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

In the present work, we investigated the structural modifications occurring during the dry heating of model whey proteins, beta-lactoglobulin and alpha-lactalbumin. Samples were adjusted to pH 6.5, water activity a(w) = 0.23 and dry heated at 100 degrees C for up to 24 h, 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 alpha-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 alpha-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 Asp(64). The non-native species were not favored in the case of beta-lactoglobulin as they represented less than 18% of non-aggregated proteins. (C) 2013 Elsevier Ltd. All rights reserved.