Impact of pH, ferrous ions, and tannic acid on lipid oxidation in plant-based emulsions containing saponin-coated flaxseed oil droplets.

The influence of pH (pH 3, 5 and 7), ferrous ions (0 or 100 μM Fe2+), and tannic acid (0 to 0.1% TA) on the rate of lipid oxidation in plant-based emulsions containing quillaja saponin-coated flaxseed oil droplets was studied. Tannic acid formed complexes with Fe2+ whose properties depended on TA:Fe2+ ratio and pH. Emulsions were incubated at 37 °C in the dark, and changes in their particle size, surface potential, appearance, microstructure, and lipid oxidation status were monitored over time. The initial ζ-potential and mean particle diameter of the emulsions were -68 mV and 0.18 μm, respectively. In the absence of TA, the particle size increased appreciably during storage due to droplet coalescence, as rapid oxidation occurred. In the presence of TA, the emulsions were more resistant to both droplet aggregation and lipid oxidation, as a result of its strong ferrous ion-binding properties. The lipid oxidation rate increased with decreasing pH, which was attributed to an increase in ferrous ion's water-solubility and activity in acidic solutions. The addition of Fe2+ greatly accelerated lipid oxidation, but the oxidation rate was decreased by also adding TA. These results suggest that tannic acid is an effective antioxidant in emulsions, which can be attributed to its ferrous ion-chelation properties.