, but better than the apparent hot-air drying. From scanning electron microscopy of the micro-structure of film can be observed that the hot-air drying of materials in close tissue contraction, and the freeze-drying and low-temperature adsorption drying of the tissue more porous materials.
     Therefore, the dry method, the choice of drying conditions, not only to speed up the drying rate, for biological products and food, but also should be taken into account in particular the quality and morphology of products.
3 Conclusion
     1) In this paper, the experimental conditions, the adsorption of low-temperature drying potatoes after rehydration rate (93.78%) was significantly higher than the hot air drying (70.03%), slightly lower than the freeze-drying (99.98%).
     2) low-temperature adsorption drying of potato, hot air drying and freeze-drying rate after the loss of vitamin C were 14.39%, 36.67% and 35.97%. Adsorption of low-temperature drying to preserve Vitamin C content was superior to hot air drying and freeze-drying.
     3) using scanning electron microscopy analysis showed that: freeze-drying materials to keep a good surface morphology and internal cell cavity structure, low-temperature adsorption drying of fresh material have also maintained some of the channel, hot air drying on the surface of cells, the greatest damage.