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.
This research aimed to induce repulsive gelation in Citrem-stabilized O/W emulsions by creating a secondary layer of chitosan around the droplets. A range of chitosan concentration (0-0.25wt%) and degree of...
Summary
Liquid nanoemulsions are shown to transform into viscoelastic gels by reducing droplet size, increasing interfacial repulsive barrier between the nanodroplets and therefore increasing the effective oil volume fraction. The repulsive gelation in nanoemulsions can be achieved at a significantly lower oil volume fraction compared to conventional emulsion gels, making the nanoemulsion gel an attractive material for various low‐fat food applications. Gelation in nanoemulsions stabilized by anionic small molecule emulsifier and polymeric protein are compared in terms of gel strength, average droplet size, effective oil volume fraction, and long‐term gel stability. It is expected that higher stability and large surface area of nanoscale droplet size can further extend the application of nanoemulsion gels in the field of functional foods, cosmetics and pharmaceuticals.
Conventional emulsion gels with sub-micrometre droplet size are part of many foods
where the disperse-phase induced gelation can be achieved at higher oil volume fraction
(f) through random jamming. Interestingly, random jamming can also be induced at a lower
f by reducing the droplet size (r) to nanoscale and increasing the interfacial
shell-layer thickness (d). Specifically, interfacial compositions can play a significant
role in elevating d, when reduction in r becomes a limiting factor. In this study, two
different interfacial compositions (low molecular weight emulsifier (Citrem) or whey
protein isolate (WPI)) were used to prepare the single-layer 40wt% O/W nanoemulsions (r
~ 200 nm) using high-pressure homogenization. To increase the d, the second layer of
polysaccharide (chitosan or pectin) was deposited on single-layer-stabilized
nanodroplets using the layer-by-layer (LbL) electrostatic deposition technique.
Deposition of chitosan transformed a liquid Citrem-stabilized single-layer emulsion into
repulsive bilayer weak emulsions gel, indicated by 15 times increase in storage modulus
(G¢). Oppositely, deposition of pectin second layer on WPI-stabilized nanodroplets
showed a much stronger elastic bilayer gel. A self-standing viscoelastic gel obtained
for the WPI-pectin bilayer nanoemulsions could be attributed to the close-packed
structure (random jamming) of nanodroplets compared to the lack of that in
Citrem-chitosan-stabilized bilayer nanoemulsions at similar f (0.36). The electrostatic
and steric repulsive forces (from the WPI and pectin) significantly contributed to
achieving the dispersed phase-induced gelation by elevating d and effective oil volume
fraction (feff) of nanodroplets. The deposition of the second layer, chitosan and
pectin, on nanodroplets also led to 32% and 57% reduction of in vitro lipid
digestibility, respectively, compared to single-layer nanoemulsions. Overall, this study
showed that the dispersed-phase induced gelation (at a lower oil content) and controlled
digestion in nanoemulsions can be achieved by manipulating the interfacial
compositions.
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