Effect of temperature on moisture desorption isotherms of kheer

Desorption isotherms of kheer, a rice based partially heat concentrated and sweetened Indian milk dessert, were obtained in the temperature range of 10-40°C. The desorption curves exhibited sigmoid shape corresponding to type II, typical of many foods. There was generally a negative temperature effect on EMC at low aw, but curves at 25°C and 40°C showed inversion above water activity of 0.60 implying a higher equilibrium moisture content at higher temperature. Of the five sorption models tested, the GAB model gave the best fit at all the three temperatures. Besides monolayer moisture, properties of sorbed water viz. number of adsorbed monolayers, bound or nonfreezable water, density of sorbed water and surface area of sorption were also obtained. Isosteric heat of sorption obtained by applying Clausius-Clapeyron equation decreased exponentially with the increasing moisture content.