The effect of the concentration of protein, calcium and lactose on heat-induced changes in the surface hydrophobicity and interactions of milk proteins were examined. It was observed that the protein-protein interactions increased with increasing concentration of protein during the heat treatment of milk, and the micelles reorganised into a more compact structure. However, the heat stability of milk increases with decreasing concentration of protein and the changes that occur in protein structure are less, apparently in heat treatment. It was observed that the critical temperature, when the drastic changes in protein surface hydrophobicity start, was higher in lactose concentrated-milk than others. The heat stability of milk increased with increasing concentration of lactose. An increase in the calcium ion concentration of milk caused an increase in colloidal stability during the heat treatment. It is found that when calcium concentration of milk decreased, the critical temperature needs for hydrophobic interactions between the proteins increased. Overall changes occurred above 80 degreesC. (C) 2004 Elsevier Ltd. All rights reserved.