After storage of UHT milk at 37 degreesC resp. 50 degreesC, yoghurt was prepared. For a storage temperature of 37 degreesC, breaking strength of the yoghurt samples increased from 2.7 to 5.8 N with increasing storage duration of the UHT milk. A plateau is reached after 17 days of storage. This increase in breaking strength correlates with a significant increase in non-reducible casein oligomerization from 14% for fresh UHT milk to 25% measured using size exclusion chromatography under reducing and denaturing conditions and calculated as sum of predominantly formed dimers and trimers at the total casein fraction. At a storage temperature of 50 degreesC, a less increase in breaking strength from 2.7 to 4.6 N with a plateau after 17 days was observed while casein oligomerization increased to 63%. After acid hydrolysis, only lysinoalanine and histidinoalanine were detected in the caseinate samples via amino acid analysis. The quantified concentration of lysinoalanine and histidinoalanine could not explain the observed casein oligomerization. Thus, unknown crosslinked amino acids must have been formed during storage, inducing significant changes in the functional properties of milk proteins.