Oil-in-water (5 wt%) nanoemulsions were prepared with different concentration (2.5-10 wt%) of sodium caseinate as a sole emulsifier and their long-term storage stability was investigated for 6 months. Previous studies associated with sodium caseinate looked only into nanoemulsion formation; hence the challenges with long-term stability were not addressed. All nanoemulsions displayed an average droplet size <200 nm, which remained unchanged over 6 months. However, all of them displayed rapid creaming due to unabsorbed protein induced depletion flocculation, whose extent increased with protein concentration, although the cream layer formed was weak and re-dispersible upon gentle mixing. Microstructural analysis of the cream layer showed compaction of flocculated nanodroplet network with time leaving the aqueous phase out. Calculation of depletion interaction energy showed an increase in inter-droplet attraction with protein concentration and decrease with a reduction in droplet size, making the nanoemulsions more resistant to flocculation than conventional emulsions. This work aids in understanding the dependence of protein concentration on long-term stability of sodium caseinate-stabilized nanoemulsions.
Keywords: Creaming; Depletion flocculation; Long-term stability; Nanoemulsion; Photocentrifuge; Sodium caseinate.