Thermodynamic analysis and modeling of water vapor adsorption isotherms of roasted specialty coffee (Coffee arabica L. cv. Colombia)

The experimental assessment and computer modeling of the water vapor adsorption isotherms of roasted specialty coffee is addressed in this study. Thus, both coffee beans and ground coffee of medium (850 μm) and fine (600 μm) particle sizes were analyzed over a range of water activities of between 0.1 and 0.9 and at temperatures of 25, 35, and 45 °C. The adsorption isotherms were determined using the dynamic dew point (DDI) method. The computer modeling of adsorption isotherms was addressed in order to describe the influence of the water activity and temperature on the equilibrium moisture content. Furthermore, the hygroscopic capacity of roasted coffee was analyzed by differential thermodynamic analysis. Experimental results and modeling showed that the high level of moisture adsorption found in the ground coffee was related to a large adsorption area, suggesting that specialty coffee should preferably be stored as beans. The Peleg empirical model was the most suitable at representing both type III upward concave adsorption behavior and the effect of temperature on the adsorption isotherms. Differential thermodynamic analysis revealed an increase in the water adsorption energy at low equilibrium moisture content, while negative Gibbs free energy values revealed the spontaneity of the adsorption process.