Abstract:Sorption isotherms of freeze-dried mango (variety Tainong No.1) were determined at 25℃ and the data were modeled by GAB model. A J-shaped sorption isotherm of mango was presented and the GAB monolayer moisture contents were observed to be 0.107g/g. The unfreezable and freezable water contained in freeze-dried mango was examined to develop the state diagram. The state diagram of mango was developed using glass transition line(glass transition temperature vs solids content), freezing curve(initial freezing point vs solids content) and maximal-freeze-concentration condition. Freezing points and glass transition temperature were measured by differential scanning calorimetry (DSC) as a function of water content. Freezing points were fitted according to the Clausius—Clapeyron model and the parameter E was determined as 0.071 by using non-linear optimization technique. Glass transition was fitted to Gordon—Taylor model. The constants Tgs and k were estimated as 15.7℃ and 4.42, respectively. Glass transition increased with the increase of solids content, which was due to the plasticizing effect of water in mango solids. Freezing points decreased as total solids content increased. The maximal-freeze-concentration condition was calculated to be at a solid content of 84% with characteristic temperature of end point of freezing (T′m) being -32.8℃ and characteristic glass transition temperature (T′g) being -52.9℃. These characteristics revealed that the quantities of unfreezable water were 0.16g/g indicating the amount of reactive water. The state diagram of mango can be used in evaluating the storage stability as a function of temperature and moisture content. Moreover, it can also be used to design drying and freezing processes.
