Characterization of catechin-α-lactalbumin conjugates and the improvement in β-carotene retention in an oil-in-water nanoemulsion

• ALA-catechin conjugates were prepared by free radical grafting method.
• Covalent ALA-catechin conjugates were confirmed by SDS–PAGE and ESI–MS.
• The antioxidative properties of ALA were improved with catechin conjugation.
• The BC retention in nanoemulsion was improved by ALA-catechin conjugates.

The goal of this study was to prepare and characterize α-lactalbumin (ALA)-catechin conjugates as a novel emulsifier in improving the retention of β-carotene (BC) in nanoemulsions via a free radical method. Covalent modification was observed and at least one catechin molecule was binding with ALA according to ESI–MS results. Far-UV CD indicated that the secondary structure of ALA was changed after conjugation. The Z-average particle diameters of nanoemulsions stabilized with ALA and ALA-catechin conjugates were 158.8 and 162.7 nm, respectively. The increase of mean particle size and the degradation of BC at 50 °C were both larger than at 25 °C during 30 days storage. BC retention stabilized with ALA-catechin conjugates was appreciably greater than ALA (control), which was attributed to the increase of ALA’s radicals-scavenging and free metal ion binding ability after grafting with catechin. The chemical antioxidant activities of ALA-catechin conjugates were increased with increasing concentrations from 0.1 to 1.0 mg/ml. In general, labile phytochemicals, like BC, can be protected against oxidation during storage by proteins-polyphenols conjugates without any side effects.