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Changes in the molecular structure and association of milk proteins lead to many desirable (under controlled conditions) or undesirable characteristics of dairy products. Several methods have been used to study the structure of milk proteins and changes therein in different environments. Whey proteins are an excellent model for secondary structure studies using circular dichroism (CD), Fourier-transform infrared spectroscopy (FTIR) and tertiary structure studies using X-ray crystallography and nuclear magnetic resonance (NMR). However, caseins, the most abundant protein class in milk, are far more difficult to characterize. The tertiary structure of caseins cannot be observed by X-ray crystallography due to the inability to crystallize caseins. However, NMR is an appropriate approach for structural elucidation. Thus far, NMR was applied on specific peptides of individual caseins of the molecules including phosphoserine centers and colloidal calcium phosphate. The literature focuses on these parts of the molecule due to its importance in building the sub-unit particles involving individual caseins and calcium phosphate nanoclusters. This review focuses on present structural studies of milk proteins using NMR and their importance in dairy processing.

This study investigated structural changes in β-casein as a function of temperature (4 and 20 °C) and pH (5.9 and 7.0). For this purpose, nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy were used, in conjunction with chemometric analysis. Both temperature and pH had strongly affected the secondary structure of β-casein, with most affected regions involving random coils and α-helical structures. The α-helical structures showed great pH sensitivity by decreasing at 20 °C and diminishing completely at 4 °C when pH was increased from 5.9 to 7.0. The decrease in α-helix was likely related to the greater presence of random coils at pH 7.0, which was not observed at pH 5.9 at either temperature. The changes in secondary structure components were linked to decreased hydrophobic interactions at lower temperature and increasing pH. The most prominent change of the α-helix took place when the pH was adjusted to 7.0 and the temperature set at 4 °C, which confirms the disruption of the hydrogen bonds and weakening of hydrophobic interactions in the system. The findings can assist in establishing the structural behaviour of the β-casein under conditions that apply as important for solubility and production of β-casein.