No abstract

The ability of a food ingredient, sodium stearoyllactylate (SSL), to stabilise oil-in-water (O/W) emulsions against coalescence was investigated, and closely linked to its capacity to act as a Pickering stabiliser. Results showed that emulsion stability could be achieved with a relatively low SSL concentration (≥0.1 wt%), and cryogenic-scanning electron microscopy (cryo-SEM) visualisation of emulsion structure revealed the presence of colloidal SSL aggregates adsorbed at the oil-water interface. Surface properties of SSL could be modified by altering the size of these aggregates in water; a faster decrease in surface tension was observed when SSL dispersions were subjected to high pressure homogenisation (HPH). The rate of SSL adsorption at the sunflower oil-water interface also increased after HPH, and a higher interfacial tension (IFT) was observed with increasing SSL concentration. Differential scanning calorimetry (DSC) enabled a comparison of the thermal behaviour of SSL in aqueous dispersions with SSL-stabilised O/W emulsions. SSL melting enthalpy depended on emulsion interfacial area and the corresponding DSC data was used to determine the amount of SSL adsorbed at the oil-water interface. An idealised theoretical interfacial coverage calculation based on Pickering emulsion theory was in general agreement with the mass of SSL adsorbed as predicted by DSC.

Aqueous dispersions of tripalmitin particles (with a minimum size of 130 nm) were produced, via a hot sonication method, with and without the addition of food-grade emulsifiers. Depending on their relative size and chemistry, the emulsifiers altered the properties of the fat particles (e.g. crystal form, dispersion state and surface properties) by two proposed mechanisms. Firstly, emulsifiers modify the rate and/or extent of polymorphic transitions, resulting in the formation of fat crystals with a range of polarities. Secondly, the adsorption of emulsifiers at the particle interface modifies crystal surface properties. Such emulsifier-modified fat particles were then used to stabilise emulsions. As the behaviour of these particles was predisposed by the kind of emulsifier employed for their manufacture, the resulting particles showed different preferences to which of the emulsion phases (oil or water) became the continuous one. The polarity of the fat particles decreased as follows: Whey Protein Isolate > Soy Lecithin > Soy Lecithin + Tween 20 > Tween 20 > Polyglycerol Polyricinoleate > no emulsifier. Consequently, particles stabilised with WPI formed oil-in-water emulsions (O/W); particles stabilised solely with lecithin produced a highly unstable W/O emulsion; and particles stabilised with a mixture of lecithin and Tween 20 gave a stable W/O emulsion with drop size up to 30 μm. Coalescence stable, oil-continuous emulsions (W/O) with drop sizes between 5 and 15 μm were produced when the tripalmitin particles were stabilised with solely with Tween 20, solely with polyglycerol polyricinoleate, or with no emulsifier at all. It is proposed that the stability of the latter three emulsions was additionally enhanced by sintering of fat particles at the oil-water interface, providing a mechanical barrier against coalescence.

The fabrication of protein-polysaccharide complexes via electrostatic interactions was investigated with a naturally cationic protein, potato protein isolate (PoPI), and an anionic polysaccharide, κ-carrageenan (κC), at unadjusted pH conditions. Moreover, the emulsifying capacity of these electrostatic complexes (PoPI-κC) was assessed. PoPI-κC complexes were prepared with a fixed concentration of PoPI (1 wt. %), and varying concentrations of κC (0.01-0.5 wt. %), using gentle agitation, followed by sonication to fabricate the complexes. The physicochemical properties of PoPI-κC complexes was assessed in terms of size and surface charge, measured using light scattering techniques and electrokinetic potential, respectively. The emulsifying performance of emulsions prepared with PoPI-κC complexes was assessed as a function of κC, and to PoPI, with respect to initial emulsion droplet size, emulsion stability, interfacial tension and optical microscopy. Addition of κC to a 1 wt. % PoPI solution yielded the formation of submicron (~120 nm) electrostatic complexes up to a κC concentration of ≤ 0.0375 wt. %. Higher concentrations of κC yielded micron sized complexes (> 10 µm). Emulsions prepared with PoPI-κC complexes yielded comparable emulsion droplet sizes to that of PoPI alone, with the exception of complexes prepared with κC in the range of 0.05-0.07 wt. %. Larger emulsion droplets were observed, as these complexes possessed an electrokinetic potential close to the isoelectric point, resulting in aggregation. Emulsions prepared with PoPI-κC complexes possessed marginally enhanced long-term stability in comparison to emulsions prepared with PoPI alone.

Aside from single active microencapsulation, there is growing interest in designing structures for the coencapsulation and codelivery of multiple species. Although currently achievable within solid systems, significant challenges exist in realizing such functionality in liquid formulations. The present study reports on a novel microstructural strategy that enables the coencapsulation and corelease of two actives from oil-in-water emulsions. This is realized through the fabrication of sodium caseinate/chitosan (NaCAS/CS) complexes that in tandem function as encapsulants of one active (hydrophilic) but also as ("Pickering-like") stabilizers to emulsion droplets containing a secondary active (hydrophobic). Confocal microscopy confirmed that the two coencapsulated actives occupied distinct emulsion microstructure regions; the hydrophilic active was associated with the NaCAS/CS complexes at the emulsion interface, while the hydrophobic active was present within the oil droplets. Aided by their segregated coencapsulation, the two actives exhibited markedly different corelease behaviors. The hydrophilic active exhibited triggered release that was promoted by changes to pH, which weakened the protein-polysaccharide electrostatic interactions, resulting in particle swelling. The hydrophobic secondary active exhibited sustained release that was impervious to pH and instead controlled by passage across the interfacial barrier. The employed microstructural approach can therefore lead to the segregated coencapsulation and independent corelease of two incompatible actives, thus offering promise for the development of liquid-emulsion-based formulations containing multiple actives.