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

This study highlights differences in conformational and physicochemical characteristics of bovine skim milk and micellar casein from cows of different beta-casein phenotypes. These genetic variants have been one of the pre -dominant topics among dairy researchers due to their differences in beta-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 (H-1 NMR) spectroscopies combined with chemo-metric 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 degrees C. Results showed variation in the secondary structure of all three genetic variants independent of temperature. Moreover, the main differences involved a higher level of beta-turn and alpha-helical structures in A1/A1 beta-casein milk, while intermolecular beta-sheets were more numerous in A1/A2 beta-casein milk, whereas random or polyproline II (PPII) structures were more common in A2/A2 beta-casein milk. Temperature slightly affected these differences, which was due to the dissociation of beta-casein from the micelle at low temperature. In addition, A2/A2 beta-casein milk and its micellar casein had a larger average particle size, which resulted in a lower negative zeta-potential. The A2/A2 beta-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.