Sorption isotherms of potato slices dried and texturized by controlled sudden decompression T. Iguedjtal, N. Louka * , K. Allaf Laboratoire Maı ˆtrise des Technologies Agro-Industrielles, Po ˆle Sciences and Technologies, Universite ´ de La Rochelle, 17042 La Rochelle Cedex 1, France Received 6 April 2006; received in revised form 12 April 2007; accepted 29 June 2007 Available online 20 July 2007 Abstract Controlled sudden decompression to a vacuum (DIC) was used to confer a porous structure to partially dehydrated foods by expand- ing them and facilitating the drying process at lower water content. Moisture sorption isotherms of Bintje potato slices hot-air dried and texturized by DIC were determined and compared using a gravimetric method. The samples were stored in a temperature-controlled chamber at 20, 30 or 40 °C; and relative humidity was controlled between 5% and 90%. Both adsorption and desorption isotherms had a sigmoidal shape (type II). The sorption capacity decreased with increasing temperature at a given water activity. Hysteresis was more pronounced for texturized potatoes than for hot-air dried samples. Non-linear regression analysis was used to fit five moisture sorption isotherm models. The Peleg, Kats and Kutarov, GAB and BET (a w less than 0.5) equations all accurately characterized the sorption behaviour. Texturizing by DIC increased the surface area of potato slices by 45% relative to heat air drying. The net isosteric heat of sorption was found to increase with decreasing moisture content and was higher for texturized potatoes than hot-air dried samples. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Sorption isotherms; Surface area; Isosteric heat; Texturizing; Potatoes; Drying 1. Introduction The preservation and stabilization of the main charac- teristics of foods (taste, crispness,...,etc.) during storage often requires control of their moisture content. Several preservation processes have been developed in order to extent the shelf-life of foodstuffs by lowering the avail- ability of water to micro-organisms and inhibiting some chemical reactions. While moisture content is an impor- tant criterion to judge food quality (Arslan & Tog ˘rul, 2005), water activity (a w ), is an essential additional parameter to describe water availability and mobility in foods. Establishing the relationship between moisture content (X e ) and a w which is known as the sorption iso- therm, is important to understand the stability of food- stuffs (Labuza & Hymann, 1998). Sorption isotherms of foods, which describe the thermo- dynamic equilibrium state of water, are extremely impor- tant, for the design and optimization of drying equipment, for prediction of quality, stability, shelf-life and calculating moisture changes which may occur during storage. (Al-Muhtaseb, McMinn, & Magee, 2004; Durak- ova & Menkov, 2004; Katz & Labuza, 1981; Kaymak-Ert- ekin & Gedik, 2004). The moisture isotherms also allow the isosteric heat of sorption to be defined. The isosteric heat of sorption pro- vides useful insight into the food microstructure as well as a theoretical interpretation of physical phenomenon occurring at the water–food interface (Arslan & Tog ˘rul, 2005; Aviara & Ajibola, 2002; McMinn & Magee, 2003; Mulet, Garcia-Pascual, Sanjuan, & Garcia-Reverter, 2002; Rizvi, 1995; Tolaba, Peltzer, Enriquez, & Pollio, 2004). A number of models have been proposed in the litera- ture to describe moisture isotherms. Chirife and Iglesias 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.06.028 * Corresponding author. Tel.: +33 5 46 45 86 14; fax: +33 5 46 45 86 16. E-mail address: nlouka@univ-lr.fr (N. Louka). www.elsevier.com/locate/jfoodeng Available online at www.sciencedirect.com Journal of Food Engineering 85 (2008) 180–190