Controlling polydispersity in sodium caseinate-stabilized nanoemulsions to investigate jamming transitions and improve rheological properties

Sodium caseinate-stabilized O/W emulsions are widely used in various food applications to enhance creaminess and stability. A critical phenomenon that could significantly influence the stability and rheology of these food products is repulsive gelation, which occurs due to the random jamming of droplets, restricting the movement and forming a gel-like structure. For monodispersed emulsion, the critical oil volume fraction (ϕ_c) is 0.64. However, real emulsions are polydisperse, requiring a higher and unpredictable amount of oil for jamming due to the lack of knowledge on the influence of polydispersity on random jamming. Therefore, this study aims to control polydispersity with the help of size-fractionation and investigate its effect on jamming transition as a function of oil volume fraction (ϕ). Sodium-caseinate-stabilized nanoemulsions (d_3,2 271 nm, relative polydispersity (RP) 0.61) were prepared using a high-pressure homogenizer, which was ultracentrifuged to obtain a cream-plug at the top, where the oil droplets were deposited according to their size. The cream-plug was sliced to recover pseudo-monodispersed droplets with a range of sizes and RP, then diluted with water to achieve nanoemulsions with ϕ ranging from 0.3-0.6. The effective volume fraction (ϕ_eff) of these nanoemulsions was subsequently calculated based on the droplet size and the thickness of the interfacial repulsive shell layer. Additionally, strain-sweep rheology was performed on these size-fractionated nanoemulsions to estimate their gel strength. Results indicated that elastic moduli increased with ϕ_eff, progressing through viscous and glassy states to electrostatic and interfacial jamming regions. The entropic-electrostatic-interfacial (EEI) model was used to predict ϕ_c for size-fractionated nanoemulsions at different RP. In the future, the RP will be further reduced through multiple size-fractionations, and the ϕ_c will be predicted as a function of RP. These findings will provide insights into tailoring nanoemulsions with enhanced rheology and stability, leading to repulsive gels with predictively reduced oil content.