Evaluation of the interactions between rosmarinic acid and bovine milk casein

Abstract

Polyphenols can interact with proteins, which gives rise to a significant loss of their biological properties. The objective of this research was the study of interactions in model systems composed of the polyphenol rosmarinic acid (RA) and bovine milk α-s1-casein, β-casein and κ-casein. Radical cation quenching assay (ABTS, 2,2′-azinobis-3-ethyl-benzothiazoline-6-sulfonic-acid), optical density, liquid chromatography (RP-HPLC, reverse phase-high performance liquid chromatography, and SEC, size exclusion chromatography), dynamic light scattering (DLS) and zeta-potential, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were used for the screening of the interactions at 0, 3 and 24 h of storage time and at the refrigeration temperature 4 °C. Interactions were assessed at the pH of the complexes in water, 6.8, and at acidic pH 3 and 4.5. Results showed the occurrence of non-covalent interactions such as hydrophobic, hydrogen bonding and dipole–dipole. Radical cation quenching activity of RA significantly decreased in the presence of caseins, meaning that the amount of free RA diminished. Higher and the same degree of interaction were observed for α-s1-casein and β-casein. Complex dimensions were different depending on pH, time and on the primary and secondary structure of caseins. Interactions were shown to be favoured at the lowest pH, where complexes are biggest, and reversible at all pH conditions tested. The results of this study must be complemented with the analysis of more complex systems to take into account the effect of other milk components – lipids, sugars and minerals – on the interaction of RA.