P NMR spectroscopic investigations of caseins treated with microbial transglutaminase

Caseins - the main constituents of bovine milk proteins - self-assemble into large supramolecular aggregates, so-called casein micelles. The enhancement of the stability of casein micelles is advantageous with respect to technological milk processing. A promising approach to accomplish this goal is the cross-linking of caseins using microbial transglutaminase (mTG). The present paper describes the combined use of liquid- and solid-state ³¹P NMR spectroscopy as well as dynamic light scattering in order to characterize the influence of an mTG treatment upon the structure of micelles in ultrahigh temperature (UHT)-treated skim milk at a molecular level. Liquid-state ³¹P NMR spectroscopy was applied to characterize milk, milk serum and casein dispersions. A narrow SerP signal in the liquid-state ³¹P NMR spectra of UHT-treated milk is shown to be due to casein molecules in the milk serum whereas the casein molecules bound in the micelles give rise to broad signals. Most of the caseins contribute to a 3 kHz broad background signal which can be visualized in the spectrum of suspensions of re-dispersed micellar material derived from UHT-treated milk. Treatment with mTG results in a cross-linking of caseins, which could be followed by liquid-state ³¹P NMR spectroscopy. Especially, the cross-linking of β-casein was demonstrated by quantitative liquid-state ³¹P NMR experiments. Furthermore, the stability of cross-linked micellar aggregates against EDTA could be investigated by liquid-state ³¹P HR NMR in combination with dynamic light scattering (DLS). Solid-state ³¹P NMR was used to show that the motional state of the κ-caseins located at the outer surface of the micelles derived from UHT-treated milk is not significantly changed by the applied mTG treatment.