Understanding appearance quality improvement of jujube slices during heat pump drying via water state and glass transition

Heat pump drying (HPD) has been used as one of the promising drying methods in food industry. To understand the color improvement and shrinkage inhibition of jujube slices during HPD, water state and glass transition temperature (T-g) were determined, macroscopic state diagram was further established. Jujube slices dried by HPD presented significantly lower color difference (Delta E) and shrinkage ratio than hot air drying. The temperature difference (AT = T-T-g) between the material temperature (T) and T-g showed significant effect on color and shrinkage of jujube slices. As the value of Delta T reduced, the Delta E change of jujube slices decreased. T-g increased significantly with the decrease of immobilized water. With the increase of T-g, solid matrix mobility of jujube slices decreased and the shrinkage tended to be stable. Macroscopic state diagram showed that critical moisture content of jujube slices dried by HPD which glass transition would happen at 25 degrees C was 0.061 g/g.