Biocompounds retention after drying of red raspberry fruits by infrared energy and hot airIntroduction

Abstract. Drying keeps as a cheap, feasible and suitable preservation method for fruits. Berries have been recognized as an excellent source of important biocompounds, and it is desirable to preserve them, in order to get the benefits of these biocomponents all the year and for further applications. The objective of this study was to evaluate the retention of biocompounds in raspberries after two methods of drying: infrared and hot air. The drying was carried out in batches of 50 g of raspberries. Fruits were infrared-dried at 55ºC using a lamp with 200 W of power, placed at 11 cm of the sample and with air at 2 m/s. For hot air, raspberries were dried at 85ºC with an air velocity of 5 m/s. The anthocyanin content was determined by pH differential method using sodium borate buffer at pH 1.0 and pH 4.5; total flavonoids content was measured with reagent AlCl3 in methanol, using quercetin as standard. The dimensions, color, moisture content and water activity were determined in the fresh and dried berries. The drying kinetics was modeled using empirical models. Infrared drying took 180 min, while hot air drying had duration of 360 min to reduce the moisture content to 2±0.1% w.b. and water activity between 0.528 and 0.651. Anthocyanin content in fresh raspberries (91.8% of moisture content, w.b.) was 2.74 mg/g dry solids, which decreased with the drying. Hot air drying retained 37.7% of the anthocyanin content (1.35 mg/g dry solids), while infrared drying resulted in 30.3% of retention (0.83 mg/g dry solids). Flavonoids content in fresh berries was 97.43 mg/g dry solids, which increased in the dried berries, being higher in fruits dried with hot air (477.94 mg/g dry solids) than in infrared-dried samples (431.23 mg/g dry solids). A good fit was obtained between the experimental data and the data calculated by the models, with correlation coefficients higher than 0.90. Lower color and dimension changes were achieved with the hot air method. Thus, hot air drying at 85ºC of raspberries resulted in better retention of anthocyanin content, as well as in higher flavonoid content, in comparison with infrared drying.