Integrated multi-dimensional micro-structure reveals the changes in moisture migration and quality attributes during drying of apple slices with ultrasonic pretreatment

Optimizing drying efficiency and enhancing product quality are crucial for industrial apple dehydration processing. This study focused on multi-dimensional micro-structure changes of apple slices with ultrasonic pretreatment (US). The results indicated that US pretreatment significantly enhanced mass transfer by depolymerizing cell wall pectin to increase water-soluble pectin (72.15 to 90.35 mg/g AIR) and chelate-soluble pectin (35.31 to 54.69 mg/g AIR), converting bound water to free water, disrupting the structural integrity of the cells, thereby reducing drying time by 13.31-38.82 % for hot-air drying and 32.60-36.88 % for infrared drying. Concurrently, the microscopic structural changes induced by US treatment enhanced rehydration capacity and minimized bio-active compound's loss, particularly chlorogenic acid and malic acid. Although the reduction in total phenols and flavonoids resulted in decreased antioxidant activity, appropriate US treatment enhanced color by activating polyphenol oxidase (PPO). Consequently, optimal US pretreatment is essential for enhancing both the drying efficiency and quality of apple slices.