Hot air and sun drying of grape leather (pestil)

Pestil, a well known fruit leather in Turkey. was prepared from boiled grape juice and starch mixture by using traditional technique. Drying of pestil was carried out by hot air drying and sun drying. The factors investigated in hot air drying were air temperature (55, 65 and 75 degreesC), sample thickness (S-1 = 0.7 1, S-2 = 1.53. S-4 = 2.20 and S4 = 2.86 mm) and air velocity (V-1 = 0.86, V-2 = 1.27 and V-3 = 1.82 m/s). The effects of drying time. temperature and slab thickness on moisture content of pestil during drying were significant (P < 0.05) and of air velocity was not (P > 0.05). Depending on sample thickness and air temperature, the drying time ranged between 50-140 min to achieve the commercial moisture content of pestil (0.12 kg H2O/kg DS) in air drying. Whereas, sun drying took 180-1500 min. Almost all samples dried in the falling rate period. except S-2, S-3, S-4 of sun drying and S-4 at 55 degreesC. The latter had a short (negligible) constant rate period. Effective moisture diffusivity values were estimated from Fick's diffusion model. These values were between 3.00-37.6 x 10(-11) m(2)/s for hot air drying and 1.93-9.16 x 10(-11) m(2)/s for sun drying. Activation energy value of water diffusion was calculated using an Arrhenius-type equation. The estimated values were 21.7, 16.5. 12.0 and 10.3 kJ/mol for S-1, S-2, S-3 and S-4. respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.