Use of pestil as an edible film and coating

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°C), sample thickness (Si=0.71, S2=1.53, S3=2.20 and S4=2.86 mm) and air velocity (Vi=0.86, V2=1.27 and V3=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 (11 % wet basis) in air drying. Whereas, sun drying took 180 to 1500 min. Almost all samples dried in the falling rate period, except S2, S3, S4 of sun drying and S4 at 55°C. The latter had a short (negligible) constant rate period. Effective moisture IIIdiffusivity values were estimated from Fick's diffusion model. These values were between 3.00-37.6x1 0"11 m2/s for hot air drying and 1.93-9.16x10"11 m2/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 Si, S2. S3 and S4, respectively. The Hunter colour parameters (L, a, b) of grape juice during boiling and of pestil samples during drying were investigated. It was found that most of the colour destructed during boiling. All Hunter parameters changed significantly. However, either during hot air or sun drying, only Hunter-a value changed. The colour destruction reaction followed a zero order reaction kinetics. The water vapour permeability (WVP) properties of pestil at three different temperatures (15, 25 and 37°C) and four relative humidity gradients (20, 50, 70 and 100% RH) were tested. It was found that water vapour permeability was strongly dependent on relative humidity. Pistachio nut meat was coated with pestil solution and kept at 15°C and three different relative humidities. Results showed that when the coated pistachio nut samples were kept at about 55% relative humidity, weight of the samples were almost constant. The coated samples absorbed moisture at high relative humidity (76.46%) and lost water at low relative humidity (33.33%) environments. Key words : Grape, pestil, drying, colour change, edible film, water vapour permeability, pistachio nut meat.