Ultrasound-assisted osmotic dehydration pretreatment before pulsed fluidized bed microwave freeze-drying (PFBMFD) of Chinese yam

Abstract

The effect of ultrasound assisted osmotic dehydration (USOD) with 5% salt solution as the osmotic solution with pulsed fluidized bed microwave freeze-drying (PFBMFD) of Chinese yam (Dioscorea opposita Thunb.) was studied. Yam cubes were subjected to osmotic dehydration with different ultrasonic power (1.52, 2.28 and 3.04 W/g) or without ultrasound for 10 min, and the mass transfer, water status and distribution, dielectric properties and microstructure were measured. The drying characteristics and quality attributes (color and total soluble polyphenols content, total flavones content and antioxidant capacity) of the pretreated and dried yams as well as texture, flavor and microstructure of dried yams were also studied. The USOD treatment increased the mass transfer, increased loss factor and made the water more mobile. Cell size increases were observed on the micrograph because of the application of higher ultrasonic power. Thus, the acceleration of drying and energy savings were achieved. The Page model was successfully fitted to the drying curves. Furthermore, PFBMFD dried yams with USOD pretreatment had good color and texture, reduced astringency and sourness, as well as improved antioxidant capacity when compared with fresh dried samples.