Adsorption isotherms and isosteric heat in Coffea arabica beans

Abstract

The knowledge of the adsorption hygroscopic equilibrium in coffee is pertinent for the adequacy of its storage, thus ensuring the quality of the product to the end consumer. The present study aimed to determine the adsorption isotherms of Arabica coffee (Acauã Novo cultivar) beans at the temperatures of 30 °C, 40 °C, 50 °C and 60 °C, as well as the integral adsorption isosteric heat. The equilibrium moisture contents were obtained by the gravimetric static method, using water activities between 0.10 and 0.83 (decimal). From the obtained data, several mathematical models were fitted to represent the adsorption isotherms. The statistical criteria for selecting the best model were the coefficient of determination, estimated mean standard error, relative mean error and distribution of residues. The Peleg model was the one that best represented the adsorption isotherms of the Arabica coffee beans under the studied conditions. The isotherms showed a sigmoidal type II shape. For a constant water activity, an increase in the temperature promotes a decrease in the equilibrium moisture content of Arabica coffee beans. In a moisture content range between 0.0191 and 0.2534 (dry base), the isosteric heat values ranged from 4,042.19 kJ kg-1 to 2,395.48 kJ kg-1.