Influence of power ultrasound application on drying kinetics of apple and its antioxidant and microstructural properties

• Apple drying assisted by power ultrasound has been studied.
• Acoustic energy affected the diffusion and external mass transfer coefficients.
• Increasing drying temperature reduces quality loss if ultrasound is not applied.
• Shrinkage pattern was similar in samples dried with and without ultrasound.

The effect of drying air temperature and power ultrasound (US) application on the drying curves of apples (var. Granny Smith) and on the quality (total polyphenol (TPC) and flavonoid (FC) contents, the antioxidant activity (AA) and microstructure after drying) have been evaluated. Drying curves were studied at air temperatures of 30 °C, 50 °C, and 70 °C; without ultrasound (AIR) and using two levels of ultrasonic power: 18.5 kW/m3 (AIR + US1) and 30.8 kW/m3 (AIR + US2), being the drying time significantly (p < 0.05) reduced as these two operational variables increased. A diffusional model, taking into account the influence of drying temperature and US power on both the diffusion and the external mass transfer coefficients allowed the accurate simulation of the drying curves (MRE = 5.8 ± 2.1%). TPC and FC were affected by drying temperature and US application. In AIR samples, the increase of drying temperature led to lower losses of TPC, FC, and AA. The US application (AIR + US1 and AIR + US2) involved lower TPC and FC losses in comparison to AIR samples only at 30 °C. The use of US promoted notorious changes in the microstructure (SEM observation) of apple samples, in comparison to the samples dried without US at all the temperatures considered, due to the stress developed by ultrasonic waves.