Physicochemical properties of lactoferrin stabilized oil-in-water emulsions: Effects of pH, salt and heating

Lactoferrin is a globular protein from bovine milk with an unusually high isoelectric point (pI > 8), which may lead to novel functional properties in foods and other products because it is cationic across a wide pH range. In this study, we investigated the influence of pH (2–9), NaCl addition (0–200 mM), CaCl2 addition (0–200 mM), and thermal processing (30–90 °C, 20 min) on the stability of lactoferrin (LF) stabilized oil-in-water emulsions. At ambient temperature, the emulsions were stable to droplet aggregation at low pH (pH ≤ 6), but exhibited some aggregation at pH ≈ pI (pH 7–9). The thermal stability of the emulsions depended on pH, holding temperature, and thermal history. When LF-coated droplets were heated in distilled water, and then their pH was adjusted in the range 2–9, they were highly unstable to aggregation at pH 7 and 8. On the other hand, when the pH was altered in the range 2–9 first, and then they were heated, the LF-coated droplets were highly unstable to aggregation at pH ≥ 5 when heated above 50 °C. The stability of the emulsions to salt addition depended on pH and salt type, which was attributed to counter-ion binding and electrostatic screening effects. For NaCl, emulsions were stable from 0 to 200 mM at pH 3 and 9, but aggregated at ≥100 mM at pH 6. For CaCl2, emulsions were stable from 0 to 200 mM at pH 3, but aggregated with ≥150 mM CaCl2 at pH 6 and 9. These results have important implications for the formulation and production of emulsion-based products using lactoferrin as an emulsifier.