Comparison of three microwave-assisted drying methods on the physiochemical, nutritional and sensory qualities of re-structured purple-fleshed sweet potato granules

Purple-fleshed sweet potato (PFSP) with high anthocyanin level was attractive to health concern. Three drying methods, namely 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. Products texture, colour, anthocyanin level and energy consumption were investigated. Drying periods varied among drying methods. MWSBD needed the least time to finish the drying process. Different drying methods had a significant effect (P < 0.05) on texture and anthocyanin level. Maximum penetration force of MWSBD treated sample was lowest (14.99 N). Anthocyanin level of MWSBD products was not as high as that of MWFD products, but colour and appearance were fine. MWFD products owned good crispness through sensory evaluation. Energy consumption of MWFD was about two times higher than that of MWVD. Final results indicated that MWSBD may be an alternative way to MWFD with measurements to maintain the anthocyanin level.