Different proteomic profiles of sweet whey and rennet casein obtained after preparation from raw versus heat-treated skimmed milk

Abstract

Proteomic profiles of sweet whey and rennet casein fractions prepared from raw or pasteurised (72 °C for 15 s), skimmed milk were studied by proteomics coupled with the detection of protein spots by MALDI TOF mass spectrometry. Proteins were analysed by a modification of the traditional, two-dimensional gel electrophoresis (2-DGE) method under non-reducing running conditions, which potentially permits the visualisation of disulphide-linked protein complexes formed in response to pasteurisation. Separated proteins were stained with Coomassie blue. The relative spot volumes obtained after 2-DGE of fractions from raw or pasteurised milk were compared. A number of different spots in the rennet casein and sweet whey fractions were found to vary in response to pasteurisation. Some of these represented higher molecular mass complexes that increased in the chymosin-precipitated casein fraction, and they were identified by mass spectrometry to contain αS1-casein. Certain fragments of αS1-casein, probably generated as a result of chymosin cleavage, increased in the whey after pasteurisation. The whey content of proteose peptone component 3 (PP3) or lactophorin decreased after pasteurisation, which could indicate increased association of PP3 with for example the milk fat globule membrane after pasteurisation. This shows that gel-based proteome analysis can be used in the characterisation of subtle variations in protein composition of milk fractions that occur as a consequence of pasteurisation.