Effect of Temperature on the Water Adsorption Isotherms of Sultana Raisins

The water adsorption isotherms of Sultana raisins were determined using the static method of saturated salt solutions at 20°, 25°, 30°, and 35°C. The equilibrium moisture content at water activities up to 0.70 decreased as the temperature was increased from 20° to 35°C. At higher water activities the moisture content increased sharply as the temperature was increased, resulting in crossing of the isotherm curves. This behavior is attributed to the increased solubility of fruit sugars at higher temperatures. The experimental data were fitted well with the 3‐parameter GAB and Halsey equations, and best with the 5‐parameter D'Arcy‐Watt equation. The thermodynamic implications are discussed.