Intensification of Predrying Treatments by Means of Ultrasonic Assistance: Effects on Water Mobility, PPO Activity, Microstructure, and Drying Kinetics of Apple

The effects of ultrasound assistance during a predrying treatment (soaking) on water mobility, polyphenoloxidase activity, microstructure, and the drying kinetics of apple (var. Granny Smith) have been studied. Predrying treatments were carried out using three different soaking media, distilled water, apple juice, and citric acid, at 25 A degrees C during 5 min, without the assistance of ultrasound (S) and then with two acoustic densities: 2.1 W/cm(2) (U1), and 12.9 W/cm(2) (U2). Transverse relaxation times (T-2) of vacuole and cytoplasm water of treated samples were longer than those in untreated samples (fresh apples) especially when the soaking assisted by ultrasound was used, resulting in greater mobility of internal water and easier water removal during convective drying. SEM observation showed that the ultrasound assistance promoted changes in the apple microstructure of treated samples regardless of the soaking medium. The polyphenoloxidase activity was affected by the soaking medium and the use of ultrasound during treatment, being higher reduction in samples treated with apple juice and citric acid, when the highest acoustic density was used (40 and 58 %, respectively). The effective water diffusion during hot air-drying (50 A degrees C, 1 m/s) was higher in apples treated with and without power ultrasound than that in the fresh ones, causing a significant decrease in the drying time. This figure was higher when the soaking was carried out using citric acid and at an acoustic density of 12.9 W/cm(2).