Effects of acetyl grafting on the structural and functional properties of whey protein microgels

Whey protein microgels (WPMs) are self-assembled sub-micron particles, with diverse applications in food science such as emulsion and foam stabilization. This study evaluated the influence of acetylation at different acetic anhydride-to-protein molar ratios (AA-to-protein MRs) on structural characteristics, colloidal stability, and technological functionality of WPMs. N-acetylation degree progressively increased with increasing the AA-to-protein MR and reached 96% when AA-to-protein MR was 3270. O-acylation increased significantly only after N-acylation extensively (96%) occurred. Acetylation process increased the randomness of proteins in WPMs (assessed by circular dichroism and Fourier transform infrared spectroscopies) as well as conferred a less compact structure (assessed by intrinsic fluorescence spectroscopy). The isoelectric point (pI) of WPMs shifted from roughly 4.5 to 3.0-3.9 depending on the acetylation extent; the higher the acetylation degree, the lower was the pI. Acetylation at any extent did not improve the colloidal stability of WPMs at their corresponding pI. However, the emulsification activity of WPMs at pH values between 5.0 and 7.0 increased with increasing the acetylation extent. The highest foaming and oil absorption capacities of WPMs at pH 7.0 was obtained at a N-acetylation extent of 40%.