Effects of ultrasound pretreatment on water state and hot-air drying characteristics of kiwifruit

drying is one of the most commonly used food processing methods. It can prolong the shelf life by reducing water activity and inhibiting quality degradation caused by microbial and enzyme reactions. In order to further clarify the effect of ultrasonic pretreatment on the quality of kiwifruit, the effects of ultrasonic power (200-600 W) and ultrasonic time (10-30 min) on the water state and distribution and nutritional components (total soluble solids, titratable acids, vitamin C, total phenols and total flavonoids) of kiwifruit slices were studied.In this paper, drying characteristics and moisture migration during subsequent hot-air drying, as well as the microstructure, color and texture characteristics of dried samples were investigated.The water state, distribution and migration of kiwifruit slices were monitored by Low Field Nuclear Magnetic Resonance (LF-NMR) and Magnetic Resonance Imaging (MRI), and the microstructure of kiwifruit slices was observed by Scanning Electron Microscope (SEM).The results of LF-NMR relaxation showed that there were three water components in kiwifruit, labeled T21, T22 and T23, respectively. They represented the water with the lowest fluidity in the cell wall, the water in the cytoplasm and extracellular space, and the water with the highest fluidity in the vacuole. Ultrasonic pretreatment could lead to the splitting of T23 relaxation peak and the decrease of T23 peak amplitude, suggesting the redistribution of water in kiwifruit slices. At the same time, the greater the ultrasonic power, the longer the ultrasonic time, and the more obvious the drying effect. Compared with untreated samples, ultrasonic pretreatment reduced the contents of total soluble solids, titratable acids, vitamin C, total phenols and total flavonoids in kiwifruit slices. The LF-NMR results of hot air drying showed that the water redistribution caused by ultrasonic pretreatment could accelerate the moisture migration during hot air drying. MRI found that the brightness area of kiwi slices decreased with the extension of drying time, and the red gradually decreased, indicating the decrease of water during drying. In addition, MRI images also showed that the moisture on the surface of kiwifruit slices was first removed during hot air drying, and then the internal moisture migrated to the surface for evaporation. Scanning Electron Microscope (SEM) images showed that micro channels and folds were formed in kiwifruit slices after ultrasonic pretreatment. These channels and folds led to the changes of water state and distribution, and accelerated the migration and removal of water during hot air drying. Compared with the control sample, ultrasonic pretreatment increased the drying rate of hot air drying process by 7.6% - 17.5%. In addition, ultrasonic pretreatment had no significant effect on the total color difference of dried kiwifruit slices (P>0.05), but decreased the hardness, cohesiveness, chewiness and resilience of dried kiwifruit slices (P<0.05). The results showed that ultrasonic pretreatment could accelerate the hot air drying speed and improve the texture characteristics of kiwifruit by changing the microstructure, water state and distribution of kiwifruit, but it would cause the loss of some nutrients. On this basis, the pretreatment conditions of ultrasound power 400 W for 20 min or 600 W for 10 min were more suitable. The research results provide a reference for the application of ultrasonic pretreatment in the hot-air drying of kiwifruit.