Physical and material properties of an emulsion-based lipstick produced via a continuous process.

OBJECTIVES: Water-in-oil emulsions in lipsticks could have the potential to improve moisturizing properties and deliver hydrophilic molecules to the lips. The aim of this work was to investigate the effect of a continuous process (scraped surface heat exchanger (SSHE) and pin stirrer (PS)) on the physical and material properties of an emulsion-based lipstick by altering the processing conditions of both the SSHE and PS. METHODS: Emulsion formation was achieved using a SSHE and PS. Emulsions were analysed using nuclear magnetic resonance restricted diffusion (droplet size), texture analysis and rheology (mechanical properties). RESULTS: Results showed that a higher impeller rotational velocity (IRV) (1500 r.p.m.) and a lower exit temperature (52°C) produce the smallest droplets (~ 4 µm), due to greater disruptive forces and a higher viscosity of the continuous phase. The addition of a PS reduces the droplet size (14-6 µm) if the SSHE has a low IRV (500 r.p.m.), due to greater droplet disruption as the emulsion passes through the PS unit. Results also show that if the jacket temperature of a SSHE is 65°C, so that crystallization occurs in both process and post-production, droplets can be integrated into the network resulting in a stiffer wax network (G' - 0.12, in comparison to 0.02 MPa). This is due to small crystals creating a shell around water droplets which can form connections with the continuous network forming a structured network. The addition of a pin stirrer can disrupt a formed network reducing the stiffness of the emulsion (0.3-0.05 MPa). CONCLUSION: This work suggests the potential use of a continuous process in producing an emulsion-based lipstick, particularly when wax crystals are produced in the process. Future work should consider the moisturizing or lubricating properties of wax continuous emulsions and the release of hydrophilic compounds from the aqueous phase.

Effect of emulsifier type and concentration, aqueous phase volume and wax ratio on physical, material and mechanical properties of water in oil lipsticks.

OBJECTIVES: Water-in-oil emulsions in lipsticks could have the potential to improve moisturizing properties and deliver hydrophilic molecules to the lips. The aims of this work were (i) to investigate the effect of emulsifier type (polymer vs. monomer, and saturated vs. unsaturated chain) and concentration on droplet size and (ii) to investigate the effect of wax ratio (carnauba wax, microcrystalline wax, paraffin wax and performalene) and aqueous phase volume on material properties (Young's modulus, point of fracture, elastic modulus and viscous modulus). METHODS: Emulsion formation was achieved using a high shear mixer. RESULTS: Results showed that the saturated nature of the emulsifier had very little effect on droplet size, neither did the use of an emulsifier with a larger head group (droplet size ~18-25 µm). Polyglycerol polyricinoleate (PGPR) resulted in emulsions with the smallest droplets (~3-5 µm), as expected from previous studies that show that it produces a thick elastic interface. The results also showed that both Young's modulus and point of fracture increase with increasing percentage of carnauba wax (following a power law dependency of 3), but decrease with increasing percentage of microcrystalline wax, suggesting that the carnauba wax is included in the overall wax network formed by the saturated components, whereas the microcrystalline wax forms irregular crystals that disrupt the overall wax crystal network. Young's modulus, elastic modulus and viscous modulus all decrease with increasing aqueous phase volume in the emulsions, although the slope of the decrease in elastic and viscous moduli is dependent on the addition of solid wax, as a result of strengthening the network. CONCLUSIONS: This work suggests the potential use for emulsions in lipstick applications, particularly when PGPR is used as an emulsifier, and with the addition of solid wax, as it increases network strength.

Design and application of water-in-oil emulsions for use in lipstick formulations.

The addition of water to lipsticks in the form of a water-in-oil emulsion is an attractive opportunity for cosmetics manufacturers to deliver hydrophilic molecules to the consumers, as well as improving the moisturizing properties. In this work, the effect of the emulsifier type and water content on the structural properties of the designed products was investigated. It has been shown that PGPR leads to smaller droplets than the other emulsifiers tested. This was attributed to the ability of PGPR to form elastic interfaces that slow the coalescence between droplets during the process. It was also observed that crystals of wax tend to form structures at the interface upon cooling that prevent coalescence during storage. These structures also prevent leakage of water into the continuous phase. No effect of the water content on the melting properties of the emulsions was observed. Upon addition of more than 10% water, softening of the material was measured, due to the overall decrease in solid content. Addition of crystalline material (hard paraffin) was successfully used to reinstate the material properties.

Swelling of high acyl gellan gum hydrogel: Characterization of network strengthening and slower release.

This study examined the mechanism of swelling for high acyl (HA) gellan gum and the impacts on the hydrogel mechanical properties and the release of a model drug (glucose). Controlling the material properties and the release of entrapped drugs during use in aqueous environments, such as the stomach or bodily fluids, are crucial in designing functional applications. Swelling of HA gellan gum was controlled by varying the osmotic environment with salts and solvents, and effects on the gel network were characterized by uniaxial compression tests, DSC, and rheology. Low ionic strength solutions caused the greatest degree of swelling (up to 400 %) and corresponded to a more brittle gel with a greater modulus and greater network enthalpy. Swelling slowed the release of glucose by decreasing the diffusion flux. The osmotic environment was found to produce different functional properties, and it is crucial to consider these changes in the design of formulations.

The hydrophobic modification of kappa carrageenan microgel particles for the stabilisation of foams.

HYPOTHESIS: Polysaccharides such as kappa carrageenan are often utilised in fat replacement techniques in the food industry. However, the structural role they can provide within a product is limited by their hydrophilic nature. Hydrophilic particles can be surface-activated by hydrophobic modification e.g. in-situ interaction with a surfactant. This can drastically improve foam stability by providing a structural barrier around bubble interfaces offering protection against disproportionation and coalescence. Hence, it should be possible to bind negatively charged kappa carrageenan particles with a cationic surfactant through electrostatic interaction, in order to alter their surface properties. EXPERIMENTS: Lauric arginate was mixed with kappa carrageenan microgel particles at various concentrations and the potential electrostatic interaction was studied using zeta potential, turbidity and rheological measurements. Mixtures were then aerated and foaming properties explored, in particular the location of the particles. FINDINGS: Lauric arginate was successfully bound to kappa carrageenan microgel particles. Consequently, particles were surface-activated and adsorbed at the air/water interface, as shown by optical and confocal microscopy. Foam half-life peaked at an intermediate surfactant concentration, where there was sufficient surfactant to coat particle surfaces but the concentration was low enough to prevent the formation of large aggregates unable to adsorb at the a/w interfaces.

Temperature influences on network formation of low DE maltodextrin gels.

Gelation of maltodextrin (DE 2) was examined over a range of temperatures to understand the behaviour within mixed-gel systems. Maltodextrin solutions were prepared at 95 °C and held at temperatures between 5 °C and 60 °C for four days. Bulk gel properties and the underlying microstructure were analysed using fracture strength, proton relaxation time, and differential scanning calorimetry (DSC). Holding at lower temperatures led to a greater gel strength with a brittle and crumbly texture. Analysis of the microstructure showed that gelation at 10 °C versus 60 °C produced a greater number of aggregates (melting enthalpy 14.5 J/g versus 3.4 J/g) and structuring of a higher melting entropy (45 mJ/g K versus 10 mJ/g K). A thermal hysteresis with signs of structure corresponding to both holding temperatures was also measured. Elevated temperature was hypothesized to decrease the amount of smaller molecular weight chains participating in aggregation by shifting from the helix to coil form.

Interfacial tension in aqueous biopolymer-surfactant mixtures.

The current study offers a first insight into the interfacial properties of pullulan-sodium dodecyl sulphate (SDS) aqueous two-phase systems (ATPS) in the presence of sodium chloride (NaCl). The effect of composition on the interfacial tension (sigma) in these ATPS was investigated over a wide range of pullulan, SDS and NaCl concentrations. An increase in the interfacial tension was observed with increasing pullulan and SDS concentrations and a small increase was also observed as the NaCl concentration was increased. In both cases the interfacial tension increases were closely related to the phase behaviour of these systems; as a consequence of increasing the pullulan, SDS and/or NaCl concentrations, the system moves further away from the critical point. In all systems interfacial tensions (of the order of muN/m) were comparable with those reported for polymer-polymer ATPS. Interfacial tensions sigma can be well correlated with the difference in pullulan and SDS concentrations between the phases (DeltaC pul and DeltaC SDS) and also the tie-line length (TLL); all yield straight lines on a log-log scale.

Fabrication, characterisation and stability of oil-in-water emulsions stabilised by solid lipid particles: the role of particle characteristics and emulsion microstructure upon Pickering functionality.

The quest to identify and use bio-based particles with a Pickering stabilisation potential for food applications has lately been particularly substantial and includes, among other candidates, lipid-based particles. The present study investigates the ability of solid lipid particles to stabilise oil-in-water (o/w) emulsions against coalescence. Results obtained showed that emulsion stability could be achieved when low amounts (0.8 wt/wt%) of a surface active species (e.g. Tween 80 or NaCas) were used in particles' fabrication. Triple staining of the o/w emulsions enabled the visualisation of emulsion droplets' surface via confocal microscopy. This revealed an interfacial location of the lipid particles, hence confirming stabilisation via a Pickering mechanism. Emulsion droplet size was controlled by varying several formulation parameters, such as the type of the lipid and surface active component, the processing route and the polarity of the dispersed phase. Differential scanning calorimetry (DSC) was employed as the analytical tool to quantify the amount of crystalline material available to stabilise the emulsion droplets at different intervals during the experimental timeframe. Dissolution of lipid particles in the oil phase was observed and evolved distinctly between a wax and a triglyceride, and in the presence of a non-ionic surfactant and a protein. Yet, this behaviour did not result in emulsion destabilisation. Moreover, emulsion's thermal stability was found to be determined by the behaviour of lipid particles under temperature effects.

Mechanism and dynamics of conformational ordering in xanthan polysaccharide.

The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting.

Functional food microstructures for macronutrient release and delivery.

There is a need to understand the role of fat, protein and carbohydrate in human health, and also how foods containing and/or structured using these macronutrients can be designed so that they can have a positive impact on health. This may include a reduction in fat, salt or sugar, the protection and targeted release of micronutrients or active ingredients from/to particular parts of the digestive system, improvement of gastrointestinal health or satiety enhancing properties. Such foods can be designed with various macro- and microstructures that will impact on macronutrient release and delivery. These include simple and double emulsions, the use of Pickering particles and shells, nanoparticles, liposomes, gelled networks, fluid gels and gel particles, foams, self-assembled structures, and encapsulated systems. In order to design foods that deliver these benefits understanding of how these structures behave in the gastrointestinal tract is also required, which should involve utilising both in vitro and in vivo studies. This review aims to draw together research in these areas, by focusing on the current state of the art, but also exciting possibilities for future research and food development.

Dynamic experimentation on the confocal laser scanning microscope: application to soft-solid, composite food materials.

Confocal laser scanning microscopy (CLSM) is used to follow the dynamic structural evolution of several phase-separated mixed biopolymer gel composites. Two protein/polysaccharide mixed gel systems were examined: gelatin/maltodextrin and gelatin/agarose. These materials exhibit 'emulsion-like' structures, with included spherical particles of one phase (i.e. polymer A) within a continuous matrix of the second (i.e. polymer B). Compositional control of these materials allows the phase order to be inverted (i.e. polymer B included and polymer A continuous), giving four basic variants for the present composites. Tension and compression mechanical tests were conducted dynamically on the CLSM, with crack/microstructure interactions investigated using a notched compact tension geometry. Gelatin/maltodextrin composites exhibit a 'pseudo-yielding' stress/strain response in both tension and compression, when the gelatin-rich phase is continuous, which was attributed to debonding of the particle/matrix interface. This behaviour is significantly less apparent for both the gelatin/agarose composites, and the maltodextrin continuous gelatin/maltodextrin composites, with these materials responding in a nominally linear elastic manner. Values of the interfacial fracture energy for selected compositions of the two biopolymer systems were determined by 90 degrees peel testing, where a gelatin layer was peeled from either a maltodextrin or agarose substrate. For biopolymer layers 'cast' together, a value of 0.2 +/- 0.2 J m-2 was obtained for the fracture energy of a gelatin/maltodextrin interface, while a significantly higher value of 6.5 +/- 0.2 J m-2 was determined for a gelatin/agarose interface. The interfacial fracture energy of the two mixed systems was also determined following an indirect elastomer composite debonding model. An interfacial fracture energy of approximately 0.25 J m-2 was determined using this approach for the gelatin continuous gelatin/maltodextrin composite, which compares favourably with the value calculated directly by peel testing (i.e. approximately 0.2 J m-2). A somewhat higher value was estimated for the gelatin continuous gelatin/agarose system (1.0-2.0 J m-2), using this model, although there are severe limitations to this approach for this mixed gel system. In the present case, it is believed that the differing mechanical response of the two mixed biopolymer systems, when the gelatin phase is continuous, arises from the order of magnitude difference in interfacial fracture energy. It is postulated that polymer interdiffusion may occur across the interface for the gelatin/agarose system, to a significantly greater extent than for interfaces between gelatin and maltodextrin, resulting in a higher interfacial fracture energy.

Ultrastructural evidence for intramolecular double stranding in iota-carrageenan.

Kinetic studies of primary processes of conformational ordering in gel-forming biopolymers have suggested that a change in mechanism from intermolecular to intramolecular multistrand formation occurs on lowering the concentration of biopolymer. We report here ultrastructural observations consistent with intramolecular double stranding in a carbohydrate polymer, iota-carrageenan, by arresting this process of primary conformational ordering by an ultra-rapid freeze fixation technique. High-resolution transmission electron microscopy (TEM) revealed isolated iota-carrageenan chains showing a range of morphologies (linear, circular, and hairpin) consistent with intramolecular stranding. Control experiments in which iota-carrageenan was frozen in the disordered form revealed longer and thinner strands.

Simulation of endo-PG digest patterns and implications for the determination of pectin fine structure.

Novel stepwise approaches to the calculation of enzyme digest patterns are described and used in the validation of a computer simulation. Results obtained using the simulation show that, while a previously proposed model of endo-PG action captures some of the salient features of this enzymes behaviour, it is not sufficient to successfully predict experimental digest patterns from pectic substrates. Subsequently, it has been shown that a modified model incorporating existing information regarding subsite architecture and speculative site tolerances for esterified residues, goes someway towards improving the situation.

Interfacial tension in phase-separated gelatin/dextran aqueous mixtures.

The effect of solute concentrations on interfacial tension was investigated in phase-separated mixtures of dextran and gelatin over a range of concentrations that covered different tie-lines and different positions on one tie-line. The investigations were carried out using equilibrated gelatin-rich and dextran-rich phases in a computer-controlled Couette device at 40 degrees C (above the gelation point of gelatin) and interfacial tensions were measured using the retracting drop method. The results show that the interfacial tension can be related to the length of the tie-line or to the difference in the concentration of dextran (or gelatin) in the separated phases. Interfacial tension increases as either of these parameters increases. For concentrations lying on any single tie-line, the interfacial tension is constant and independent of the concentration of biopolymers. Also, the addition of small amounts of low molecular weight dextran to a dextran-rich phase does not significantly affect the interfacial tension between the gelatine-rich and dextran-rich phases. Experimental results were also compared with theoretical predictions of the interfacial tension using a Flory-Huggins based analysis of the measured tie-line data. Reasonable agreement was found between predicted and measured values, indicating that this approach captures the basic physics of the system.

A molecular model for the formation and properties of fluid gels.

The effects of biopolymer gelation in a shear field are discussed. Gel particles are produced if the gelation mechanism involves an aggregation step. Particular attention is paid to the molecular events of ordering and aggregation upon cooling, investigating the differences in such processes as a result of shearing during gelation. A model is proposed which follows the conception of the particles, their growth, physical properties and stability.

Fat crystallisation at oil-water interfaces.

This review focuses on recent advances in the understanding of lipid crystallisation at or in the vicinity of an interface in emulsified systems and the consequences regarding stability, structure and thermal behaviour. Amphiphilic molecules such as emulsifiers are preferably adsorbed at the interface. Such molecules are known for their ability to interact with triglycerides under certain conditions. In the same manner that inorganic crystals grown on an organic matrix see their nucleation, morphology and structure controlled by the underlying matrix, recent studies report a templating effect linked to the presence of emulsifiers at the oil/water interface. Emulsifiers affect fat crystallisation and fat crystal behaviour in numerous ways, acting as impurities seeding nucleation and, in some cases, retarding or enhancing polymorphic transitions towards more stable forms. This understanding is of crucial importance for the design of stable structures within emulsions, regardless of whether the system is oil or water continuous. In this paper, crystallisation mechanisms are briefly described, as well as recent technical advances that allow the study of crystallisation and crystal forms. Indeed, the study of the interface and of its effect on lipid crystallisation in emulsions has been limited for a long time by the lack of in-situ investigative techniques. This review also highlights reported interfacial effects in food and pharmaceutical emulsion systems. These effects are strongly linked to the presence of emulsifiers at the interface and their effects on crystallisation kinetics, and crystal morphology and stability.

Interfacial behaviour of sodium stearoyllactylate (SSL) as an oil-in-water pickering emulsion stabiliser.

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.

Boundary lubrication by sodium salts: a Hofmeister series effect.

Boundary lubrication plays an important role in the function of sliding surfaces in contact. Of particular interest in this study, boundary regime tribology is relevant for understanding textural attributes perceived during oral consumption of food, where the tongue squeezes and slides against the hard palate. This work investigates aqueous lubrication of a sliding/rolling ball-on-disc contact by sodium anions of the Hofmeister series in both water and guar gum solutions. Low concentrations (0.001 M) of strongly kosmotropic salts provide reduced friction coefficients in both systems (water and guar gum solutions), although a different mechanism prevails in each. Surface-bound hydrated ions are responsible in the case of water, and salt-promoted adsorption of hydrated-polymer chains dominate with guar gum. In each system, friction decreases in accordance with the Hofmeister series: iodide, nitrate, bromide, chloride, fluoride, phosphate and citrate. The addition of salt has little impact on solution of bulk viscosity, and so this work demonstrates that significant boundary lubrication can be provided without surface modification and with lubricants of viscosity similar to that of water.

Competitive adsorption of surfactants and hydrophilic silica particles at the oil-water interface: interfacial tension and contact angle studies.

The effect of surfactants' type and concentration on the interfacial tension and contact angle in the presence of hydrophilic silica particles was investigated. Silica particles have been shown to have an antagonistic effect on interfacial tension and contact angle in the presence of both W/O and O/W surfactants. Silica particles, combined with W/O surfactant, have no effect on interfacial tension, which is only dictated by the surfactant concentration, while they strongly affect interfacial tension when combined with O/W surfactants. At low O/W surfactant, both particles and surfactant are adsorbed at the interface, modifying the interface structure. At higher concentration, interfacial tension is only dictated by the surfactant. By increasing the surfactant concentration, the contact angle that a drop of aqueous phase assumes on a glass substrate placed in oil media decreases or increases depending on whether the surfactant is of W/O or O/W type, respectively. This is due to the modification of the wettability of the glass by the oil or water induced by the surfactants. Regardless of the surfactant's type, the contact angle profile was dictated by both particles and surfactant at low surfactant concentration, whereas it is dictated by the surfactant only at high concentration.

O/W emulsions stabilised by both low molecular weight surfactants and colloidal particles: The effect of surfactant type and concentration.

The stability against coalescence of O/W emulsions in the presence of both surfactants and colloidal particles was investigated. In particular the effect of the surfactant type and concentration in these emulsifier mixtures on the O/W emulsions' stability was studied. Two types of surfactants were selected; those that have the ability to stabilise O/W emulsions on their own (O/W surfactants) and those that cannot (W/O surfactants). Tween 60 and Sodium Caseinate were selected as the O/W surfactants and lecithin as the W/O surfactant. Oil-in-water emulsions prepared with both particles and any of the three surfactants were stable against coalescence but, depending on the type of surfactant, the behaviour of the systems was found to depend on surfactant concentration. The droplet sizes of emulsions stabilised by mixed emulsifier systems containing low concentrations of O/W surfactants (Tween 60 or Sodium Caseinate) were smaller than those solely stabilised by either the surfactant or particles alone. At intermediate O/W surfactants concentrations, the droplet sizes of the emulsions increased. Further increases in the O/W surfactants' concentration, resulted in the complete removal of particles from the interface with the system now behaving as a surfactant-only stabilised emulsion. The behaviour of emulsions stabilised by emulsifier mixtures containing W/O surfactants was not dependent on the concentration of surfactant: no removal of particles was observed.