Cross-linking of proteins and other changes in UHT milk during storage at different temperatures

Many chemical changes take place in UHT milk during storage. In this work UHT milk was stored at different temperatures (5°C, 20°C, 37°C and 45°C) for 12 weeks and assessed for pH, browning, proteolysis and protein changes by HPLC, PAGE and lysinoalanine determination. There was a greater decrease in pH and increase in browning, due to Maillard reactions, and proteolysis in samples stored at 37°C and 45°C than in samples stored at 5°C and 20°C; these changes were positively related to both time and temperature of storage. The RP-HPLC chromatogram of the proteins in the UHT milk samples stored at 45°C changed considerably during storage and at the end of 12 weeks' storage was completely different from that of a sample stored at 5°C, indicating that considerable change takes place in the proteins of samples stored at elevated temperature. Lysinoalanine concentration, an indicator of covalent, non-disulfide cross-linking of proteins, increased with storage temperature and time. The cross-linking was also observed by reduced SDS-PAGE from which the percentages of cross-linked milk proteins were estimated to be 1.1%, 1.3%, 7.0% and 36.8% after 12 weeks' storage at 5°C, 20°C, 37°C and 45°C, respectively.