Research note Application of the Guggenheim, Anderson and De Boer model to correlate water activity and moisture content during osmotic dehydration of apples Fr  ed eric Prothon, L  ılia M. Ahrn e * SIK, The Swedish Institute for Food and Biotechnology, Department of Environment and Process Engineering, Box 5401, 40229 G€ oteborg, Sweden Received 24 September 2002; accepted 6 April 2003 Abstract Water activity and moisture content are the most valuable characteristics for assessing the stability of dried foods. The aim of this work was to evaluate if a desorption isotherm model could be used to describe the relationship between the water activity and moisture content in the product during osmotic dehydration. This model can lead to a better understanding of how water activity of a product can be reduced during osmotic dehydration. The Guggenheim, Anderson and De Boer (GAB) model was chosen because it has been proven to best fit the desorption and adsorption of foods over a large range of water activities. The studied material was apple pieces, and the osmotic medium with a water activity of 0.939 was prepared with sucrose. The osmotic dehydration process was studied by collecting moisture content and water activity data at different temperatures and times. The model was shown to fit well the experimental points at temperatures 25, 45 and 55 °C. At 65 °C, however, the model is satisfactory if a w is below 0.970 but not above. The effect of temperature can be expressed on the C-parameter of the GAB model. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: GAB model; Osmotic dehydration; Water activity prediction; Temperature 1. Introduction In the context of minimal processing, the measure- ment and prediction of water activity provide the best available tool for evaluating the stability of foods. The sorption isotherm, consisting of a graphic representation of water activity (a w ) against moisture content at con- stant temperature, is a common way of presenting the relationship between these two parameters (McLaughlin & Magee, 1998; Rahman & Labuza, 1999). Desorption isotherms are of particular importance in the design of a food dehydration process, especially in the determina- tion of a drying end point. The end point of drying is the residual moisture content of the final product which ensures economic viability and microbiological safety, i.e. a water activity value lower than 0.60. Desorption isotherms focus therefore on low moisture or interme- diate moisture zones, because this is the range of mois- ture content of the food product after drying methods such as air-drying and freeze-drying. The effect of tem- perature on moisture sorption isotherms has also been reported in many studies (Vel asquez de la Cruz, Torres, & Mart  ın-Polo, 2001; Wang & Brennan, 1991). Numerous mathematical models have been proposed for the study of both adsorption and desorption of foods, such as the GAB, Iglesias and Chirife, BET, Oswin, and Ferro Font an models (Barbosa-Canovas & Vega-Mercado, 1996; Crapiste & Rotstein, 1982; Gekas, 1992; Wang & Brennan, 1991). There is a lack of knowledge on how the overall water activity of cellular material is evaluated near the full turgor state (Crapiste & Rotstein, 1982). The Guggenheim–Anderson–de Boer (GAB) model is reported to be the best for fitting sorption isotherm data for the majority of food products up to a w levels of approximately 0.9 (Barbosa-Canovas & Vega-Mercado, 1996; Timmermann, Chirife, & Igle- sias, 2001; Tsami, Krokida, & Drouzas, 1999). At a w > 0:9, which is the region where cellular material is still in a quasi-turgor state, the water activity was sel- dom accurately measured or studied. Osmotic dehydration is characterized by a more com- plex mass transfer process than air-drying. The transport Journal of Food Engineering 61 (2004) 467–470 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +46-31-335-56-00; fax: +46-31-83-37- 82. E-mail address: lia@sik.se (L.M. Ahrn e). 0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0260-8774(03)00119-5