WATER VAPOR ADSORPTION ISOTHERM OF IMPORTED AND DOMESTIC MODIFIED MILK POWDERS AND CALCULATION OF ISOSTERIC HEAT

The object of this study was to investigate the stability and preservation characteristics of modified milk powder by measuring adsorption isotherm of water vapor and isosteric heat. Adsorption isotherms of water vapor for modified milk powders produced by three different companies of Korea (I, Y, M) and imported products (N, S, J) were measured at 15, 25 and 35 °C using modified COST-90 (European Cooperation in the Field of Scientific and Technical Research) method. The BET-model was applied to compare the experimental value. Our experimental results showed that all isotherms were sigmoid shape. As the temperature was increased, the propensity of EMC was decreased in all samples. The domestic infant formula milk powder of I and imported one of S showed the highest monolayer value of 5.07 g and 7.51 g [H2O]/100 g [solid], respectively. Isosteric heat was calculated from the result of application of BET-model using the Clausius-Clapeyron equation. Isosteric heat was declined in all samples as EMC was higher. As the milk powder adsorbed the water vapor and the water molecule is linked in form of free water, the isosteric heat is closed to the vaporization heat of pure water.