Conformational and physicochemical characteristics of bovine skim milk obtained from cows with different genetic variants of β-casein

Abstract

This study highlights differences in conformational and physicochemical characteristics of bovine skim milk and micellar casein from cows of different β-casein phenotypes. These genetic variants have been one of the predominant topics among dairy researchers due to their differences in β-casein structure, and thus their potential effects on dairy processing and human health. For characterising differences in milk protein structure, Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (1H NMR) spectroscopies combined with chemometric analysis were used. Additionally, physiochemical properties such as mineral content, particle size, and electrostatic charge in skim milk and micellar casein samples were analysed at 4 and 20 ᵒC. Results showed variation in the secondary structure of all three genetic variants independent of temperature. Moreover, the main differences involved a higher level of β-turn and α-helical structures in A1/A1 β-casein milk, while intermolecular β-sheets were more numerous in A1/A2 β-casein milk, whereas random or polyproline II (PPII) structures were more common in A2/A2 β-casein milk. Temperature slightly affected these differences, which was due to the dissociation of β-casein from the micelle at low temperature. In addition, A2/A2 β-casein milk and its micellar casein had a larger average particle size, which resulted in a lower negative ζ-potential. The A2/A2 β-casein samples contained greater amounts of phosphorus and less calcium compared to the other genetic variants of milk and their micellar caseins. The results also indicated that a combination of FTIR and 1H NMR spectroscopies could be used to establish conformational differences in milk and micellar caseins of different genetic variants.