Structural consequences of dry heating on alpha-lactalbumin and beta-lactoglobulin at pH 6.5

Abstract

In the present work, we investigated the structural modifications occurring during the dry heating of model whey proteins, β-lactoglobulin and α-lactalbumin. Samples were adjusted to pH 6.5, water activity aw=0.23 and dry heated at 100°C for up to 24h, and the structural modifications followed by gel permeation chromatography, reverse phase-HPLC, SDS PAGE and mass spectrometry (LC–MS/MS). The dry heating treatment traps a fraction of the proteins into covalently linked soluble aggregates. Moreover, a high proportion of non-aggregated α-lactalbumin (about 73%) was converted into non-native forms. The characteristic of those non-native species was the loss of one or two water molecules per α-lactalbumin molecules. Using tandem mass spectrometric peptide mapping, these chemical modifications were found to be attributed to (i) the formation of a pyroglutamic acid from the N-terminal glutamic acid and (ii) the formation of an internal cyclic imide at position Asp64. The non-native species were not favored in the case of β-lactoglobulin as they represented less than 18% of non-aggregated proteins.