Abstract: The effects of 5 drying methods including hot air drying, vacuum microwave drying, extrusion drying with variable temperature and pressure difference, vacuum freeze drying and vacuum drying on physical properties, nutritional composition and microstructure of sweet potato products were investigated. Hot air drying resulted in the largest harness followed by extrusion drying with variable temperature and pressure difference, vacuum microwave drying, vacuum drying and vacuum freeze drying. Hot air-dried sweet potato products exhibited the largest δE* followed by those dried by vacuum drying, extrusion drying with variable temperature and pressure difference, vacuum microwave drying and vacuum freeze drying. Sweet potato products showed no remarkable change in their crude protein or crude fiber contents before and after drying. All of the five drying methods could result in a decrease in the crude starch content and an increase in the reducing sugar content of sweet potato products, and moreover, β-carotene and VC contents were both substantially decreased. After vacuum freeze drying and extrusion drying with variable temperature and pressure difference, the excellent honeycomb structure of sweet potato products was observed with evenly widened intercellular spaces. Hot air dried and vacuum dried sweet potato products displayed a compact structure. The structure of vacuum microwave dried sweet potato products was very compact. The results collectively indicate that extrusion drying with variable temperature and pressure difference can result in both better product quality and lower production cost and is therefore suitable for industrial-scale production of dried sweet potato products.

Key words: sweet potato, drying method, physicochemical properties, quality characteristics