Hot air and sun drying of grape leather (pestil) Aysun Maskan, Sevim Kaya * , Medeni Maskan Department of Food Engineering, Engineering Faculty, University of Gaziantep, 27310 Gaziantep, Turkey Received 28 May 2001; accepted 19 September 2001 Abstract 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 (S 1 ¼ 0:71, S 2 ¼ 1:53, S 3 ¼ 2:20 and S 4 ¼ 2:86 mm) and air velocity (V 1 ¼ 0:86, V 2 ¼ 1:27 and V 3 ¼ 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 H 2 O=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 °C. 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  10 11 m 2 =s for hot air drying and 1:93–9:16  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. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Grape; Leather; Pestil; Drying 1. Introduction Food drying is one of the oldest methods of pre- serving food for later use. It is a complex operation in- volving heat and mass transfer which may cause changes in product quality. Physical changes that may occur include shrinkage, puffing and crystallisation. In some cases, desirable or undesirable chemical or biochemical reactions may occur leading to changes in colour, tex- ture, odour or other properties of the food product. Drying can either be an alternative to canning and freezing or complement these methods. Drying occurs by vaporisation of the liquid by supplying heat to the wet feedstock. Heat may be supplied by conduction (contact or indirect dryers), by convection (direct dry- ers), by radiation or volumetrically by placing the wet material in a microwave or radio frequency electro- magnetic field. Over 85% of industrial dryers are of convective type with hot air or direct combustion gases as the drying medium (Chirife, 1983; Mujumdar, 2000). The drying of fruit on a commercial scale has received a resurgence of interest during the past years few espe- cially in the production of fruit ‘‘leathers’’. Leathers are made by removing moisture from a large flat tray of wet puree until the desired cohesive ‘‘leathery’’ composition is obtained (Moyls, 1981). In many processes, improper drying may lead to irreversible damage to product quality and hence a non-saleable product. With modern dehydrators, whole fruits, fruit leathers, fruit chips and pieces can all be dried at any time of the year. Dried fruits and fruit products taste sweeter because the water has been removed thus concentrating the fruit’s flavour and calories. They can be eaten as a snack food or added to various food preparations (Karathanos & Belessiotis, 1997; Mujumdar, 2000). Because of the short harvest season and the sensitivity to storage even at refrigerated conditions, most fresh grapes and grape derivatives should be preserved in some form. Drying is among the commonly used pres- ervation methods. Sun dried fruits and fruit products are the best known of all dried foods. In Turkey, raisins, apricots, figs and fruit leathers (known as pestil) are the most popular sun dried foods. Sun drying permits one to produce a product with a rich colour, a translucent appearance and a desirable gummy texture; however, it Journal of Food Engineering 54 (2002) 81–88 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +90-342-3601105; fax: +90-342- 3601100. E-mail addresses: skaya@gul1.bim.gantep.edu.tr, skaya@gan- tep.edu.tr (S. Kaya). 0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0260-8774(01)00188-1