Moisture sorption isotherms of whole milk powder in the temperature range of 5–35 °C and critical values of water activity prediction

The study presents results of water sorption tests of whole milk powder in the temperature range of 5–35 °C and water activity (aw) from 0.11 to 0.97. The experimental procedure used was the manometric static method. Four sorption models recommended in literature sources (Chung-Pfost, Halsey, Henderson, Oswin) were analysed and evaluated with the aim of aw prediction. The modified Oswin’s equation was the best model for moisture adsorption and desorption of the whole milk powder. Critical values of the equilibrium moisture content (EMC) from the viewpoint of microorganism growth corresponding to the aw = 0.6 were calculated for the temperature range tested. The critical EMC was 7.82% and 8.51% wet basis (w.b.), for water adsorption and desorption, respectively, at the temperature of 20 ºC. Sorption capacity of samples tested decreased as temperature increased, and vice versa. The differences between the EMC values at a constant aw were small in the temperature range measured, and rehydration of the dried material resulted in hysteresis but this effect was non-significant