Moisture sorption isotherms and thermodynamic properties of tiger nuts: An oil-rich tuber

Abstract

The moisture adsorption isotherms of tiger nuts were determined by a gravimetric static method at 15, 25, 35 °C within the water activity varying from 0.113 to 0.979. Ten mathematical models were selected to fit the adsorption data using non-linear regression technique. Thermodynamic properties such as isosteric heat, sorption entropy and Gibbs free energy were evaluated. The results indicated that equilibrium moisture content of tiger nuts tended to increase with increasing water activity and decreasing temperature. The isotherm curves of tiger nuts exhibited a Type Ⅱ sigmoidal shape. Peleg model was the most suitable for describing the experimental sorption behavior, followed by the Smith and GAB models. The adsorption surface area and monolayer moisture content of tiger nuts obtained from GAB model were 166.37–149.74 m2/g solid and 4.71–4.24 H2O/100 g solid, respectively, which decreased with increasing temperature. Moreover, the isosteric heat of adsorption decreased exponentially with the increase of equilibrium moisture content, while the Gibbs free energy was opposite. Adsorption entropy was negative in value and it decreased sharply and then rose slowly as the moisture content increased, having a minimum value (6.43 J/mol K) at a moisture content of 11 g/100 g d.b.