Uht Milk Contains Multiple Forms Of Alpha(S1)-Casein That Undergo Degradative Changes During Storage

Milk proteins are susceptible to chemical changes during processing and storage. We used proteomic tools to analyse bovine alpha(S1)-casein in UHT milk. 2-D gels of freshly processed milk alpha(S1)-casein was presented as five or more spots due to genetic polymorphism and variable phosphorylation. MS analysis after phosphopeptide enrichment allowed discrimination between phosphorylation states and genetic variants. We identified a new alternatively-spliced isoform with a deletion of exon 17, producing a new C-terminal sequence, K(164)SQVNSEGLHSYGL(177), with a novel phosphorylation site at S-174. Storage of UHT milk at elevated temperatures produced additional, more acidic alpha(S1)-casein spots on the gels and decreased the resolution of minor forms. MS analysis indicated that non-enzymatic deamidation and loss of the N-terminal dipeptide were the major contributors to the changing spot pattern. These results highlight the important role of storage temperature in the stability of milk proteins and the utility of proteomic techniques for analysis of proteins in food. (C) 2012 Elsevier Ltd. All rights reserved.