Studies On Quality, Saving Energy Technology And Model Of Tandem Combined Freeze Drying-vacuum Microwave Dried Apple

So far, freeze drying (FD) is the best method for maintaing the material's quality among all drying methods. For fruits and vegetables, the color, flavor, shape and taste of FD fruits and vegetables are very good. However, high energy consumption during FD process is a major problem that limits the wide application of this method on common fruits and vegetables. FD was replaced by two-stage tandem combined drying technologies, FD and vaccum microwave dyring (VMD), in order to decrease the energy consumption and maintain the quality well.In this research, apple slices were used as raw materials and tandem combined drying technologies of FD and VMD were applied. It was found that conversion moisture content was very important factor during FD-VMD. The optimal conversion point was the end of sublimation drying stage and begin of desorption drying stage. The savings ratio of invalid energy consumption was 39.20%. Moreover, it was found that microwave power was the important factor during VMD-FD. According to the experiments, the savings ratio of invalid energy consumption was 46.49%. Although the saving energy ratio for VMD-FD was higher than that for FD-VMD, the quality of FD-VMD products was more close to FD products.For the purpose of revealing the law of dehydration during combination drying, mass transfer model for drying apple slices was studied after the technology research. The Page model could be used to fit the mass transfer process during the FD-VMD. In order to understand the temperature changing during VMD post-drying, dielectric properties of apple slices were studied and the VMD process was divided into three stages according to the dielectric loss values. The flat theory and energy conservation equation were used to fit the temperature changing for the three drying stages.The uneven character of cut apple slices was resolved by re-structure technology. The putting area of material could be expanded by mix with potato slurry, which decreases the corner number of material. This two both made the drying process in microwave field more even. The effect of MFD, FD, VMD and VD on texture, color, rehydration, microstructure and other indices was studied. It was found that the quality of MFD chips was better than that of FD chips and the drying time decreased in half. In addition, the quality of VMD chips was better than that of VD chips and the drying time for VMD decrease by 95% compared with VD process. Microwave drying not only reduces the drying time but also improves the quality of dried products. Both MFD and VMD techniques are found to be suitable for production of re-structured chips of varying grades by consideration the quality and energy consumption results.In order to reduce the rehydration ratio of combined dried apple pieces in liquid carriers, whey protein isolate was used as coating material and spouted bed drying technology was applied to coat apple pieces and dry them simutanously. It was found that the rehydration ratio decrease by 40%, the retention rate of nutrients was 90% and the shape of the coated apple pieces was the same with uncoated. In this research, WPI coating solution preparation method was also studied. It was found that higher denaturation temperature and longer time could induce the lower rehydration ratio of coated apple pieces. The best denaturation condition was 15 min at 80°C. Furthermore, adjusting pH value of coating solution did not decrease the rehydration ratio of coated apple pieces. According to the observation during rehydration and the rehydration curves, it was suggested that coating was not a single layer but small pieces attached to apple pieces. This reasoning was confirmed by later experiments.