Moisture transfer and microstructure change of banana slices during contact ultrasound strengthened far-infrared radiation drying

To explore water transfer and structure change during contact ultrasound strengthened far-infrared radiation drying (CU-FIRD) on banana slices, the effects of far-infrared radiation (FIR) temperature and contact ultrasound (CU) power on drying process, water transfer change, microstructure and pore evolution were studied. The results showed that the drying time could be shortened obviously by increasing FIR temperature and CU power. The effective moisture diffusivity values of banana slices were between 2.28 x 10(-10)-6.12 x 10(-10) m(2).s(1), which increased with the rises of FIR temperature and CU power. The moisture changes in banana slices during CU-FIRD were analyzed by low-field nuclear magnetic resonance (LF-NMR). There were three kinds of water status in banana slices including bound water, immobilized water and free water. Free water was the main water in banana, and the increases of both FIR temperature and CU power could obviously shorten the removal time of free water. The immobilized water in banana slices increased first and then decreased during the drying process, and the improvements of FIR temperature and CU power could accelerate its mobility and migration. There was no significant change for bound water, which was the main water left in banana slices after drying. The SEM and shrinkage ratio results showed that higher FIR temperature and CU power could reduce the structure shrinkage of banana samples, increase the porosity, and generate more diffusion micro-channels with larger size, so as to speed up the migration process of water in banana. CU-FIRD could improve the porous structure of banana samples, enhance the heat and mass transfer process, and accelerate the migration and diffusion of water.