The water sorption behaviour and phase transitions of dairy proteins (beta-casein and beta-lactoglobulin), dextrans (dextran6 and dextran500) and their mixtures were studied at low water content. Freeze-dried polysaccharide samples containing between 20 and 80% dairy protein were equilibrated at different water activities (a(w)) between 0.11 and 0.75, at 25 degrees C. Water sorption isotherms of pure compounds and mixtures, as well as glass transition at different water activities were determined. Crystallization of polysaccharides was also investigated. BET and Gordon and Taylor equations were used to model water adsorption isotherms and glass transition temperature behaviour, respectively. Polysaccharides showed a higher water adsorption capacity than dairy proteins in the range of aw studied, which decreased with the addition of protein. The addition of beta-casein decreased the Tg values of dextran systems. This effect was attributed to water migration from beta-casein to the polysaccharide fraction following the formation of beta-casein hydrophobic interactions. Likewise, dairy proteins provoked an increase in the temperature of dextran crystallization and a decrease in the enthalpy. This effect did not reflect the increase of dextran molecular mobility in the presence of beta-casein but could be masked by other factors, like steric hindrance. The effect of dairy proteins, especially beta-casein, on the phase transitions of polysaccharides should be considered for controlling the Maillard reaction, as well as physical and chemical changes that occur during processing and storage of food systems. (C) 2011 Elsevier Ltd. All rights reserved.