Equilibrium moisture isotherms of two-phase solid olive oil by-products: Adsorption process thermodynamics

The equilibrium moisture curves of the two-phase solid olive oil extraction by-product (SOB) are used in order to get a complete understanding of the relationship between water and the solid components within the solid. For that purpose, fundamental thermodynamics of an idealised system, in which the solid acts as an inert adsorbent and the water vapour behaviour corresponds to an ideal gas that adsorbs over the solid surface, were employed to find out an estimation of the most important thermodynamic parameters related to the water adsorption process. The net isosteric heat and entropy of water adsorption were calculated and revealed that only the water molecules in the monolayer, which are very close to the solid surface, are intensively attracted by the adsorbent agent. The interaction energy in the monolayer reached a maximum at 19 kJ mol−1, in accordance with the typical energy amount involved in physisorption phenomena. The behaviour of the water spreading pressure versus water activity was also studied, yielding in the equilibrium moisture curves in terms of spreading pressure from which it is possible to compute the net integral values of the enthalpy and entropy involved in water adsorption. These integral quantities represent in fact a real quantification of the equilibrium enthalpy and entropy of the adsorbed water molecules. The analysis of these data provides all the information concerning the degree of association between water and the two-phase SOB, the different energetical binding levels of the water and the quantity of energy required to remove this water in a drying treatment of this product.