Microwave and ultrasound pre-treatments for ‘Rocha’ Pear: impact on drying kinetics and selected quality attributes

Rocha’pear (Pyruscommunis L.) is the main cultivar produced in Portugal, and is classified as protected designation of origin (PDO).This pear fruit is characterized by its own typical and specific properties, such as pleasant flavor, crispness and also sweetness.Pears are rich in polyphenols with strong antioxidant activity,offering health promoting benefits. In order to extend its shelf life and increase availability along the year, drying processes are widely applied as a good alternative method for fruits preservation. Drying is used to improve food stability, decrease moisture content and microbial activity, and minimize physico-chemical changes during storage. However, drying process conditions (i.e. air temperature, time, and velocity) usually may lead to food quality degradation in terms of physico-chemical and nutritional attributes. In this context, drying combined with pre-treatments have been proposed in order to reduce the drying impact on products quality. The objective of this work was to evaluate the use of microwave and ultrasound pre-treatments on ‘Rocha’ pear drying kinetics and quality attributes (water activity, color, shrinkage, total phenolics, antioxidant activity, and texture and rehydration behavior of dried slabs).Three different samples were compared: a) Control, b) Microwave Pre-treatment (1540 MW, 4 min) (MW), and c) Ultrasound Pre-treatment (35 kHz, 10 min, 25°C) (US). Drying experiments were conducted on pear slabs (thickness and diameter of 6 and 38 mm, respectively) in a tray dryer with an air temperature of 60°C and a fixed velocity of 0.75 m/s. Pear samples pre-treated with MW had shorter drying time than control and ultrasound treated ones.Pear samples pre-treated with US showed the lowest colour changes and shrinkage, higher total phenolics and antioxidant activity, the highest rehydration capacity, and similar drying time to the control. This study contributes with information for the development of new drying pre-treatment conditions of ‘Rocha” pear fruits, as an alternative to traditional drying in an industrial context.