MODELING WATER ADSORPTION ISOTHERMS OF PINHAO (ARAUCARIA ANGUSTIFOLIA SEEDS) FLOUR AND THERMODYNAMIC ANALYSIS OF THE ADSORPTION PROCESS

The seeds of Araucaria angustifolia, commonly known as pinhao, are widely consumed in both Southern and Southeastern Brazil because of their high nutritious value. The literature on the technological aspects of this seed is still very scarce. Moisture adsorption isotherms of pinhao flour were determined at 10, 20, 30 and 40C using the gravimetric method. Results show that temperature has little effect on the adsorption process and the Chirife model was found to best represent the experimental data. The isosteric heat of sorption (differential enthalpy) was calculated by using the moisture adsorption isotherm and decreased as moisture content increased, becoming almost constant at 0.3 kg water/kg dry solids. The enthalpy-entropy compensation theory was applied to adsorption isotherms and the isokinetic temperature for pinhao flour was calculated by plotting the differential enthalpy versus differential entropy. It was found that the adsorption process investigated was enthalpy controlled.