Thermodynamic and kinetic study of water adsorption on whey protein

The objective of this work was to relate the water vapor adsorption kinetics on whey protein (WP) with the thermodynamic equilibrium obtained at several water activities, in order to determine the driving mechanisms of the process. The D'Arcy-Watt model was found to agree very well with the experimental data of water adsorption on WP. The mean relative deviation modulus value (P) was 3.3, 1.3 and 3.7% for 15, 30 and 45 degrees C respectively. Enthalpy-entropy compensation (Thermodynamic criterion) showed two zones: the first was entropy-controlled (Isokinetic temperature (T-B) = 56.2 +/- 1.4 K) and appeared from zero moisture to the moisture content corresponding to the minimum integral entropy (MIE), whereas the second was driven by changes in the enthalpy of water (T-B = 407.1 +/- 6.8 K) and was observed from the MIE until water activities close to 1.0. Theory of pore blockage (kinetic criterion) suggested that immediately after reaching the MIE, the water molecules blocked the micropores mouth forming a resistance it which diminished the water vapor adsorption rate.