The non-reducible (irreversible) oligomerization of casein isolated from UHT-treated skim milk after storage at various temperatures was investigated using amino acid analysis and high-performance gel-permeation chromatography. The content of polymerized casein increased from 8.2 to 11.7, 19.5, and 53.9% for UHT skim milk stored 6 months at 4 degrees C, 20 degrees C, and 37 degrees C, respectively. Lysinoalanine and histidinoalanine were formed in similiar amounts, correlating with storage time and temperature. Only 14 - 50% of non-reducible casein polymerization could be explained by intermolecular formation of LAL and HAL, indicating that significant amounts of other types of crosslinked amino acids must have been formed, most probably sugar - amine derivatives originating from the Maillard reaction. Using amino acid analysis in combination with fluorescence detection, very small amounts of pentosidine, an unnatural amino acid, derived from the reaction of one arginine and one lysine residue with ribose, could be detected for the first time.