Casein micelle with different β-casein phenotypes: Fingerprinting pH-induced structural changes using FTIR and NMR spectroscopies

Abstract

The aim of this study was to investigate the structural characteristics of casein micelles and casein particles at pH 6.7, 5.7, and 2.3 containing β-caseins A1/A1, A1/A2, and A2/A2 at 37 °C using Nuclear Magnetic Resonance, Fourier-Transform Infrared spectroscopies and chemometrics. The amount of all released caseins from the casein micelle at pH 6.7 was significantly different after the pH had been reduced to 5.7 and subsequently decreased to 2.3. Results showed variation in the structure of all three samples mainly dependent of caseins content, caseins’ phenotypes, and pH modulation. During the pH modulation, higher levels of α-helixes and intramolecular β-sheets were found in A1/A1 casein, whilst aggregated β-sheets, β-turns, and polyproline II helixes dominated in A1/A2 and A2/A2 samples. Principal Component Analysis was used to characterise and distinguish among the structure of the three caseins based on spectral data. While samples containing β-casein A2 possessed structural properties different to A1/A1 samples at pH 6.7 and 2.3, at pH 5.7 all casein micelles behaved in a similar manner. This study brings significant insights in the importance of the observed casein phenotypes on the structural arrangement of A1/A1, A1/A2, and A2/A2 casein micelles, hence defining their main conformational differences essential for the dairy industry.