A study of oil-in-water emulsions stabilized by whey proteins

Like many other emulsifiers, whey protein concentrates stabilize oil-in-water emulsions. However, the emulsifying capacity of whey proteins is affected by several factors, e. g., type of homogenizer, degree of homogenization, protein concentration, oil volume fraction, pH and ionic strength of the aqueous phase. For the present study, oilin-water emulsions were made by homogenizing known amounts of whey protein concentrate with a vegetable oil (i. e. grapeseed oil) at different pH. The emulsifying properties of whey proteins are expressed as a function of the particle size and size distribution of oil droplets as measured by light scattering, and of the surface charge density derived from the electrophoretic mobility.The whey protein concentrate was shown to have an isoelectric point at pH 4.4. Near this pH value, the oil-in-water emulsions exhibited poor stability as expected from the low surface coverage.     

Influence of the fat characteristics on the physicochemical behavior of oil-in-water emulsions based on milk proteins-glycerol esters mixtures

Oil-in-water emulsions based on 10% milk protein preparation, 0.3% mono-di-glycerides (MDG) and 8% vegetable oil were prepared for models typifying ice cream formulations. Two MDG (saturated and partially unsaturated) and four fats (oleic oil, hydrogenated and refined coconut oils, refined palm oil) were chosen to investigate the interactions occurring between the oil phase, the MDG and the milk proteins. Influence of temperature (4 °C) and ageing (24 h at 4 °C) was also tested. The emulsions were characterized for protein desorption, particle size distribution and rheological properties. The dynamic surface activity of the milk proteins and the MDG at the oil-water interface was also determined. At 20 °C, emulsions were mostly stabilized by proteins although the protein load at the globule surface strongly depended on the emulsifier and the oil phase natures. A displacement of the proteins adsorbed at the oil droplet interface by the lipid surfactant was a consequence of the temperature decrease and/or ageing step, suggesting a disruption of the interfacial protein interactions. This disruption was more or less marked depending on the physicochemical characteristics of the surfactant and the oil used (amount of crystallized matter, fatty acid chain length and unsaturation degree). In parallel, the variation of the apparent viscosity of the various emulsions upon temperature was well correlated with the solid fat content. On the whole, the results obtained suggested that not only the surfactant molecules, i.e. emulsifiers and proteins, but also the fat used in the emulsion formulation participated in the development of the interface characteristics and rheological properties.     

Physicochemical behavior of oil-in-water emulsions: influence of milk protein mixtures, glycerol ester mixtures and fat characteristics

Different emulsions based on two protein mixtures (skim milk powder (SMP) and functional dairy proteins (FDP)), two mono-di-glyceride mixtures (MDG) (saturated and partially unsaturated), three fats (hydrogenated and refined coconut oils and refined palm oil) were studied to investigate the interactions occurring between the oil phase, low molecular weight emulsifiers and proteins. Immediately following the emulsification process, high diameters of fat globules were obtained in FDP-based systems, relevant of an aggregation phenomenon. At this stage, the fat globule size characteristics were dependent on the emulsifier and fat types present in the formulation. In contrast, SMP-based emulsions were characterized by low proportions of aggregated particles regardless the formulations. Ageing (24 h at 4 °C) promoted disaggregation in FDP formulations, while SMP emulsions were well stabilized. Just after the homogenization step, less proteins were required to stabilize the globule interface in FDP systems as compared to SMP ones. Only with SMP, the amount of protein load at the fat globule surface was influenced by the oil nature and/or by the emulsifier type. A competitive adsorption of caseins, over whey proteins, was demonstrated in the case of FDP. The ageing period promoted a displacement of the proteins adsorbed at the oil droplet interface, suggesting a disruption of the interfacial protein interactions. This disruption was more marked with SMP than with FDP and, in both cases, was more or less influenced by the emulsifier and oil phase natures. The variations of the viscosity and rheological parameters (elastic and viscous moduli) were not dependent on one specific component of the formulation.     

A differential microcalorimetric study of whey proteins and their behaviour in oil-in-water emulsions

Oil-in-water emulsions (20% soya oil, 1% protein) have been prepared containing lysozyme or isolates of -lactalbumin and β-lactoglobulin from whey protein. The structural characteristics of these proteins adsorbed at an oil/water interface were determined by following their thermal transitions using differential scanning microcalorimetry. Thermograms of the proteins in the adsorbed state differed markedly from the corresponding transitions seen for the proteins in solution. This suggests that the proteins underwent substantial changes in secondary and tertiary structure upon adsorption. In general, for all the proteins studied, a net decrease in the total energy absorbed during denaturation was found when the proteins were in an adsorbed state. Both lysozyme and -lactalbumin were in part “surface denatured”, and they showed a certain degree of reversibility between solution and the adsorbed state. β-Lactoglobulin showed the highest degree of denaturation upon adsorption and the conformational change was irreversible.     

On the flow characteristics of highly concentrated oil-in-water emulsions

The laminar flow characteristics of oil-in-water emulsions with oil concentrations greater than 59% by volume have been investigated experimentally. Up to an oil concentration of 65% by volume, the emulsions exhibited power-law non-newtonian behaviour. At a higher oil concentration, of 72.21% by volume, a dramatic change in the flow behaviour of the emulsion was observed. The flow curve, i.e. shear stress vs. shear rate plot on a log-log scale, clearly exhibited the presence of a yield-stress.The rheological data on the emulsions were used to correlate the laminar pipeline transport data on the same emulsions. For power-law emulsions, values of the drop in pipeline pressure could be accurately predicted from simple rheological measurements. For a yield-stress emulsion, the experimental pipeline data deviated from the predicted values especially at low values of shear stress.     

Influence of interfacial characteristics on Ostwald ripening in hydrocarbon oil-in-water emulsions

The influence of the nature of the interfacial membrane on the kinetics of droplet growth in hydrocarbon oil-in-water emulsions was investigated. Droplet growth rates were determined by measuring changes in the droplet size distribution of 1 wt % n-tetradecane or n-octadecane oil-in-water emulsions using laser diffraction. The interfacial properties of the droplets were manipulated by coating them with either an SDS layer or with an SDS-chitosan layer using an electrostatic deposition method. The emulsion containing SDS-coated octadecane droplets did not exhibit droplet growth during storage for 400 h, which showed that it was stable to Ostwald ripening because of this oils extremely low water-solubility. The emulsion containing SDS-coated n-tetradecane droplets showed a considerable increase in mean droplet size with time, which was attributed to Ostwald ripening associated with this oils appreciable water-solubility. On the other hand, an emulsion containing SDS-chitosan coated n-tetradecane droplets was stable to droplet growth, which was attributed to the ability of the interfacial membrane to resist deformation because of its elastic modulus and thickness. This study shows that the stability of emulsion droplets to Ostwald ripening can be improved by using an electrostatic deposition method to form thick elastic membranes around the droplets.     

Self-depletion flocculation of tetralin oil-in-water emulsions

Oil-in-water emulsions of slightly soluble oils such as tetralin prepared by high-pressure homogenization and stabilized by sodium dodecyl sulfate undergo depletion flocculation induced by an initially polydisperse droplet size distribution. The smaller droplets flocculate the larger ones; the flocculation can be reversed by gentle sonication. After aging, the flocs disappear because the smaller droplets dissolve through Ostwald ripening. These effects were observed by electroacoustic measurements, supplemented by light scattering.     

Shear-induced coalescence of oil-in-water Pickering emulsions

Thermodynamic treatment of thin liquid films in Part III of this series was applied to foam films stabilized by sodium dodecyl sulfate. Miscibility of sodium chloride and sodium dodecyl sulfate in the adsorbed films at the film surfaces and transition between the black films were studied by measuring film thickness and contact angle. A discontinuous change in the thickness and a break on the contact angle vs. concentration curve appeared at the transition. Judging from the phase diagram of adsorption, sodium chloride and sodium dodecyl sulfate are a little miscible in the adsorbed films. The miscibility was ascribed to specific interaction between sodium ion and dodecyl sulfate ion in the adsorbed films. The miscibility in an adsorbed film was compared between the film surface and meniscus and between the common black and Newton black films.     

Influence of droplet characteristics on the formation of oil-in-water emulsions stabilized by surfactant-chitosan layers

The objective of this study was to establish the optimum conditions for preparing stable oil-in-water emulsions containing droplets surrounded by surfactant-chitosan layers. A primary emulsion containing small droplets (d32 approximately = 0.3 microm) was prepared by homogenizing 20 wt% corn oil with 80 wt% emulsifier solution (20 mM SDS, 100 mM acetate buffer, pH 3) using a high-pressure valve homogenizer. The primary emulsion was diluted with chitosan solutions to produce secondary emulsions with a range of oil and chitosan concentrations (0.5-10 wt% corn oil, 0-1 wt% chitosan, pH 3). The secondary emulsions were sonicated to help disrupt any droplet aggregates formed during the mixing process. The electrical charge, particle size, and amount of free chitosan in the emulsions were then measured. The droplet charge changed from negative to positive as the amount of chitosan in the emulsions was increased, reaching a relatively constant value (approximately +50 mV) above a critical chitosan concentration (C(Sat)), which indicated that saturation of the droplet surfaces with chitosan occurred. Extremely large droplet aggregates were formed at chitosan concentrations below C(Sat), but stable emulsions could be formed above C(Sat) provided the droplet concentration was not high enough for depletion flocculation to occur. Interestingly, we found that stable multilayer emulsions could also be formed by mixing chitosan with an emulsion stabilized by a nonionic surfactant (Tween 20) due to the fact the initial droplets had some negative charge. The information obtained from this study is useful for preparing emulsions stabilized by multilayer interfacial layers.     

Flocculation and coalescence of droplets in oil-in-water emulsions formed with highly hydrolysed whey proteins as influenced by starch

The effects of added unmodified amylopectin starch, modified amylopectin starch and amylose starch on the formation and properties of emulsions (4 wt.% corn oil) made with an extensively hydrolysed commercial whey protein (WPH) product under a range of conditions were examined. The rate of coalescence was calculated based on the changes in the droplet size of the emulsions during storage at 20 degrees C. The rates of creaming and coalescence in emulsions containing amylopectin starches were enhanced with increasing concentration of the starches during storage for up to 7 days. At a given starch concentration, the rate of coalescence was higher in the emulsions containing modified amylopectin starch than in those containing unmodified amylopectin starch, whereas it was lowest in the emulsions containing amylose starch. All emulsions containing unmodified and modified amylopectin starches showed flocculation of oil droplets by a depletion mechanism. However, flocculation was not observed in the emulsions containing amylose starch. The extent of flocculation was considered to correlate with the rate of coalescence of oil droplets. The different rates of coalescence could be explained on the basis of the strength of the depletion potential, which was dependent on the molecular weight and the radius of gyration of the starches. At high levels of starch addition (>1.5%), the rate of coalescence decreased gradually, apparently because of the high viscosity of the aqueous phase caused by the starch.     

Molecular dynamics simulation of cellulose-coated oil-in-water emulsions

The behaviors of cellulose chains and cellulose mini-crystal in oil-in-water emulsions were studied by molecular dynamics simulations to investigate the coating states and the structural features of cellulose in these emulsions. In oil-in-water emulsion, dispersed cellulose chains gradually assemble during the progress of the simulation, eventually surrounding the octane droplet. In case of a cellulose mini-crystal, the cellulose chain at the corner of the crystal first contacts with the octane droplet through its hydrophobic surface. The other cellulose chains along the crystal plane then gradually move toward the octane molecules. In both emulsions, the cellulose was found to interact with both water and octane surfaces with specific conformations that allow the CH groups of the glucose rings to contact with octane molecules, while the OH groups of these rings contact with water molecules to form hydrogen bonds. The cellulose chains on the octane droplet also contact with each other through lateral hydrogen bonding between chains. These interactions stabilize the emulsion formed by cellulose molecules as surfactants.     

Gelling of oil-in-water emulsions comprising crystallized droplets

We fabricate oil-in-water emulsions above the melting temperature of the oil phase (hexadecane and/or paraffin). Upon cooling, the oil droplets crystallize and the initially fluid emulsions turn into hard gels. The systems evolve by following two distinct regimes that depend on the average droplet size and on the oil nature. In some cases gelling involves partial coalescence of the droplets, i.e., film rupturing with no further shape relaxation because of the solid nature of the droplets. In some other cases, gelling occurs without film rupturing and is reminiscent of a jamming transition induced by surface roughness. We prepare blends of oils having different melting temperatures, and we show that it is possible to reinforce the gel stiffness by applying a temperature cycle that produces partial melting of the crystal mass, followed by recrystallization.     

Aging mechanisms of oil-in-water emulsions based on a bioemulsifier produced by Yarrowia lipolytica

The aging mechanisms of oil-in-water emulsions prepared with Yansan, a bioemulsifier produced by a Brazilian wild strain of Yarrowia lipolytica, IMUFRJ 50682, in glucose-based fermentation medium, were studied and compared with those prepared with Gum Arabic. Oil-in-water emulsions obtained by combining three different organic phases, perfluoro-n-hexane, n-hexadecane and toluene, with two aqueous buffers of different pH, and two bioemulsifiers, were studied through the evolution of the mean droplet size. The emulsions were prepared by sonication and their droplet size distribution was followed for 60 days at 301 K using image analysis. The results indicate that the aging mechanisms of the studied emulsions depend mainly on the bioemulsifier and on the pH of the medium. It is shown that the emulsions containing Gum Arabic age by coalescence while Yansan-based emulsions change their aging mechanisms from coalescence at pH 3 to molecular diffusion at pH 7.     

Evaporation rates of water from concentrated oil-in-water emulsions

We have investigated the rate of water evaporation from concentrated oil-in-water (o/w) emulsions containing an involatile oil. Evaporation of the water continuous phase causes compression of the emulsion with progressive distortion of the oil drops and thinning of the water films separating them. Theoretically, the vapor pressure of water is sensitive to the interdroplet interactions, which are a function of the film thickness. Three main possible situations are considered. First, under conditions when the evaporation rate is controlled by mass transfer across the stagnant vapor phase, model calculations show that evaporation can, in principle, be slowed by repulsive interdroplet interactions. However, significant retardation requires very strong repulsive forces acting over large separations for typical emulsion drop sizes. Second, water evaporation may be limited by diffusion in the network of water films within the emulsion. In this situation, water loss by evaporation from the emulsion surface leads to a gradient in the water concentration (and in the water film thickness). Third, compression of the drops may lead to coalescence of the emulsion drops and the formation of a macroscopic oil film at the emulsion surface, which serves to prevent further water evaporation. Water mass-loss curves have been measured for silicone o/w emulsions stabilized by the anionic surfactant SDS as a function of the water content, the thickness of the stagnant vapor-phase layer, and the concentration of electrolyte in the aqueous phase, and the results are discussed in terms of the three possible scenarios just described. In systems with added salt, water evaporation virtually ceases before all the water present is lost, probably as a result of oil-drop coalescence resulting in the formation of a water-impermeable oil film at the emulsion surface.     

Partitioning of para-phenylenediamines in oil-in-water emulsions

A method to measure distribution coefficients (P) of electroactive species in situ in turbid oil-in-water emulsions is demonstrated using four p-phenylenediamines (PPD) in oil-in-aqueous-gelatin emulsions of six oils at 40°C. The PPD examined represent a series in β-X-ethyl substitution (4-amino-3-methyl-N-ethyl-N-(β-X-ethyl)aniline) where X = H (2), OH (3), methylsulfonamide (4), and methoxyethyl (5), respectively, for PPD 2–5. The oils examined include di-n-butyl phthalate (DBP), N,N-diethyldodecanamide (DEDA), 1-octanol (OCA), 1-undecanol (UNA), tri-n-hexyl phosphate (THP), and dodecane (DOD). The rotating platinum disk electrode (RPDE) is used as a voltammetric probe of PPD concentration in the aqueous-gelatin phase of the emulsions. Distribution coefficients in macroscopic aqueous/oil systems are also reported, and are illustrated to correlate linearly with values determined in emulsions. The distribution coefficients (P = C_oilC_aqueus) for the different PPD decrease in the order 2 > 5 > 3 ≈ 4 for each of the six oils. The largest distribution coefficients are obtained with THP, and the smallest are obtained with DOD. The interface appears to play a significant role in modifying solute distribution in emulsions.     

Retardation of oil drop evaporation from oil-in-water emulsions

The rate of evaporation of volatile oils from oil-in-water emulsions can be strongly retarded by using a polymeric emulsion stabiliser instead of a low molar mass surfactant.     

Casein adsorption on the surfaces of oil-in-water emulsions modified by lecithin

The effects of incorporating an additional component, egg-yolk lecithin, on the properties of oil-in-water emulsions stabilized by casein have been studied. The impact of lecithin on the stability of the emulsions was studied using integrated light scattering and the casein-oil-lecithin interaction was studied with photon correlation spectroscopy combined with breakdown of the adsorbed protein layers by proteolysis. Lecithin was found to enhance the stability of the emulsions at low cascin concentrations, below the limiting surface coverage of 1 mg m⁻² of casein which is found in the absence of lecithin. Conversely, small amounts of casein also stabilized flocculating oil-lecithin emulsions. The hydrodynamic thickness of the adsorbed protein layer on the hydrophobic oil surface was modified by the presence of lecithin. When the total surface area occupied by lecithin was less than 10% (5 mg lecithin for 2 ml oil), the thickness of the adsorbed casein layer was not significantly different from that in the absence of phospholipid. At higher concentrations of lecithin, the adsorbed casein layer had a lower minimum value for the layer thickness of 6.5 nm at low casein concentration and an upper plateau value of 8 nm at saturated adsorption, compared to a low limit of 5 nm and a plateau value of 10 nm in the absence of lecithin, demonstrating that the structure of the adsorbed casein layer was changed by the presence of phospholipid.     

Ostwald ripening of oil-in-water emulsions stabilized by phenoxy-substituted dextrans

The stability of oil-in-water emulsions prepared using dextran, a natural polysaccharide, hydrophobically substituted with phenoxy groups, was studied. The evolution of the emulsion droplet size was investigated as a function of polymer concentration (Cp=0.2 to 1% w/w in a water phase) and the degree of phenoxy substitution (tau=4.2 to 15.7%). For the highest tau values, emulsions, which presented submicrometer droplets, were stable over more than 4 months at room temperature. The most substituted polymers clearly showed a better efficiency to lower the surface tension at the oil/water interface. DexP did not induce real viscosification of the continuous phase. The linearity of the particle volume variation with time, and the invariability of the volume distribution function, proved that Ostwald ripening was the main destabilization mechanism of the phenoxy dextran emulsions. The nature of the oil dispersed phase drastically affected the behavior of emulsions. While the emulsions prepared with n-dodecane presented a particle growth with time, only few size variations occurred when n-hexadecane was used. Furthermore, small ratios of n-hexadecane in n-dodecane phase reduced the particle growth due to the lower solubility and lower diffusion coefficient in water of n-hexadecane, which acted as a ripening inhibitor.     

An adsorption effect on the gel strength of dilute gelatin-stabilized oil-in-water emulsions

The incorporation of emulsion oil droplets into a gelatin gel leads to an initial increase in shear modulus at 25 °C for a gelatin concentration of 8 wt % but an initialdecrease for a concentration of 5 wt %. The latter result is consistent with a net lowering of the gelatin concentration available for gelation in the aqueous phase due to adsorption at the oil-water interface.