Effects of storage temperature on physico-chemical characteristics of semi-skimmed UHT milk

Storage of UHT milk results in physico-chemical changes, which can sometimes lead to aggregation or sedimentation of milk. In this study, close attention was paid to the reactions occurring in semi-skimmed UHT milk during 6 months of storage at 4, 20 and 40 degrees C. Overall milk characterization revealed the development of the Maillard reaction and proteolysis which led to acidification of the milk. One hundred eighty-one peptides were identified by mass spectrometry for the freshly processed UHT milk and the milks stored 6 months at 4, 20 and 40 degrees C. The cleavage sites gave information concerning the possible actors of proteolysis. Plasmin, cathepsins B, D and G, elastase and proteases from Pseudomonas fluorescens B52 were thus found to be potential contributors to enzymatic proteolysis. Non-enzymatic proteolysis induced by heat-treatment and storage was also observed. Despite these modifications, milk particles (casein micelles and homogenized fat globules) did not exhibit many changes in zeta-potential, except for the last storage time at 40 degrees C where a decrease of the absolute value of about -3 mV was observed. The decrease in size in milks stored at 20 and 40 degrees C was only about 20 nm after 6 months. Similarly, storage of UHT milk showed that the higher the storage temperature the lower the heat stability and the higher the phosphate stability. Storage leads to physico-chemical changes in milk and, although some reactions such as acidification and proteolysis are known to be destabilizing, some of them are probably stabilizing to counterbalance the negative effects. (C) 2007 Elsevier Ltd. All rights reserved.