Conversion of Viscous Oil-in-Water Nanoemulsions to Viscoelastic Gels upon Removal of Excess Ionic Emulsifier

Abstract

Viscous, flowable nanoemulsions stabilized with ionic emulsifier can be transformed into repulsively jammed elastic gels that do not flow under gravity by reducing the droplet size and increasing the interfacial repulsive shell layer thickness. However, a high concentration of emulsifier required to achieve nanodroplets could remain in the continuous phase and lead to oscillatory structural forces, thereby reducing repulsive interaction and forming flowable liquid systems. It was hypothesized that the removal of excess emulsifier from a nanoemulsion could lead to the formation of repulsive gels. Canola oil-in-water nanoemulsions, containing 40 wt % oil, were prepared with a citric acid ester of monoglyceride (Citrem) using a high-pressure homogenizer. The excess emulsifier in the aqueous phase was removed by multiple ultracentrifugation cycles, and the droplet size, rheology, and stability of the nanoemulsions were investigated as a function of excess Citrem concentration. Nanoemulsions with average droplet sizes of 222 and 150 nm were obtained with 3 and 5 wt % Citrem, respectively. The removal of excess Citrem did not change the droplet size significantly. However, the viscosity, yield stress, and storage moduli increased significantly with the reduction of excess Citrem and the decrease in droplet size, converting the flowable weak gel nanoemulsion to a strong viscoelastic gel. The calculated values of oscillatory structural forces decreased with the removal of excess emulsifier, leading to an increase in repulsive interactions and the thickness of the electric double layer. Such an increase in interdroplet separation led to an increase in the effective oil volume fraction beyond the maximum random jamming of oil droplets and the formation of a viscoelastic nanoemulsion gel.

Effect of the chitosan second layer on the gelation and controlled digestion of Citrem–chitosan bilayer emulsions

“…Therefore, by increasing the chitosan concentration, the rheology of monolayer emulsion was transformed from a flowable liquid-like material to a self-supporting elastic gel due to droplet aggregation at 0.1 wt% and finally into a repulsive bilayer viscoelastic gel at 0.25 wt%. Moreover, comparing with our previous study, 10 where we obtained repulsive gelation in a 3 wt% Citrem stabilized monolayer 40 wt% O/W emulsion, in the present case, about 6-times increase in the gel strength was observed for the chitosan-coated bilayer emulsion gels at 36 wt% oil. Therefore, by changing the composition, a further Food & Function Paper decrease in the oil concentration of the bilayer emulsions would be possible while keeping the gel strength similar to the monolayer emulsions.…”

“…It was found that two ultracentrifugation cycles were enough to remove almost all unabsorbed Citrem from the emulsion continuous phase. 10 The final Citrem-stabilized emulsion without excess emulsifier was adjusted to pH 4 using 1 N HCl. To prepare the bilayer emulsions, the second layer of positively charged chitosan with different degrees of deacetylation (DDA 93% and 50%) was deposited on Citremstabilized droplets according to Kaasgaard and Keller.…”

“…Although we were able to achieve the repulsive gelation in 5 wt% Citrem-stabilized nanoemulsions at 40 wt% at 150 nm average droplet size, 10 our focus for this study was to achieve gelation at a lower concentration of Citrem (3 wt%), hence with a larger droplet size, and lower oil content (<40 wt%) by further increasing the interfacial shell-layer thickness. One strategy to increase the interfacial shell-layer thickness is to employ a layer-by-layer (LbL) electrostatic deposition based on the sequential adsorption of an oppositely charged biopolymer to form a stable polyelectrolyte shell layer around the droplet.…”

“…9 In our previous study, we have also shown an increase in the gel strength of 3 wt% and 5 wt% Citrem (citric acid ester of mono-and diglycerides)-stabilized 40 wt% O/W nanoemulsions upon removal of excess micelles from the continuous phase due to an increase in the thickness of the repulsive barrier around the nanodroplets. 10 Such improved elastic behaviour of the nanoemulsion could make it an alternative structured material for various low-fat food applications.…”

“…It was hypothesized that at surface saturation, the adsorbed chitosan molecules would form a thicker steric barrier around the Citremstabilized droplets that would help to achieve repulsive gelation in the emulsion at lower ϕ oil and larger droplet size than that reported before for monolayer Citrem emulsions. 10 It was also hypothesized that the gastric stability of the thicker chitosan layer would also help to control lipid digestibility of the bilayer emulsions compared to the monolayer emulsion.…”