Abstract
The experiments of contact ultrasound intensified heat pump drying (CUHPD) of kiwifruit slices were conducted to investigate the effect of ultrasonic power on dehydration process and water migration of kiwifruit during CUHPD. The results clarified that contact ultrasound (CU) application could reduce the drying time of HPD significantly, and the rise of ultrasonic power had stronger reinforcing effect on dehydration rate. Weibull distribution function could simulate the dehydration process of CUHPD of kiwifruit with high precision, and the β values verified that ultrasonic power improvement could transform the moisture diffusion mechanism of CUHPD from total internal moisture diffusion control to partial internal moisture diffusion control. The Dcal values were 2.304 × 10−9~4.026 × 10−9 m2/s and increased as the rise of ultrasonic power. The scanning electron microscopy results illustrated that increasing ultrasonic power could produce more porous and spacious microstructure which was beneficial for water migration. The low-field nuclear magnetic resonance results elucidated that free water, immobilized water, and bound water in kiwifruit migrated and changed during CUHPD. Free water with the highest mobility was the most abundant water in kiwifruit, and was the first kind of water to be completely removed. With drying progressed, the contents of immobilized water and bound water gradually increased and then decreased, but they were not completely removed. The increasing in ultrasonic power was beneficial to promoting internal water migration and shortening the required dehydration time, especially for free water. Proton density images visualized that increasing ultrasonic power could significantly promote the water diffusion from internal kiwifruit outward the surface, and thereby accelerate the rate of water removal. Therefore, CU application is a suitable method to accelerate the water diffusion and moisture migration of HPD process.
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