DRYING OF APPLES IN A FLUIDIZED BED DRYER

Fluidized bed driers present some advantages over other driers, such as higher heat transfer due to the more effective contact between apples and drying air, and low operating cost. The objective of this work was to produce apple snacks in a fluidized bed, and to evaluate the use of ultrasound as a technology to improve the efficiency of the drying process. The experiments were based on a Central Composite Design (CCD): pre-treatment time in ultrasonic bath (10, 20 and 30 min), and temperature in the fluidized bed (30, 40 and 50 °C). Analysis of moisture, water activity, water gain and loss of solids were performed after ultrasonic processing and along the fluidized bed drying. The results allowed the calculation of the water diffusivity during the drying process. The effect of ultrasonics on samples generates microchannels in the apples cubes, which enhances the mass transport between the apple and water during the pretreatment and between the apple and air during drying. Values for water gain during the pretreatment ranged from 2.12 to 6.82% and the loss of solids presented a higher value (18%) on the pretreatment at 30 °C for 30 min, showing the efficiency of this pre-treatment. The results of water diffusivity in treatments at 50 ºC showed a maximum of 2.81x10-9 m²/s. The use of ultrasound pre-treatment reduced the total processing time to 220 min when drying at 30 °C, when compared to the treatment without ultrasound (280 min). Under this condition, the moisture of the apple snacks was 9.87 % and water activity was 0.450, thus being safe from microbiological growth. Ultrasound potentiated drying at 30 °C, reducing processing time, while its effects at higher drying temperatures was only slight.