Drying characteristics, microstructure, glass transition temperature, and quality of ultrasound‐strengthened hot air drying on pear slices

In order to explore the reinforcement effect of contact ultrasound on hot air drying (HAD), the effects of different ultrasonic powers and drying temperatures on drying characteristics and quality of pear slices were studied. The results showed that the increase in ultrasonic power and drying temperature could significantly shorten drying time. The drying process of pear was internal diffusion control, and ultrasound application could reduce internal diffusion resistance as well as improve dehydration rates. SEM results illustrated that ultrasound's cavitation and mechanical effects could produce more microchannels and dilate intercellular spaces in pear slices. DSC results revealed that ultrasound application during drying could improve water mobility and then reduce glass transition temperature. Both drying temperature and ultrasonic power had significant effects on total phenolic, total flavonoids, VC, and rehydration ratio, and contact ultrasound application in HAD was beneficial to increasing the nutrient component contents. Practical applications Hot air drying (HAD) is a traditional and frequently used drying method. Yet it has the defects of low thermal efficiency, lengthy time, and unacceptable product quality. Nowadays, the application of contact ultrasound strengthening technology has drawn great attention in drying enhancement. When contact ultrasound is applied in HAD of pear, ultrasonic energy is directly transmitted into materials without passing through any medium, so as to enhance the utilization and enhancement effect of the ultrasound. The ultrasound application could enlarge and even produce more microchannels in pear slices, which could accelerate moisture migration. Contact ultrasound application in drying process was beneficial to improving the drying rate as well as increasing nutrient components. Therefore, contact ultrasound-strengthened HAD technology is promising and could be applied for drying agricultural products.