The general mechanism of water sorption on foodstuffs – Importance of the multitemperature fitting of data and the hierarchy of models Sylwester Furmaniak, Artur P. Terzyk * , Piotr A. Gauden N. Copernicus University, Department of Chemistry, Physicochemistry of Carbon Materials Research Group, Gagarin St. 7, 87-100 Torun, Poland Received 9 November 2006; received in revised form 5 January 2007; accepted 5 March 2007 Available online 19 March 2007 Abstract This paper points out the importance of the multitemperature fitting procedure in description of water sorption on foodstuffs. The data tabulated in literature (water sorption at different temperatures on: chickpea seeds, lentil seeds, potato and on green peppers) were described applying the BET, GAB and recently proposed GDW models. Our results explain total failure of the first model in description of multitemperature data and the similarities between the GAB and GDW are shown. Finally the general mechanism of water sorption on foodstuffs is proposed. This mechanism can be of the GAB or GDW type, depending on the arrangement and features of the primary water sorption sites. If the geometrical constraints for creation of the BET – like type clusters do not occur on surface, and if each from primarily sorbed water molecules convert only into one secondary surface site, one can say that the mechanism follows the GAB scenario (as for example in the case of lentil seeds). Contrary, in the case of rough or porous surfaces, where there are the geometric constraints for creation of secondary sites (for example sorption on chickpea seeds), and/or where one primary site produces more than one secondary site (potato and green peppers), the mechanism of water sorption is of the GDW type. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Enthalpy of sorption; Equilibrium moisture content; Isotherm model; Water sorption mechanism; GAB; GDW 1. Introduction and the aim of the study It is well known that sorption isotherms of foodstuffs are very important for design, modelling and optimization of many processes. Different authors (for example Czepirski, Komorowska-Czepirska, & Szymon ´ ska, 2002, 2005; Hoss- ain, Bala, Hossain, & Mondol, 2001; Lewicki, 1997, 2000) pointed out the importance of those data in drying, aera- tion, predicting of stability and quality during packaging and storage of food. Therefore, different more or less advanced adsorption models have been used (with greater or smaller success) in the field of food engineering science for description water sorption data. Here very important question arises about the purpose of application of those models. Analyzing different results presented in many papers from the area of interest one can conclude that in the most of cases authors apply a model since they have (or try) to do something with experimental data (i.e. describe them applying mathematics). In many cases they do not analyze the fundamental assumptions of the applied theory, its physical validity for the studied case and so on. More- over, often the model applied to describe of a set of experi- mental data says anything about the mechanism of the adsorption process and, what is more important, the data obtained from the fitting of different models lead authors only to the conclusion that a model, say, A is better than model B. In our opinion the major features of a chosen model should be the reality and simplicity, while the major purpose of its usage is something that one can call ‘‘the pre- dictive ability”. Therefore, the evaluated (by fitting to exper- imental data) parameters of the model can be applied to predict different sets of data, for example the sorption results measured for different temperature(s). The measurements of temperature dependence of water sorption on foodstuffs is 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.03.012 * Corresponding author. Tel.: +48 056 611 43 71; fax: +48 056 654 24 77. E-mail address: aterzyk@chem.uni.torun.pl (A.P. Terzyk). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 82 (2007) 528–535