Research On Drying Characteristics And Processing Development Of Microwave Assisted Spouted Bed Drying Of Purple Fleshed Sweet Potato Granules

The thesis was aimed at developing a new processing method in food dehydration with purple-fleshed sweet potato (PFSP) by a laboratory scale microwave assisted spouted bed dryer (MWSBD). We studied the physical and chemical changes of PFSP particles during the MWSBD treatment, and made a primary recognition in the processing characteristics. Based on the analysis of dehydrated product's sensory and nutritional qualities, a processing method was finally formed and advanced. In comparison with microwave assisted freeze drying (MWFD), microwave assisted vacuum drying (MWVD), we made a further understanding and utilizing of MWSBD dehydration method. The whole study enlarged the application of MWSBD in food processing area.Color difference, anthocyanin level change, rehydration ability and hardness were selected to determine the product's quality in the direct drying of both fresh-cut and steamed PFSP cubes. Meanwhile, amylopectin starch to amylase starch ratio and swelling ability were also analyzed. Final results indicated that amylopectin starch to amylase starch ratio was as low as 53.92%, and swelling ratio of PFSP starch was only 9.75. Moisture uniformity of individual MWSBD treated product was fine, but rehydration ability and color uniformity was not ideal.Three different blanching methods, namely hot-water blanching, steaming blanching and microwave blanching were utilized in browning inhibiting in simple re-structured PFSP granules. We chose commonly used peroxidase (POD) inactivation to determine the blanching effect. POD inactivation curves, color change, anthocyanin level and subsequent MWSBD drying curve were introduced to study the effect of different blanching treatments. Final results pointed that microwave blanching of 560W, 90s, material to water ratio (2.5:1, g: mL) could maintain a high level of anthocyanin in PFSP, but the sensory acceptance of dehydrated product was not ideal. Anthocyanin level of hot water blanched sample was low.Microwave-assisted freeze-drying (MWFD), microwave-assisted vacuum drying (MWVD) and microwave-assisted spouted bed drying (MWSBD), were compared in the drying of PFSP granules to determine energy consumption and drying uniformity. Product's texture, color, anthocyanin level, energy consumption and sensory acceptance were investigated. Drying periods varied among drying methods. Final product's hardness was significantly reduced with re-structuring treatment with maize starch. Maximum penetration force of MWSBD treated sample was lowest (14.99 N), and the sensory acceptance was similar as MWFD treated ones, with good crispness. Color uniformity and moisture uniformity of individual MWSBD products were best among the three drying methods, energy consumption was relatively low. MWSBD can be used as an alternative way to MWFD with measurements to maintain the anthocyanin level.