Effect of contact ultrasonic power on moisture migration during far‐infrared radiation drying of kiwifruit

Abstract

AbstractIn order to explore the effect of direct contact ultrasound (DCU) on moisture migration during far‐infrared radiation (FIR) drying of kiwifruit, the drying characteristics, microstructure, water status, and moisture change of kiwifruit slices during drying were investigated based on scanning electron microscopy (SEM), low‐field nuclear magnetic resonance (LF‐NMR), and magnetic resonance imaging (MRI). The results showed that the drying time at ultrasonic power of 54 W was 40% shorter than that without DCU application. SEM observation verified that higher ultrasonic power could produce more and larger microcapillaries in the microstructure of kiwifruit slices, which was beneficial for moisture migration. LF‐NMR detection indicated that water mobility and distribution in kiwifruits changed during drying. The relative proportion of free water in kiwifruits decreased from 70 to 0% with the prolongation drying time at different ultrasonic powers, the removal time for free water at ultrasonic power of 54 W was 57% shorter than that without DCU application during FIR drying, and higher ultrasonic power could accelerate the shifting of water in different states. MRI results illustrated that the DCU application at higher ultrasonic power could promote moisture migration and water removal during FIR drying, and then increase drying rate.Practical applicationsDrying is a commonly utilized process method in food and agricultural industry. Far‐infrared radiation (FIR) drying, as one of the modern drying methods, has desirable advantages including high‐energy efficiency and uniform heating. However, it has unsatisfied promoting effect on internal mass transfer. Ultrasound has significant positive effect on moisture diffusion and water removal. In this study, direct contact ultrasound (DCU) was applied in FIR drying of kiwifruit slices. The study verified that the DCU application in FIR drying could promote drying rate, produce loose microstructure, and shorten drying time significantly. The increase in ultrasonic power could accelerate moisture migration and mass transfer to facilitate water removal. Therefore, DCU application with higher ultrasonic power is beneficial to increasing drying rate and reducing drying time of FIR drying, and FIR drying technology combined with DCU is promising in food industry and can be developed for other food products.