A. Correa

Prediction of water activity of osmotic solutions

Models to correlate and predict water activity in aqueous solutions of single and multiple solutes, including electrolytes, relevant for osmotic processing of foods are reviewed. During the last decade a significant number of theoretical thermodynamic models that are applicable to these systems have been developed and published. Though their use is still limited, their performance is in general very good, similar to the best traditional empirical equations. Their predictive character together with built-in capabilities to work at different temperatures and in some cases pressure suggests that an increased effort to their wide use should take place. It was found that predictions of water activity in aqueous solutions may easily be made with average relative deviations of less than 2%; this value is of the same order or in some cases less than the typical error of current instrumentation available to measure water activity.

Measurement and prediction of water activity in electrolyte solutions by a modified ASOG group contribution method

Abstract A set of new ionic type functional groups for the prediction of water activities (aw) in aqueous solutions of electrolyte solutes using the ASOG group contribution method is proposed. Previously published experimental data on water activities, osmotic coefficients and freezing temperatures for binary solutions of electrolyte salts and water at different temperatures were used to calculate interaction parameters. With such parameters, values of aw for binary (14), ternary (28) and quaternary (3) systems, at different temperatures, were predicted and compared with experimental data. This data included both published and new data, measured with an electric hygrometer, for sodium nitrate (at 20°C) and potassium nitrate (at 20°C and 30°C). Calculated average relative deviations of aw predictions using the ASOG method were 0.21%, 0.28% and 0.20% respectively.

Measurement of water activity in water-urea-“sugar” and water-urea-“polyol” systems, and its prediction by the ASOG group contribution method

Abstract Correa, A., Comesana, J.F. and Sereno, A.M., 1994. Measurement of water activity in water-urea-“sugar” and water-urea-“polyol” systems, and its prediction by the ASOG group contribution method. Fluid Phase Equilibria, 98: 189-199. Water activities in aqueous solutions of urea with sugars (glucose and fructose) and polyols (glycerol, sorbitol and mannitol) at 25°C were measured with an electric hygrometer. Concentration ranges considered in this study reached solubility limits for each solute. Correlation and prediction of water activities using the ASOG group contribution method required the use of a set of new specific groups. The interaction parameters for such new groups were calculated from new and previously published experimental data. Average percent deviations of 0.4% between experimental and predicted aw values were obtained.