EFFECTS OF TEMPERATURE AND WATER ACTIVITY ON THE SORPTION HEAT OF WHEY AND YOGURT POWDER SPRAY WITHIN THE TEMPERATURE RANGE 20–40C

Abstract

ABSTRACT Moisture sorption isotherms of whey and yogurt powder spray in the temperature range of 20–40C and water activity from 0.4 to 0.99 were determined using gravimetric method. Five sorption models were evaluated. The Henderson's and Chung–Pfost's equation gave the best fit for sorption data of whey and yogurt powder spray, respectively. The isosteric heat of sorption was defined and then calculated using corresponding sorption model and Clausius–Clapeyron equation. An exponential function fitted the experimental heat of sorption values at various moisture contents (MCs) for both milk powder products. The curves on diagrams show that the sorption heat decreases when the MC is increased. Calculated values decreased, 47.03 kJ/mol and 46.37 kJ/mol for whey and yogurt powder spray, respectively, to the values approaching the heat of vaporization of pure water. Values of the heat of desorption were higher than those of adsorption; the difference increases with the MC decrease. PRACTICAL APPLICATIONSHeat of sorption is a valuable tool for food scientists and technologists because it shows, usually in a graphical form, the energy consumption for water desorption and adsorption in the specific temperature range and furthermore the temperature dependence of water activity (Aw). Values of heat of sorption, which approach the heat of vaporization of pure water, are also indicative of free moisture content (MC) in the material. Whey and yogurt powder spray are important spray dryer products. They are significant export articles, too. The minimal storage time of these foods is commonly longer than 1 year. To guarantee the long‐term quality of these spray dryer products means to analyze and to monitor in practice their basic thermodynamic parameters such as Aw and temperature or heat of sorption, MC and parameters of near ambient air, too.