Fitting of different models for water vapour sorption on potato starch granules L. Czepirski a,* , E. Komorowska-Czepirska a , J. Szymon Âska b a Faculty of Fuels and Energy, University of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Cracow, Poland b Department of Chemistry, Agriculture University, al. Mickiewicza 24/28, 30-059 Cracow, Poland Abstract Water vapour adsorption isotherms for native and modi®ed potato starch were investigated. To obtain the best ®t for the experimental data several models based on the BETapproach were evaluated. The hypothesis that water is adsorbed on the starch granules at the primary and secondary adsorption sites as well as a concept considering the adsorbent fractality were also tested. It was found, that the equilibrium adsorption points in the examined range of relative humidity (0.03±0.90) were most accurately predicted by using a three-parameter model proposed by Kats and Kutarov. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Water sorption; Bio-based adsorbents; BET model; Fractal dimension Parameters of isothermal equilibrium sorption pro- cess for food products are very important from both scienti®c and practical points of view. Carbohydrate biopolymers (like starch, cellulose, hemicellulose) adsorb water vapour from air, alcohols or organic vapours easily and selectively [1,2]. These bio-based adsorbents containing different classes of chemical compounds are chemically heterogeneous. Moreover, theyarealsophysicallyheterogeneousbecauseofboth crystallite and amorphous phases are present in their molecules. In practice, equilibrium data are necessary to cal- culate parameters of the processes like drying and packaging, which depend mainly on the product water activity. In many cases, the obtained isotherm can represent, with reasonable accuracy, the sorption data for the whole range of water activities. The aim of this study was to determine the water sorption isotherms and to evaluate the utility of var- ious adsorption models for describing water sorption on potato starch granules. The samples of native and modi®ed by a deep freezing/thawing process (accord- ing to the procedure described by Szymon Âska and co- workers [3,4]) potato starch were used in this work. Water vapour adsorption isotherms were measured volumetrically at 298 K using a device for simulta- neous determination of isotherms and kinetics of adsorption [5]. Before the experiment the samples were outgassed at 343 K to residual pressure about 10 4 mmHg. The typical example of water sorption isotherm for potato starch granules is given in Fig. 1. The pre- sented isotherm belongs to II type (according to the IUPAC classi®cation) and describes the polymolecu- lar water vapour sorption in a wide range of relative pressure. At low relative pressure water molecules penetrate the starch granule surface, where they form a monomolecular layer, while at higher pressure the Applied Surface Science 196 (2002) 150±153 * Corresponding author. Tel.: 48-12-617-21-41; fax: 48-12-617-20-66. E-mail address: czepir@uci.agh.edu.pl (L. Czepirski). 0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0169-4332(02)00050-8