A quartz crystal microbalance (QCM) has been used to study the fouling of chromium and hydrophobically modified gold surfaces when heated with milk proteins. Solutions of pure beta-lactoglobulin and a commercial skimmed milk powder were studied at 3 wt%, neutral pH, and before and after heating the solutions to 80 degrees C. The ease of removal of the adsorbed protein by rinsing with buffer and 1 wt% Tween 20 was also studied. The beta-lactoglobulin behaved rather similarly on the hydrophobic gold and chromium surfaces: Tween 20 was not particularly effective in removing this protein after heating. On the other hand, Tween 20 seemed more efficient at removing the heated skimmed milk protein from the hydrophobic gold surface, but less efficient at removing the skimmed milk from the chromium surface (which also exhibited the highest adsorbed amounts of either protein). On chromium, trypsin followed by buffer removed almost all the beta-lactoglobulin but had little effect on the adsorbed layers from skimmed milk. These changes are interpreted in terms of the hydrodynamic thickness of the adsorbed films and lead to the conclusion that the QCM I is a highly sensitive way of monitoring adsorbed film properties during heating, cooling, and detergent action.