A novel fiber-optic photometer for in situ stability assessment of concentrated oil-in-water emulsions

The purpose of this research was to evaluate a novel fiberoptic photometer for its ability to monitor physical instabilities occurring in concentrated emulsions during storage. For this, the fiber-optic photometer was used to measure transmission of oil-in-water emulsions stabilized with hypromellose (HPMC) as a function of oil volume fraction and droplet size distribution (DSD). To detect physical instabilities like creaming and coalescence, the transmissivity of the samples was studied at 2 different hight levels over a certain period of time. The corresponding droplet size distributions were determined by laser diffraction with PIDS. Transmissivity was found to depend on the number of dispersed droplets and thus is sensitive to both the variation of phase volume fraction as well as the emulsions droplet size distribution. At constant DSD, light transmission decreased linearly with increasing oil content within a large interval of phase volume fractions from 0.01 to 0.3. At constant phase volume fraction, an increase in droplet size increased light transmission. Investigation of creaming on emulsions with different droplet size distributions showed changes in the initial delay times and creaming velocities. In contrast to creaming phenomenon coalescence can be identified by height independent changes of the transmissivity. In conclusion, transmissivity of oil-in-water emulsions observed by the novel fiber-optic photometer is sensitive to phase volume fraction, droplet size distribution, and thus can be used as a tool for stability studies on concentrated emulsions. Published: August 31, 2007     

Optimization of Nanoemulsion Fabrication Using Microfluidization: Role of Surfactant Concentration on Formation and Stability

Nanoemulsions have some important potential advantages over conventional emulsions for certain commercial applications due to their optical clarity, high physical stability, and ability to increase the bioavailability of lipophilic bioactives. In this study, the factors influencing droplet size and stability in nanoemulsions fabricated from a hydrocarbon oil and an anionic surfactant were examined. Octadecane oil-in-water nanoemulsions were produced by a high pressure homogenizer (microfluidizer) using sodium dodecyl sulfate (SDS) as a model anionic surfactant. The influence of homogenization pressure, number of passes, and surfactant concentration was examined. The droplet size decreased with increasing homogenization pressure, number of passes, and surfactant concentration. Nanoemulsions with low turbidity and small droplet diameters (≈62 nm) could be produced under optimized conditions. Interestingly, nanoemulsions containing relatively high surfactant levels were highly susceptible to creaming when they were only passed through the homogenizer a few times, which was attributed to depletion flocculation. These results show the importance of optimizing surfactant levels to produce small droplets that are also stable to creaming.     

Impact of Phytic Acid on the Physical and Oxidative Stability of Protein-Stabilized Oil-in-Water Emulsions

The effects of phytic acid on the physical and oxidative stability of flaxseed oil-in-water emulsions containing whey protein-coated lipid droplets were investigated. The surface potential, particle size, microstructure, appearance, and oxidation of these emulsions were monitored when they were stored at pH 3.5 and 7.0 for 25 days in the dark (37 °C). The phytic acid and protein-coated lipid droplets had similar charges (both negative) at pH 7.0, but had opposite charges (negative and positive) at pH 3.5. At pH 7.0, the addition of phytic acid had no impact on the physical stability of the emulsions but significantly improved their oxidative stability, which was attributed to its ability to sequester pro-oxidant transition metals (iron ions). At pH 3.5, extensive droplet aggregation and creaming occurred in the emulsions containing phytic acid, which was ascribed to charge neutralization and ion bridging. The oxidative stability of the acidified emulsions, however, still increased after addition of phytic acid, which was again attributed to its ability to chelate iron ions. Interestingly, the antioxidant activity of phytic acid decreased as its level was increased. Our results suggest that phytic acid may be used as a natural antioxidant to improve the oxidative stability of food emulsions containing polyunsaturated fatty acids, but its level must be carefully controlled.

     

Influence of Nanoemulsion Addition on the Stability of Conventional Emulsions

Interest in using nanoemulsions as delivery systems for lipophilic food ingredients is growing due to their high optical clarity, good physical stability, and ability to increase bioavailability. Nanoemulsion-based delivery systems may need to be incorporated into food matrices that also contain conventional emulsions. The aim of this work was to evaluate the effect of adding nanoemulsions (d?<?200 nm) to conventional emulsions (d?>?200 nm) on the creaming stability and microstructure of the mixed systems. Droplet flocculation and rapid creaming was observed when the nanoemulsion concentration exceeded a particular level: the critical flocculation concentration (CFC) was 3.75 % and 0.25 % (v/v) for conventional emulsions with average droplet diameters of 350 and 250 nm, respectively. Confocal microscopy indicated that there was appreciable droplet flocculation, and the fraction of individual droplets with diameters?<?100 nm decreased after 14 days storage, which was probably due to Ostwald ripening and/or coalescence. The results of the present study might have important implications for the incorporation of nanoemulsion-based delivery systems into food products containing larger fat droplets, such as dressings, sauces, or beverages.     

Hydrophobically modified alginate for emulsion of oil in water

Dodecanol was covalently coupled to sodium alginate (NaAlg) via ester functions using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC-HCl) as a coupling reagent to provide an amphiphilic dodecanol alginate (DA) for subsequent use in oil-in-water (O/W) emulsion application. The structure of DA was confirmed by FT-IR spectrometry. The stability of the emulsions prepared with different concentrations (0.3-1.2 wt%) of DA or 1.0 wt% NaAlg was evaluated by measuring droplet size, microstructure, viscosity and creaming. The results showed that the emulsions containing 1.0 wt% NaAlg, 0.3 and 0.5 wt% DA were unstable and the emulsions containing 0.8-1.2 wt% DA presented better stability during storage.     

Influence of pH and ionic strength on formation and stability of emulsions containing oil droplets coated by beta-lactoglobulin-alginate interfaces

Emulsions of 0.1 wt % corn oil-in-water containing oil droplets coated by beta-lactoglobulin (0.009 wt % beta-Lg, 5 mM phosphate buffer, pH 7.0) were prepared in the absence and presence of sodium alginate (0 or 0.004 wt %). The pH (3-7) and ionic strength (0-250 mM NaCl) of these emulsions were adjusted, and the particle charge, particle size, and creaming stability were measured. Alginate adsorbed to the beta-Lg-coated droplets from pH 3 to 6, which was attributed to electrostatic attraction between the anionic polymer and cationic patches on the droplet surfaces. Droplets coated by beta-Lg-alginate had better stability to flocculation than those coated by beta-Lg alone, especially around the isoelectric point of the adsorbed proteins and at low ionic strengths (< 100 mM NaCl). At pH 5, alginate molecules desorbed from the droplet surfaces at high salt concentrations due to weakening of the electrostatic attraction.     

Stability of the oil-in-water type triacylglycerol emulsions

Lipid emulsions with saturated triacylglycerols (TAGs) with 4 to 10 carbons in each acyl chain were prepared to study how the oil component alters the stability of the lipid emulsions when phosphatidylcholines were used as emulsifiers. The average droplet size of the emulsions became smaller as the chain length of the TAG increased. For a given oil, emulsion with smaller droplets was formed with an emulsifier having higher HLB value. The influence of HLB values on the droplet size was biggest for the tributyrin (C4) emulsions. For the tricaprylin (C8) emulsions, droplet size was identical at given emulsifier concentrations regardless of HLB values. The HLB value and the concentration of the emulsifiers also affect the droplet size of the emulsions. The emulsions with smaller average droplet size were more stable than with bigger size for 20 days. The oil and water (o/w) interfacial tension is inversely proportional to the initial droplet size of the emulsion.     

Modulation of pH Sensitivity of Surface Charge and Aggregation Stability of Protein-Coated Lipid Droplets by Chitosan Addition

The impact of a cationic polyelectrolyte on the pH sensitivity of the electrical charge and aggregation stability of protein-coated lipid droplets was examined. One percent (w/w) corn oil-in-water emulsions containing lipid droplets coated by β-lactoglobulin [0.05% (w/w) β-Lg, 10 mM acetate buffer, pH 3] were prepared in the absence (“primary” emulsions) and presence (“secondary” emulsions) of chitosan (0 to 0.05 wt%). The pH (3 to 8) of these emulsions was adjusted, and the particle charge, particle size, creaming stability, and microstructure were measured. Chitosan adsorbed to the β-Lg-coated droplets from pH 4.5 to 7.5, which was attributed to electrostatic attraction between the cationic polyelectrolyte and anionic patches on the droplet surfaces. Droplets coated by β-Lg–chitosan had better stability to flocculation than those coated by β-Lg alone around the isoelectric point of the adsorbed proteins (pH 4.5 to 5.5), which was attributed to increased electrostatic and steric repulsion between the droplets. We have shown that chitosan may be used to modulate the electrical characteristics and stability of protein-coated lipid droplets, which may be useful in the design and formation of delivery systems for use in the food, pharmaceutical, and other industries.     

Microorganism viability influences internal phase droplet size changes during storage in water-in-oil emulsions

Water-in-oil emulsions provide an alternative for long-term stabilization of microorganisms. Maintaining physical stability of the emulsion and cell viability is critical for large-scale application. Water-in-oil (W/O) emulsions were prepared with the biolarvacide Lagenidium giganteum and the green alga Chlorella vulgaris. Physical stability was measured via light scattering measurements of the internal phase droplets and cell viability was measured by plating and enumerating colony forming units. Emulsions were demonstrated to stabilize L. giganteum and C. vulgaris for more than 4 months without refrigeration. Introducing nutrients into the internal phase of W/O emulsions without cells had no significant effect on changes in aqueous phase droplet size dynamics. Internal phase droplet size changes that occurred over time were greater in the presence of cells. Increases in droplet size were correlated with cell death indicating measurement of internal phase droplet size changes may be an approach for monitoring declines in cell viability during storage.     

Factors Affecting Foamed Emulsions Prepared with an Extract from <Emphasis Type="Italic">Quillaja saponaria</Emphasis> Molina: Oil Droplet Size,pH and Presence of Beta-Lactoglobulin

Oil is well-known to act as antifoam and to destabilize foam lamellae by bridging between two adjacent foam bubbles. It was hypothesized that an optimal oil droplet size exists with respect to the stability of a foamed emulsions, where the oil droplets are sufficiently small to postpone bridging and the amount of free surfactant is sufficient to stabilize the oil/water-interface and the air/water-interface. Emulsions with 0.3% Quillaja saponin and a median oil drop-let size between 0.2 and 2.0 μm were prepared under varying homogenization conditions and characterized in a dynamic foam analyzer. Results confirmed the above mentioned hypothesis. Stability of the foamed emulsions considerably increased with increasing pH, which was attributed to electrostatic repulsion between oil droplets and the effect on the balance between disjoining pressure and capillary pressure. In a binary system containing proteins and saponins, stability of foamed emulsions can be further increased when emulsifiers are added sequentially. When the emulsion is stabilized by β-LG and QS is added after emulsification stability of the foamed emulsion is distinctly higher compared to systems, where QS and β-LG are added prior to emulsification. Future studies should deepen our understanding of these complex dispersed systems by investigating the molecular interactions including other proteins and additional food constituents.     

Phase Transition Stability of Cocoa Butter Emulsions

The impact of tempering-crystallization on microstructure and stability of water-in-cocoa butter (w/o) emulsions was analyzed using differential scanning calorimetry (DSC). The type and volume fraction of the disperse phase, and cooling rate during DSC analysis were systematically varied. Freshly prepared emulsions were additionally characterized by microscopy and laser diffraction. Fresh cocoa butter emulsions were composed of small and well dispersed droplets of an average size of 2.24 μm and 1.96 μm for water and 50 % sucrose solution as disperse phase, respectively. The thermograms revealed that the dissolved sugar lowered freezing and melting temperature and, dependent on volume fraction, the dispersion in the oil phase led to a change in solidification behavior. The temperature at the solidification peaks gives qualitative information about droplet size whereas width and number of exothermic events are related to particle size distribution (mono/polydispersity and mono/multimodality) and microstructure. Emulsions with water as dispersed phase show a clear shift of the freezing peaks of the disperse phase which points on modified emulsion microstructure because of droplet coalescence, which is more pronounced at higher volume fraction and lower cooling rate. Emulsions with sucrose solution as dispersed phase showed the greatest stability, wherein the volume fraction and the cooling rate does not matter. The results allow conclusions about the mechanisms of crystallization processes in cocoa butter emulsions resulting as network crystallization.     

Flow cytometry: a promising technique for the study of silicone oil-induced particulate formation in protein formulations

Subvisible particles in formulations intended for parenteral administration are of concern in the biopharmaceutical industry. However, monitoring and control of subvisible particulates can be complicated by formulation components, such as the silicone oil used for the lubrication of prefilled syringes, and it is difficult to differentiate microdroplets of silicone oil from particles formed by aggregated protein. In this study, we demonstrate the ability of flow cytometry to resolve mixtures comprising subvisible bovine serum albumin (BSA) aggregate particles and silicone oil emulsion droplets with adsorbed BSA. Flow cytometry was also used to investigate the effects of silicone oil emulsions on the stability of BSA, lysozyme, abatacept, and trastuzumab formulations containing surfactant, sodium chloride, or sucrose. To aid in particle characterization, the fluorescence detection capabilities of flow cytometry were exploited by staining silicone oil with BODIPY 493/503 and model proteins with Alexa Fluor 647. Flow cytometric analyses revealed that silicone oil emulsions induced the loss of soluble protein via protein adsorption onto the silicone oil droplet surface. The addition of surfactant prevented protein from adsorbing onto the surface of silicone oil droplets. There was minimal formation of homogeneous protein aggregates due to exposure to silicone oil droplets, although oil droplets with surface-adsorbed trastuzumab exhibited flocculation. The results of this study demonstrate the utility of flow cytometry as an analytical tool for monitoring the effects of subvisible silicone oil droplets on the stability of protein formulations.     

Antifreeze Emulsions Produced from Linoleic Acid and Water Using Enzymatically Modified Gelatin

An enzymatically modified gelatin with covalently attached leucine dodecyl ester, referred to as EMG-12, was used as a surfactant to prepare emulsions with different properties by changing the surfactant concentration, oil volume fraction, and pH in the water phase. The emulsions generally resisted the freezing of their constituent bulk water at approximately ?10°C, but similar emulsions produced with soy protein isolate, casein, or Tween-80 as control agents were less resistant. The freezing (or unfreezing) of the bulk water in these emulsions depended on the kind of agent used, not on the emulsion properties such as average area of the oil/water interface, stability against coalescence, and stability against creaming. The emulsion produced with EMG-12, like that produced with polyglycerol stearate, tended to maintain its unfrozen state even in the presence of silver iodide crystals added as heterogeneous ice-nuclei. The significance of producing such an antifreeze emulsion is discussed from the standpoint of cryopreservation of cold-sensitive food and biological systems.     

Effects of Lecithin Addition in Oil or Water Phase on the Stability of Emulsions Made with Whey Proteins

The effects of lecithin addition in oil or water phase on the stability of oil-in-water emulsions made with 0.1 wt% whey protein and 10 wt% n-tetradecane at neutral and acidic pH were studied by monitoring the gravitational creaming and phase separation. The effects of lecithin addition on the interfacial behavior of β-lactoglobulin were also studied to compare with the results of emulsion stability. At neutral pH, crude phosphatidylcholine (PC) from egg yolk or soybean increased the stability of the emulsion made with protein and lowered the interfacial tension of protein films more effectively than pure egg PC. A more remarkable effect on both the emulsion stability and the interfacial tension was found when crude PC was added in the oil phase rather than in the water phase. The purity of lecithins and the way to add them are suggested to be very important to make a stable emulsion with protein. On acidic pH (4.5 or 3.0), the increased creaming or phase separation in a whey protein-stabilized emulsion, but the lowered interfacial tension of β-lactoglobulin films, were found upon the addition of pure or crude PC in oil or water phase. These results suggest that in acidic pH, densely packed films may be formed on a planar oil–water interface, but not on adsorbed layers around oil droplets in an emulsion.     

Evaluation of Electrostatic Interaction between Lysolecithin and Chitosan in Two-Layer Tuna Oil Emulsions by Nuclear Magnetic Resonance (NMR) Spectroscopy

The main objective of this work was to investigate the electrostatic interaction between lysolecithin and chitosan in two-layer tuna oil-in-water emulsions using nuclear magnetic resonance (NMR) spectroscopy. The influence of chitosan concentration on the stability and properties of these emulsions was also evaluated. The 5 wt% tuna oil one-layer emulsion (lysolecithin-stabilized oil droplets without chitosan) and two-layer emulsions (lysolecithin-chitosan stabilized oil droplets) containing 5 wt% tuna oil, 1 wt% lysolecithin and various chitosan concentrations (0.025–0.40 wt%) were prepared. The one-dimensional (1D) ³¹P and ¹H NMR spectra of emulsions were then recorded at 25 °C. The results showed that addition of chitosan affected the stability and properties of lysolecithin-stabilized one-layer emulsions. The ³¹P NMR peak of the choline head group on lysolecithin molecules disappeared when chitosan was added at concentrations above neutralization concentration (> 0.05 wt%). The ¹H NMR peak intensity monitoring free amino groups (?NH ₃ ⁺) of chitosan showed a strong positive linear relationship to the chitosan concentration with a high correlation coefficient (R² ≈ 0.99). This ¹H NMR peak in emulsions could not be detected for chitosan in emulsions lower than saturation concentration (< 0.15 wt%). These phenomena indicate an electrostatic interaction between lysolecithin and chitosan at droplet surface in emulsion and were consistent with the results from zeta-potential measurements. The T ₂* relaxation time of the choline head group (N-(CH ₃)₃) signal of lysolecithin also confirmed that lysolecithin-chitosan electrostatic interaction occurs at the surface of oil droplets in two-layer emulsions. The results suggest that NMR spectroscopy can be used as an alternative method for monitoring the electrostatic interaction between surfactant and oppositely charged electrolytes or biopolymers in two-layer emulsions.     

Influence of processing factors on the stability of model mayonnaise with whole egg during long-term storage

Mayonnaise-like oil-in-water emulsions with different stabilities—evaluated from the degree of macroscopic defects, e.g., syneresis—were prepared by different formulations and processing conditions (egg yolk weight, homogenizer speed, and vegetable oil temperature). Emulsions prepared with lower egg yolk content were destabilized for shorter periods. The long-term stability of emulsions was weakly related to initial properties, e.g., oil droplet distribution and protein coverage at the interface. Protein aggregation between oil droplets was observed and would be responsible for the instability of emulsions exhibited by the appearance defects. SDS-PAGE results for adsorbed and unadsorbed proteins at the O/W interface suggested that predominant constituents adsorbed onto the interface were egg white proteins as compared with egg yolk components when the amount of added egg yolk was low. In present condition, egg white proteins adsorbed at the O/W interface could be a bridge of neighboring oil droplets thereby causing flocculation in emulsions.     

Influence of Ionic Surfactants on the Microstructure of Heat-Set β-Lactoglobulin-Stabilized Emulsion Gels

Using confocal microscopy, we studied the effect of heating (up to 85°C) on the microstructure of β-lactoglobulin-stabilized emulsions (20 vol% oil, pH 6.8) containing excess protein (total protein content 13.2%). Two different fluorescent dyes were used to separately visualize the oil droplets and the protein. In overlay micrographs, their location with respect to each other could then be determined. In the presence of a low salt concentration, flocculation of the emulsion without surfactant was inhibited, by a mechanism analogous to the “salting-in” of aqueous protein solutions. Addition of the anionic surfactant sodium dodecyl sulfate (SDS) caused weak flocculation, probably as a result of the formation of protein−SDS complexes. The final heat-set emulsion contained distinct pores for a surfactant/protein ratio of R = 1, but no pores for R = 2. Addition of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) caused strong aggregation, as indicated by microscopic observation of the concentrated emulsion and light scattering of the diluted emulsion. For R = 1 with CTAB, there were aggregates consisting of oil droplets and excess protein. At R = 2, almost all the excess protein was aggregated into separate protein flakes. In the final emulsion gels containing CTAB, the protein was more spread out. Differing structural behavior with anionic and cationic surfactants has been interpreted in terms of different protein−surfactant interactions in aqueous solution and at the oil−water interface, both before and after protein denaturation.     

Tracking of the kinetic stability of 2 types of total nutrient admixtures containing different lipid emulsions

The physical stability of 2 types of total nutrient admixtures was studied as a function of storage time and temperature. One of them contained only structured triglycerides and the other exclusively long-chain triglycerides as lipid components. To evaluate the possible changes in the kinetic stability of the emulsions and in the surface characteristics of the droplets during storage, particle size analysis, zeta potential, and dynamic surface tension measurements were performed. To follow any chemical decomposition processes that occurred during storage, the pH of the emulsions was also monitored. The mean droplet size of emulsions prepared with lipids containing exclusively long-chain triglycerides showed a remarkable increase after 4 days of storage, in contrast with that of the mixtures containing structured lipids. A combination of size distribution, zeta potential, and dynamic surface tension measurements proved to be useful for an adequate tracking of the kinetic stability of total nutrient admixtures. Structured triglycerides not only provide advantageous metabolic effects but improve the physical stability of total parenteral nutrition admixtures.     

In Vitro Characterization and Mosquito (Aedes aegypti) Repellent Activity of Essential-Oils-Loaded Nanoemulsions

The nanoemulsions composed of citronella oil, hairy basil oil, and vetiver oil with mean droplet sizes ranging from 150 to 220 nm were prepared and investigated both in vitro and in vivo. Larger emulsion droplets (195–220 nm) shifted toward a smaller size (150–160 nm) after high-pressure homogenization and resulted in higher release rate. We proposed that thin films obtained from the nanoemulsions with smaller droplet size would have higher integrity, thus increasing the vaporization of essential oils and subsequently prolonging the mosquito repellant activity. The release rates were fitted with Avrami’s equations and n values were in the same range of 0.6 to 1.0, implying that the release of encapsulated limonene was controlled by the diffusion mechanism from the emulsion droplet. By using high-pressure homogenization together with optimum concentrations of 5% (w/w) hairy basil oil, 5% (w/w) vetiver oil (5%), and 10% (w/w) citronella oil could improve physical stability and prolong mosquito protection time to 4.7 h due to the combination of these three essential oils as well as small droplet size of nanoemulsion.