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Association of enzymatically and non-enzymatically functionalized caseins analyzed by size-exclusion chromatography and light scattering techniques.

The influence of covalent protein modifications resulting from the Maillard reaction (glycation) of casein and lactose on the non-covalent association behavior of the protein was studied. Non enzymatic cross-linking with methylglyoxal (MGO) and glutardialdehyde (GTA) as well as enzymatic cross-linking with microbial transglutaminase (mTG) were investigated for comparison. Molar mass, particle size and conformational characteristics of casein associates as well as the extent of intraparticle protein cross-linking were examined utilizing size-exclusion chromatography (SEC) combined with UV detection and static and dynamic light scattering. Cross-linking resulted in the stabilization of a certain fraction of casein associates, with particle sizes of approximately 30 nm in radius of gyration (Rg), and promoted an incorporation of further casein molecules into those particles yielding molar masses (Mw) of 1.0 to 1.2×106 g/mol. When caseins were additionally conjugated with lactose during early Maillard reaction, a further growth of the associates up to approximately 50 nm in Rg with a Mw of 2.1×106 g/mol was observed. Furthermore, a transition from a rather elongated towards anisotropic or microgel-like conformations were induced by MGO- and mTG-modification or glycation, respectively. Associates consisting of caseins cross-linked with GTA appeared to preserve the native particle conformation.