Drying characteristics and moisture migration of ultrasound enhanced heat pump drying on carrot

The study aims at reveal the enhanced effect of ultrasound on heat pump drying (HPD) process. Carrot was used as experimental material to determine its water removal, conversion and distribution during ultrasound-enhanced HPD. Carrots were dried at HPD temperatures of 40, 50, 60 °C and ultrasound powers of 0, 40, 80 W. Scanning electron microscopy (SEM) and low-field nuclear magnetic spectroscopy (LF-NMR) were applied to analyze the moisture distribution conditions status and moisture migration of carrot under 5 different drying conditions. The results showed that it displayed apparent strengthening effect by applying contact ultrasound (CU) during HPD, and higher temperature and CU power could obviously promote dehydration process. The SEM results revealed that the intensification effect of ultrasound and the increase of temperature could cause the improvement of micropores’ number inside carrot slices, which was conducive to the migration of internal moisture. LF-NMR analysis illustrated that the largest content of free water was removed first in CU-HPD process and the removal speed was accelerated with the increasing of drying temperature and ultrasound power. The immobilized water rose first and then fell, while the bound water hardly changed. Magnetic resonance imaging results could visually demonstrate the change and migration of water inside carrot slices, and CU had a strengthening effect on HPD, but this intensifying effect reduced significantly with the propagation of ultrasound in carrot slices. Therefore, CU application during HPD could achieve positive effect on water migration and moisture removal.