Enhancing oxidative stability of flaxseed oil emulsions by forming double-layers of protein-polyphenol conjugate and a polysaccharide

This study addresses the challenge of lipid oxidation in flaxseed oil emulsions by developing a double-layer emulsion system with a protein-polyphenol conjugate as the stabilizing-antioxidant first layer and a polysaccharide as the protective second layer. Soluble faba bean proteins were conjugated with tannic acid using the free-radical grafting method and the conjugate was used to stabilize flaxseed oil-in-water emulsions at pH 3. The positively charged primary emulsion droplets were electrostatically coated with a negatively charged pectin at pH 3 generating a double-layer emulsion. The conjugate-stabilized double-layer emulsion exhibited an average aggregate size of 9.4 μm (charge -20.9 mV). Despite the large aggregate size, the emulsion remained stable due to a higher viscosity and a high negative charge. Stability tests under a freeze-thaw cycle and high temperatures revealed minimal size changes, confirming their stability against environmental stresses. Lipid oxidation studies revealed that conjugate-stabilized double-layer emulsions had significantly lower peroxide and p-Anisidine values after 28 days at 60°C compared to the control protein-stabilized double-layer emulsions. The efficiency of the pectin as the secondary layer to protect the interfacial tannic acid from external radical attack was also confirmed using electron paramagnetic resonance spectroscopy. To further protect the emulsified flaxseed oil, freeze-drying was used to convert the double-layer emulsions into powder. The reconstituted emulsions showed similar droplet size, surface charge, and dispersibility similar to the original emulsions, indicating the structural integrity of the oil droplets during freeze-drying. Upon storage at 60°C for 28 days, the peroxide and anisidine values of the conjugate-stabilized double-layer emulsion powder were 20% and 70% lower than the corresponding emulsion. Therefore, the freeze-dried flaxseed oil powder developed via conjugated protein-polyphenol and coated with pectin showed significant potential for improving oxidative stability, making it a suitable ingredient for delivering omega-3-rich oil in various food applications.