Formation and stability of single and bi-layer nanoemulsions using WPI and lactoferrin as interfacial coatings under different environmental conditions

Abstract

Electrostatic deposition of lactoferrin (LF) on the stability of nanoemulsions stabilized by whey protein isolate (WPI) was investigated as a function of pH (2–10) and LF concentration (0.25–5% w/w). The WPI-stabilized nanoemulsions prepared by emulsification and solvent evaporation were diluted with LF solutions to form a series of bi-layer emulsions at different pH levels. At pH 2 and 4, the bi-layer nanoemulsions were stable regardless of LF concentrations but underwent droplet aggregation at pH 5 near to the pI of WPI. At pH 6, electrostatic interactions were favored for the deposition of LF onto the WPI-coated droplets. However, the emulsions were prone to droplet aggregation at low concentrations of LF (0.5–1% w/w), presumably due to bridging flocculation. Thus, a sufficiently high level of LF was necessary to stabilize the emulsions. At pH 7, 8, 9 and 10, there was sufficient electrostatic attraction between the protein molecules to form WPI-LF bi-layer at the droplet surface and the formed emulsions were stable. Based on the results, a stable bi-layer nanoemulsion can be formed using 3% (w/w) LF at ≥ pH 6. The incorporation of LF in bi-layer emulsions improved the stability of single layer WPI-stabilized emulsions at pH near the pI of WPI (pH 4–6) and salt addition (0–90 mM CaCl2) but the bi-layer emulsions were susceptible to droplet aggregation at temperatures above 60 °C. This study showed the overall potential benefits of having of a WPI-LF bi-layer to improve the stability of WPI-stabilized nanoemulsions.