Delaying lipid oxidation in emulsions by localizing lentil protein-polyphenol conjugates at the oil-water interface

Abstract

Polyphenols are naturally derived antioxidants. In the present study, the ability of polyphenols to prevent lipid oxidation was used in synergism with the surface activity of lentil proteins to fabricate flaxseed oil-in-water emulsions. The conjugation of lentil protein isolate with polyphenols (quercetin, rutin and ellagic acid) was achieved by a simple grafting technique involving high alkaline pH of 9.0. The resulting lentil proteins-polyphenol conjugates were characterized by their structural and functional properties. The extent of covalent binding in lentil protein was 21.0%, 11.7% and 4.4% for quercetin, ellagic acid and rutin molecules, respectively. Results from Fourier transform infrared (FTIR) spectroscopy revealed an increase in random coils and a subsequent decrease in the β-sheet for all conjugates. Surface hydrophobicity of the conjugates was lower than control lentil proteins suggesting possible involvement of protein hydrophobic groups towards covalent bonding with polyphenols. The antioxidant activity of lentil proteins-polyphenol conjugates was 0.30 to 1.75-fold higher for reducing power assay and 0.05 to 0.08-fold higher for free radical scavenging than the control lentil proteins. Further, the prepared conjugates were also successful in lowering the interfacial tension of oil-water interface and therefore, were employed for stabilizing oil-in-water emulsions. Lentil protein isolate-quercetin and lentil protein isolate-ellagic acid conjugates were selected for emulsifying flaxseed oil and to retard its oxidation in a location-dependent manner. The emulsions stabilized using the conjugates showed superior oxidative stability compared to those stabilized using the original lentil protein isolate or control lentil proteins with an equivalent amount of quercetin or ellagic acid in their respective aqueous phases, thereby demonstrating the advantage of placing the polyphenols at the oil droplet surface. However, emulsions prepared using conjugates showed limited stability towards changes in environmental factors such as pH, ionic strength, and heat. Results from accelerated gravitational separation analysis showed emulsions made using conjugates had similar instability indices as those made using lentil proteins only. The distribution of the conjugates at the surface of the oil droplets was observed using confocal laser scanning microscopy. Overall, the results from this study demonstrated the advantage of localizing antioxidants at the oil-water interface of an emulsion in better protecting the lipid core against oxidation.