Sorption isotherms and moisture sorption hysteresis of intermediate moisture content banana Zhengyong Yan, Maria J. Sousa-Gallagher * , Fernanda A.R. Oliveira Department of Process and Chemical Engineering, University College, Cork, Ireland Received 10 November 2006; received in revised form 12 October 2007; accepted 12 October 2007 Available online 22 October 2007 Abstract Sorption isotherms of dried or intermediate moisture content (IMC) products are most important to model moisture uptake during storage and distribution. Working isotherms of IMC bananas were determined at five different temperatures (10, 15, 20, 30 and 40 °C). Desorption and adsorption isotherms were determined at 10 and 40 °C. All the sorption curves were found to be Type II, with non-zero moisture content (approximately 3–9% db for different temperatures) when the water activity was zero. Different models were fitted to the data and it was found that the best results were obtained with a modified Freundlich equation (valid from 0.06 to 0.76 water activity). The dependence of the models constants on temperature was investigated and the secondary models were built accounting for the effect of a w and temperature on the samples moisture content (R 2 = 99%). Hysteresis was observed at 10 °C and 40 °C, but the effect was greater at 10 °C. The net isosteric heat of sorption (working isotherms) varied from 0.85 kJ g 1 mol 1 to 7.67 kJ g 1 mol 1 , decreasing with increasing moisture content. This effect was well described by an exponential function (R 2 > 99%). These results provide reliable experimental data on sorption isotherms which is important for designing an optimized packaging system. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Banana; Hysteresis; Isosteric heat of sorption; Modelling; Sorption isotherm 1. Introduction As most microorganisms would not grow in foods with a w below 0.6, a w of food products would need to keep less than 0.6 to extend product shelf life. Therefore, it is essen- tial for dried or intermediate moisture content (IMC) prod- ucts to know the sorption characteristics of these products in this range of water activity (0 to 0.6), and sorption iso- therms can then be used to predict the shelf life of packaged moisture-sensitive products by modelling moisture uptake during storage and distribution. Sorption isotherms are usually classified according to their shape in five different types: I, II, III, IV and V (Bru- nauer et al., 1940; Basu et al., 2006). Type I curves are con- vex upward throughout whereas type II curves are concave upwards throughout. Types II, IV and V isotherms show one or more inflection points. Various mathematical mod- els have been proposed in literature to describe sorption isotherms. Some were developed with a theoretical basis to describe adsorption mechanisms (e.g. GAB and BET) (Brunauer et al., 1938; Van den Berg and Bruin, 1981), whereas others are just empirical or a simplification of more elaborate models. In some ranges of water activity, sorption isotherms can be approximated to linear equa- tions. Dried food products usually show isotherms of Type II or III. It should be noted that in some cases these equa- tions predict non-zero moisture content for zero water activity, e.g. Chung and Pfost equation (Chung and Pfost, 1967), Smith equation (Smith, 1947), Iglesias and Chirife equations (1) and (2) (Iglesias and Chirife, 1978, 1981), modified Chung–Pfost equation (Pfost et al., 1976), Halsey equation (Halsey, 1948) and modified Halsey equation (Iglesias and Chirife, 1976). Some of equations take into account the effect of temperature, e.g. modified Chung– Pfost equation, modified Henderson equation (Thompson 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.10.009 * Corresponding author. Tel.: +353 21 490 3594; fax: +353 21 427 0249. E-mail address: m.desousagallagher@ucc.ie (M.J. Sousa-Gallagher). www.elsevier.com/locate/jfoodeng Available online at www.sciencedirect.com Journal of Food Engineering 86 (2008) 342–348