Effect of temperature on water sorption equilibrium of onion (Allium cepa L.)

Results of the experimental measurement of sorption isotherms at different temperatures obtained by the standard gravimetric method showed the typical sigmoid shape of products rich in carbohydrates. The temperature had the expected effect predicted by the theory of physical adsorption i.e the quantity of sorbed water at a given water activity increased as the temperature decreased. The amount of adsorbed or desorbed water per unit weight of solid depends on the equilibrium temperature. The increase of the temperature results in increase of water activity for the same moisture content which in increases the rates of reactions leading to quality deterioration. As the water activity is raised beyond the intermediate region, water is sorbed by low molecular weight constituents resulting in shifting the isotherm upward. The values of the net isosteric heal of desorption were found to be higher than those or adsorption, which indicates that the binding energy for the latter is higher. At low moisture content the heats of desorption is much higher than that of adsorption.