Ultrasonically enhanced low-temperature drying of apple: Influence on drying kinetics and antioxidant potential

Low-temperature air drying represents an alternative means to hot air drying of better retaining the sensory, nutritional and functional properties of foods. However, reducing the air temperature to figures below the product's freezing point involves low drying rates, which largely places constraints on any further industrial application. The main aim of this work was to evaluate the feasibility of using power ultrasound to improve the low-temperature drying of apple, considering not only the kinetic effects but also the influence on the antioxidant potential of the dried apple. For that purpose, apple (Malus domestica cv. Granny Smith) cubes (8.8 mm side) were dried (2 m/s and a relative humidity of under 10%) at low temperatures (-10, -5, 0, 5 and 10 degrees C) with (20.5 kW/m(3)) and without ultrasound application. The drying kinetics were modeled by considering the diffusion theory, negligible shrinkage and cubic geometry. In the dried apple, total phenolic and flavonoid contents and antioxidant capacity were measured. The application of power ultrasound sped up the drying kinetics at every temperature tested, achieving drying time reductions of up to 77%, which was linked to the improvement in diffusion and convective mass transport. In overall terms, ultrasound application involved a greater degradation of polyphenol and flavonoid contents and a reduction of the antioxidant capacity, which was related to the cell disruption caused by the mechanical stress of acoustic waves. (C) 2014 Elsevier Ltd. All rights reserved.