Investigation of the lactosylation of whey proteins by liquid chromatography-mass spectrometry

Heat treatment of milk induces a reaction between the milk proteins and lactose, resulting in lactosylated protein species. The lactosylation of the two major whey proteins alpha-lactalbumin and beta-lactoglobulin was investigated by reversed phase liquid chromatography-mass spectrometry (LC-MS). Three sample series, consisting of aqueous model solutions of each whey protein separately and in mixture and whole milk, were heated for different time periods, and the progression of the lactosylation reaction was monitored. The observed degrees of lactosylation and the reaction kinetics showed that the lactosylation of beta-lactoglobulin was not influenced by the presence of other components, whereas the lactosylation of alpha-lactalbumin was enhanced in whole milk compared to the aqueous model systems. An in-depth evaluation of the LC-MS data yielded information regarding changes of physicochemical properties of the whey proteins upon lactosylation. Whereas retention time shifts indicated changes in hydrophobicity for both alpha-lactalbumin and beta-lactoglobulin, changes in the charge state distribution denoting conformational alterations were observed only for beta-lactoglobulin. The analysis of different liquid and solid milk products showed that the lactosylation patterns of the whey proteins can be used as indicators for the extent of heat treatment.