In this study the fluorescent probe binding method was used to explain the changes on the surface of milk protein caused by ultrafiltration. Raw skimmilk samples were concentrated 2-fold and 3-fold by ultrafiltration. The hydrophobicity of the surface sites of milk proteins in retentates and unconcentrated milk was examined. It was found that the number of surface hydrophobic sires and affinity to the hydrophobic/fluorescent marker decreased with ultrafiltration. The modification of protein functionality was explained as a changed protein surface hydrophobicity. This study suggests that the renneting pattern for milk is changed by this modification and can be one of the main reasons for some problems in hard cheese manufacturing from retentate. (C) 2000 Elsevier Science Ltd. All rights reserved.