The effect of heat treatment on the structure of the milk protein system has been investigated in unconcentrated bovine skimmilk and its twofold and threefold ultrafiltered retentates. A variation in hydrophobic binding sites of proteins which gives information on hydrophobicity was observed. The energy of activation (E,) values of the samples were compared with each other in the initial phase. The protein surface hydrophobicity of the samples was plotted versus time for four different temperatures. The Ea values which calculated for each temperature were compared with each other in unconcentrated skimmilk and the retentates. This kinetic approach suggests that the heat-induced structural modification of milk proteins is related to the concentration factor of the ultrafiltration treatment. (c) 2005 Elsevier Ltd. All rights reserved.