The effect of raw milk quality (total and psychrotrophic bacterial and somatic cell counts, proteinase and plasmin activity) and UHT temperature (145 or 150 degrees C for 4 s) on proteolysis in UHT milk processed by a direct (steam -injection) system was investigated during storage at 25 degrees C for 180 d. High proteinase activity was measured in low-quality raw milk, which had high somatic cell count, bacterial count and plasmin activity. The levels of 12% trichloroacetic acid-soluble and pH 4.6-soluble nitrogen in all milk samples increased during storage, and samples produced from low-quality milk at the lower UHT temperature (145 degrees C showed the highest values. Bitterness in UHT milk processed from low-quality milk at 145 degrees C increased during storage-. gelation occurred in that milk after 150 d. The RP-HPLC profiles of pH 4.6-soluble fraction of the UHT milk samples produced at 150 degrees C showed quite small number of peaks after 180 d of storage. Sterilization at 150 degrees C extended the shelf-life of the UHT milk by reducing proteolysis, gelation and bitterness. (c) 2006 Elsevier Ltd. All rights reserved.