Covalent phenolic acid-grafted β-lactoglobulin conjugates: Synthesis, characterization, and evaluation of their multifunctional properties

In our work, covalent conjugates of β-Lactoglobulin (βLg) with different phenolic acids (PhAs) including caffeic (CA), ferulic (FA), sinapic (SA), and gallic (GA) acids were synthesized by using carbodiimide chemistry. The resulting βLg-PhA conjugates were characterized by matrix-assisted laser desorption ionization-time of flight mass spectrometry, Fourier transform infrared spectroscopy, and differential scanning colorimetry. Results showed that four kinds of PhAs were covalently bonded with the amino group of βLg at the grafting mole ratios of 7:1, 1:1, 1:1, and 2:1, respectively. βLg-PhA conjugates contained more α-helix and less β-sheet secondary structure than unmodified βLg. The DPPH and ABTS radical scavenging indexes of βLg-PhA conjugates were significantly higher than that of native βLg (by 2.18–2.55- and 2.66–3.12-fold, respectively) and βLg/PhA mixtures (1.15–2.01- and 1.13–1.47-fold, respectively). Fish O/W emulsions stabilized by βLg-PhA conjugates, excepting βLg-GA conjugate, exhibited smaller droplet size and narrower PDI, as well as better storage and oxidation stability compared with βLg-stabilized emulsion. βLg covalently grafted with CA, SA, and FA were endowed with improved surface-active property and antioxidant activity over native βLg. Accordingly, βLg-PhA conjugates, particularly βLg-CA, βLg-SA, and βLg-FA conjugates, were demonstrated to have great potential as novel group of multifunctional stabilizers in fabrication of emulsion-based deliver system.