Effect of contact ultrasound power on internal moisture migration of pear slices during ultrasound enhanced hot air drying

Abstract: Ultrasound strengthening technology has gained more and more attention in drying research, however, the strengthening mechanism of ultrasound on water status and moisture migration during drying has been unclear until now. In order to investigate the strengthening effect of contact ultrasound on moisture migration of pear slices during hot air drying process, the experiments of ultrasound reinforced hot air drying on pear slices were conducted with different ultrasound powers. The scan electric microscope was used to observe the surface microstructure of dried pear slices at different ultrasound powers. The low-field nuclear magnetic resonance (LF-NMR) technology and nuclear magnetic resonance imaging (MRI) technology were applied to analyze the influence of ultrasound power on internal water state and moisture migration of pear slices during hot air drying. The results showed that hot air drying of pear slices belongs to internal diffusion control, and the reduction ratios of drying time were 60.0%, 58.3% and 57.8% when contact ultrasound of 48 W was applied in hot air drying of pear slices at 35, 45 and 55 ℃, respectively, which indicated that the application of contact ultrasound could significantly accelerate drying process. The drying time reduced from 1440 min to 1260, 960, 780, 600 min when contact ultrasound assistance at 12, 24, 36, 48 W was applied in hot air drying of pear slices at 45 ℃, which showed that higher ultrasound power could lead to higher water removal rate and shorter drying time. The results of scan electric microscope showed that the application of ultrasound could improve the number of micro-capillaries as well as enlarge the size of micro-capillaries, and higher ultrasound power produced looser microstructure of pear surface that was contacted with ultrasound radiation board, which was favorable to moisture migration and water diffusion. The LF-NMR results showed that the peak amplitudes of inversion spectrum kept decreasing during the drying process and higher ultrasound power led to faster decreasing rate of the amplitudes, which represented that free water, immobilized water and bound water inside pear slices changed and migrated during drying process, and higher ultrasound power could accelerate the migration of the 3 kinds of water. Free water, with the greatest mobility and the highest content, was the first kind of water for total removal. The removal time of free water was 720, 660, 600, 480 and 360 min at ultrasound powers of 0, 12, 24, 36 and 48 W, respectively. Ultrasound showed great strengthening effect on free water removal, which indicated that ultrasound could achieve good energy transmission and strong water turbulence at the existence of free water. The contents of immobilized water and bound water decreased during the drying, however, the immobilized water and bound water inside pear slices could not be removed completely. The peak areas became smaller with the increase of ultrasound power, which indicated that the increase in ultrasound power could improve water mobility and reinforce moisture migration. The MRI results showed the change of moisture content and water distribution at different ultrasound powers during the drying process, and higher ultrasound power caused faster moisture reduction, which indicated that the MRI images could visually illustrate the change and transformation of water inside pear slices and the application of ultrasound could accelerate internal moisture migration significantly and increase drying rate. The research can present reference to the theoretical study and technical application of ultrasound enhanced hot-air drying technology.