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.     

Direct quantification of protein partitioning in oil-in-water emulsion by front-face fluorescence: avoiding the need for centrifugation

The quantification of proteins adsorbed at the oil-in-water interface is often difficult since it requires separation of fat globules from the aqueous phase that may damage the fat globule size and/or modify the interfacial composition. Front-face fluorescence spectroscopy was used to characterize the protein partitioning between the aqueous and oil phases of emulsions without separating these two phases. Different emulsions based on skim milk powder (SMP), two mono- and di-glyceride (MDG) mixtures (saturated and partially unsaturated), and three fats (hydrogenated and refined coconut oils and refined palm oil) were studied. The impact of an ageing period (24 h at 4 °C) was also investigated to typify the first step of ice cream processing. The emulsions were characterized for protein partitioning, immediately following emulsification and after ageing, using the Bradford spectrophotometric method, applied to the aqueous phase recovered after emulsion centrifugation. In parallel, the emulsions were characterized by their tryptophan emission fluorescence spectra. The area of the peaks at 333 nm, of the fourth-derivative fluorescence spectra corresponding to the amount of proteins present in the aqueous phase of emulsions, was well correlated with the Bradford measurements (r² = 0.91). This amount was also calculated from the fluorescence calibration curve obtained with SMP in solution. In conclusion, front-face fluorescence spectroscopy appeared to be a powerful and simple technique allowing the quantification of different populations of protein in an emulsified system, i.e., in the aqueous phase and loaded at the fat globule interface.     

Spreading of partially crystallized oil droplets on an air/water interface

The influence of crystalline fat on the amount and rate of oil spreading out of emulsion droplets onto either a clean or a protein-covered air/water interface was measured for β-lactoglobulin stabilized emulsions prepared with either anhydrous milk fat or a blend of hydrogenated palm fat and sunflower oil. At a clean interface, liquid oil present in the emulsion droplets was observed to completely spread out of the droplets unimpeded by the presence of a fat crystal network. Further, the presence of a fat crystal network in the emulsion droplets had no effect on the rate of oil spreading out of the droplets. At a protein-covered interface, the spreading behavior of emulsion droplets containing crystalline fat was evaluated in terms of the value of the surface pressure (Π_AW) at the point of spreading; Π_AW at spreading was unaffected by the presence of crystalline fat. We conclude it is unlikely that the role of crystalline fat in stabilizing aerated emulsions such as whipped cream is to reduce oil spreading at the air/water interface. However, the temperature of the system did have an effect: spontaneous spreading of emulsion droplets at clean air/water interfaces occurred for systems measured at 5 °C, but not for those measured at 22 or 37 °C. Thus, temperature may play a more important role in the whipping process than commonly thought: the entering and spreading of emulsion droplets was favored at lower temperatures because the surface pressure exerted by protein adsorbed at the air/water interface was reduced. This effect may facilitate the whipping process.     

Effects of heat on physicochemical properties of whey protein-stabilised emulsions

The effect of heating has been studied for whey protein-stabilised oil-in-water emulsions (25.0% (w/w) soybean oil, 3.0% (w/w) whey protein isolate, pH 7.0). These emulsions were heated between 55 and 95 °C as a function of time and the effect on particle size distribution, adsorbed protein amount, protein conformation and rheological properties was determined. Heating the emulsions as a function of temperature for 25 min resulted in an increase of the mean diameter (d₃₂) and shear viscosity with a maximum at 75 °C. Heating of the emulsions at different temperatures as a function of time in all cases resulted in a curve with a maximum for d₃₂. A maximum increase of d₃₂ was observed after about 45 min at 75 °C and after 6–8 min at 90 °C. Similar trends were observed with viscosity measurements. Confocal scanning laser micrographs showed that after 8 min of heating at 90 °C large, loose aggregates of oil droplets were formed, while after 20 min of heating compact aggregates of two or three emulsion droplets remained. An increase of the adsorbed amount of protein was found with increasing heating temperature. Plateau values were reached after 10 min of heating at 75 °C and after 5 min of heating at 90 °C. Based on these results we concluded that in the whole process of aggregation of whey protein-stabilised emulsions an essential role is played by the non-adsorbed protein fraction, that the kinetics of the aggregation of whey protein-stabilised emulsions follow similar trends as those for heated whey protein solutions and that upon prolonged heating rearrangements take place leading to deaggregation of initially formed large, loose aggregates of emulsion droplets into smaller, more compact ones.     

Physical properties of diglycerol esters in relation to rheology and stability of protein-stabilised emulsions

The surface activity and lyotropic phase behaviour of concentrated diglycerol-esters of fatty acids with chain length of C14, C16, C18 and C18:1 (cis-oleic acid) are investigated. Diglycerol-esters show a much stronger reduction in the interfacial tension at a low concentration (0.01–0.1%) than corresponding monoglycerides. The diglycerol-esters form lamellar mesophases above their Krafft point, and no other types of mesophases are found in the temperature region examined (0–80°C). The lamellar phases show a limited swelling capacity, corresponding to a water layer thickness of ≈24 Å, which is found when the ratio of diglycerol-ester to water is 60:40, or lower. At high water concentrations (>90%) multi-lamellar liposomes are formed. The diglycerol-monooleate form lamellar phases in water in the temperature region from zero to 80°C. This is in strong contrast to the corresponding glycerol-monooleate, which forms cubic and reversed hexagonal mesophases in water. Oil in water emulsions are stabilised by diglycerol-esters by formation of liquid crystalline interfacial films around the oil droplets, which can be seen in polarised light microscopy. In presence of milk proteins in the aqueous phase the emulsion stability is depending on the protein to emulsifier ratio. At 40°C a mixed interfacial film of diglycerol-monooleate (DIGMO) and protein is present at the oil–water interface, but when cooled to 5°C, the proteins are displaced by DIGMO. This behaviour affects the stability and rheological properties of emulsions stored at low temperatures.     

Stabilisation of emulsions using hydrophobically modified inulin (polyfructose)

Oil-in-water (O/W) emulsions were prepared using a hydrophobically modified inulin surfactant, INUTEC^®SP1. The quality of the emulsions was evaluated using optical microscopy. Emulsions, prepared using INUTEC^®SP1 alone had large droplets, but this could be significantly reduced by addition of a cosurfactant to the oil phase, namely Span 20. The stability of the emulsions was investigated in water, in 0.5, 1.0 and 2 mol dm⁻³ NaCl as well as 0.5, 1.0, 1.5 and 2 mol dm⁻³ MgSO₄. All emulsions containing NaCl did not show any strong flocculation or coalescence up to 50 °C for almost 1 year storage. With MgSO₄ they were stable up to 50 °C and 1 mol dm⁻³. The stability of the emulsions against strong flocculation and coalescence could be attributed to the conformation of the polymeric surfactant at the O/W interface (multipoint attachment with several loops) and the strong hydration of the polyfructose chain in such high electrolyte concentrations. This was confirmed using cloud point measurements, which showed absence of any cloudiness up to 100 °C and at NaCl concentrations reaching 4 mol dm⁻³ and MgSO₄ reaching 1 mol dm⁻³. These high cloud points in electrolyte solutions could not be reached with polyethylene glycol. This clearly demonstrated the superiority of INUTEC^®SP1 surfactant as an emulsion stabiliser when compared with surfactants based on polyethylene glycol. Viscoelastic measurements showed a gradual increase in the storage modulus G′ with storage time both at room temperature and 50 °C. This was indicative of weak flocculation and absence of coalescence. The weak flocculation of the emulsions could be attributed to the presence of an energy minimum, G_min, in the energy–distance curve.     

Spreading of oil from protein stabilised emulsions at air/water interfaces

Spreading of a drop of an emulsion made with milk proteins on air/water interfaces was studied. From an unheated emulsion, all oil molecules could spread onto the air/water interface, indicating that the protein layers around the oil globules in the emulsion droplet were not coherent enough to withstand the forces involved in spreading. Heat treatment (90 °C) of emulsions made with whey protein concentrate (WPC) or skim milk powder reduced the spreadability, probably because polymerisation of whey protein at the oil/water interface increased the coherence of the protein layer. Heat treatment of emulsions made with WPC and monoglycerides did not reduce spreadability, presumably because the presence of the monoglycerides at the oil/water interface prevented a substantial increase of coherence of the protein layer. Heat treatment of caseinate-stabilised emulsions had no effect on the spreadability. If proteins were already present at the air/water interface, oil did not spread if the surface tension (γ) was <60 mN/m. We introduced a new method to measure the rate at which oil molecules spread from the oil globules in the emulsion droplet by monitoring changes in γ at various positions in a ‘trough’. The spreading rates observed for the various systems agree very well with the values predicted by the theory. Spreading from oil globules in a drop of emulsion was faster than spreading from a single oil drop, possibly due to the greater surface tension gradient between the oil globule and the air/water interface or to the increased oil surface area. Heat treatment of an emulsion made with WPC did not affect the spreading rate. The method was not suitable for measuring the spreading rate at interfaces where surface active material is already present, because changes in γ then were caused by compression of the interfacial layer rather than by the spreading oil.     

Milk protein interfacial layers and the relationship to emulsion stability and rheology

The properties of milk protein-stabilised, oil-in-water emulsions are determined by the structure and surface rheology of the adsorbed layer at the oil-water interface. Analysis of the segment density profiles normal to the surface show differences in the structure between adsorbed layers of disordered casein and globular whey protein. Systematic studies of stability and rheology of model oil-in-water emulsion systems made with milk proteins as sole emulsifiers give insight into the relation between adsorbed layer properties and bulk emulsion stability. Of particular importance are effects of pH, temperature, calcium ions and protein content. Colloidal interactions between adsorbed layers on different surfaces can be inferred from an analysis of dynamic collisions of protein-coated emulsion droplets in shear flow using the colloidal particle scattering technique. The role of competitive adsorption on emulsion properties can be derived from experiments on systems containing mixtures of milk proteins and small-molecule surfactants. Shear-induced destabilisation is especially influenced by the presence of fat crystals in the emulsion droplets. Aggregated gel network properties are dependent on the balance of weak and strong interparticle interactions. In heat-set whey protein emulsion gels, the rheological behaviour is especially sensitive to surfactant type and concentration. Rearrangements of transient caseinate-based emulsion gels can have a profound influence on the quiesent stability behaviour. Computer simulation provides a general link between particle interactions, microstructure and rheological properties.     

Emulsification using micro porous glass (MPG): surface behaviour of milk proteins

To investigate the emulsifying properties and adsorption behaviour of high molecular amphiphilic substances such as proteins, it is important to maintain the native status of the used samples. The new method of micro porous glass (MPG) emulsification could offer an opportunity to do this because of the low shear forces. The oil-in-water emulsions were produced by dispersing the hydrophobic phase (liquid butter fat or sunflower oil) through the MPG of different average pore diameters (d_p=0.2 or 0.5 μm) into the flowing continuous phase containing the milk proteins (from reconstituted skim milk and buttermilk). The emulsions were characterised by particle size distribution, creaming behaviour and protein adsorption at the hydrophobic phase. The particle size distribution of protein-stabilised MPG emulsions is determined by the pore size of MPG, the velocity of continuous phase (or wall shear stress σ_w) and the transmembrane pressure. A high velocity of =2 m s⁻¹ (σ_w=13.4 Pa) and low pressure (pressure of disperse phase slightly exceeded the critical pressure Δp_TM=4.5 bar of 0.2 μm-MPG) led to the smallest droplet diameter. As a consequence of average droplet diameters of d₄₃>3.5 μm creaming was observed without centrifugation in all MPG emulsions after 24 h, but no coalescence of the oil droplets occurred. The study of protein adsorption showed that the MPG emulsification at low shear forces resulted in lower protein load values (2.5±0.5 mg m⁻²) than pressure emulsification (11.5±1.0 mg m⁻²). In addition, the various emulsification conditions (MPG or pressure homogenization) led to differences in the relative proportions of casein fractions, whey proteins and milk fat globule membranes (MFGM) at the fat globule surfaces.     

The Composition of the Milk Fat Globule Surface Alters the Structural Characteristics of the Coagulum

The effects of the composition of a fat globule surface in reconstituted milks on the properties of rennet-induced coagulums were studied by rheological measurements and by front-face fluorescence spectroscopy in combination with a multivariate statistical method to investigate, at a molecular level, the evolution of the structure during the milk coagulation process. Reconstituted milks used in this study were prepared from different fat-in-water emulsions stabilized by whole skim-milk proteins, beta-casein, or beta-lactoglobulin. Coagulation of milk reconstituted with natural fat globules was also investigated. The study showed that the fat droplet/water interface influences the rheological properties (G' modulus) of the reconstituted milks during the coagulation process. The tryptophan fluorescence emission spectra of proteins were recorded during the kinetics of coagulation. The results of the principal component analysis performed on the spectral data showed a discrimination in the different systems investigated. It was shown that the fluorescence properties of protein tryptophans and, consequently, the structures of the protein networks were different for the investigated systems. The development of fluorescence transfer between protein tryptophans and fat-globule vitamin A during the coagulation kinetics agreed with the interactions between the protein network and fat globules. Copyright 2001 Academic Press.     

Characterisation of spray-dried emulsions with mixed fat phases

Fat encapsulation in spray-dried protein-stabilised emulsions is known to depend on the choice of protein, the emulsion droplet size, and the melting point of the fat. However, the fat encapsulation may also depend on the fat crystal habit. Fats may crystallise in three different forms , β′ and β, of which the β-form is thermodynamically stable. The -form is obtained in rapidly cooled fats, and it can then transform into the β′-form during storage, and this crystal form is finally transformed into the β-form. In order to investigate the effect of different fat phases on the spray-dried emulsions, two solid fats were studied: fully hardened rapeseed oil (β-stable) and fully hardened palm oil (β′-stable). The solid fats were used on their own or in mixtures with rapeseed oil, in order to provide fat phases with different properties. The emulsion composition was chosen as to mimic the composition of whole milk, i.e. 40% lactose, 30% sodium caseinate and 30% fat on a dry weight basis. The dried powders were stored under dry conditions at 4 or 37 °C in order to investigate the changes in the fat crystals and surface composition of the powders with time. The surface composition was analysed using electron spectroscopy for chemical analysis. Evaluation of the data showed that surface coverage of fat varied depending on the composition of the fat phase. The ratio of lactose to protein remained constant, which implies that the fat was present as ‘islands’ on a surface composed of lactose and protein. The hardened palm oil crystallised initially in the - or β′-form (depending on the ratio of hardened fat to oil), and during storage, the crystal form gradually changed into the β′-form. In powders containing hardened rapeseed oil only the stable β-form was found, even in fresh samples. The surface coverage of fat was reduced after storage, whereas the ratio of lactose to protein at the surface remained unchanged. The emulsion droplet size in emulsions prepared at a low homogenisation pressure was considerably increased after spray-drying and reconstitution, whilst the emulsion droplet size was well preserved in emulsions prepared at high homogenisation pressure.     

Effects of milk protein type and pre-heating on physical stability of whipped and frozen emulsions

Various milk protein mixtures were used to manufacture complex emulsions differing by their total protein concentration (1, 2.25 and 3.5%) and by their weight proportion of casein (from 0 to 75%) or whey proteins (WP) (containing from 10 to 80% β-lactoglobulin). Beside those changes in protein concentration and composition, impact of milk protein heat treatment, which was applied before emulsification, was also investigated. The resulting structuration effects on the corresponding emulsions were determined through changes in the particle size distribution and in the amount of adsorbed protein at the fat globule surface. Furthermore, fat destabilisation under a whipping and freezing steps was underlined as functions of the processing parameters (protein concentration and composition, and application or not of an additional heat treatment), and it was discussed in terms of the resulting amount of displaced protein from the oil–water interface.     

Effect of the type of the oil phase on stability of highly concentrated water-in-oil emulsions

The water-in-oil high internal phase emulsions were the subject of the study. The emulsions consisted of a super-cooled aqueous solution of inorganic salt as a dispersed phase and industrial grade oil as a continuous phase. The influence of the industrial grade oil type on a water-in-oil high internal phase emulsion stability was investigated. The stability of emulsions was considered in terms of the crystallization of the dispersed phase droplets (that are super-cooled aqueous salt solution) during ageing. The oils were divided into groups: one that highlighted the effect of oil/aqueous phase interfacial tension and another that investigated the effect of oil viscosity on the emulsion rheological properties and shelf-life. For a given set of experimental conditions the influence of oil viscosity for the emulsion stability as well as the oil/aqueous interfacial tension plays an important role. Within the frames of our experiment it was found that there are oil types characterized by optimal parameters: oil/aqueous phase interfacial tension being in the region of 19–24 mN/m and viscosity close to 3 mPa s; such oils produced the most stable high internal phase emulsions. It was assumed that the oil with optimal parameters kept the critical micelle concentration and surfactant diffusion rate at optimal levels allowing the formation of a strong emulsifier layer at the interface and at the same time creating enough emulsifier micelles in the inter-droplet layer to prevent the droplet crystallization.     

Effect of surfactant sucrose ester on physical properties of dairy whipped emulsions in relation to those of O/W interfacial layers

Dairy foams were manufactured on a pilot plant with various sucrose ester contents. Oil-in-water emulsions were produced by high-pressure homogenisation of anhydrous milk fat (20 wt%) with an aqueous phase containing skim milk powder (6.5 wt%), sucrose (15 wt%), hydrocolloids (2 wt%), and sucrose esters. Sucrose ester content was varied from 0 to 0.35 wt%. Firmness and stability of dairy foams were determined. The fraction of protein associated with emulsion fat droplets and the compression isotherms of those droplets were determined as a function of sucrose ester content. With less than 0.1 wt% sucrose ester, no foam could be produced. The most firm and stable foams were obtained with ca. 0.1 wt% sucrose ester. The fraction of protein associated with emulsion droplets suddenly falls from 60% at a sucrose ester content lower than 0.1125% down to ca. 10-20% for higher surfactant content. Compression isotherms of emulsion droplets at the air-water interface show that, in the presence of surfactant, emulsion droplets disrupt and spread at the interface whilst without surfactant they become dispersed. This means that the presence of sucrose ester causes some destabilisation of fat droplet interfacial layers. There is hence an optimal sucrose ester content that allows some destabilisation of the oil-water interface without concomitant protein displacement from that interface. Consequently, with the recipe and manufacturing process used to produce dairy foams, there exists a compromise in sucrose ester content with regards to manufacture and shelf-life of dairy foams.     

Silicone-protein surfactants; stability of water-in-silicone oil emulsions

Small scale water-in-silicone oil emulsions were readily prepared using high speed mixers. Two surfactant systems were studied: a comb-type silicone-polyether surfactant, and a surfactant system employing a mixture of the surface active protein human serum albumin (HSA, in the internal phase) and an alkoxysilane-modified silicone TES-PDMS in the silicone oil (continuous phase). Little difference in particle sizes was noted between the two surfactant types for a given mixing protocol, but dual-blade turbulent mixing led to relatively monodisperse particles of approximately 2–5 m in diameter while high speed Dremel mixers led to bimodal particle distributions. Prior to spontaneous demulsification of the latter emulsions stabilized by HSA/TES-PDMS (the 3225C emulsions remain stable), they proved very difficult to break. The addition of dibutyltin dilaurate to the HSA/TES-PDMS-stabilized emulsions led to catastrophic collapse of the emulsion and formation of a silicone elastomer at the bulk water/oil interface. This makes unlikely the possibility that silicone elastomers, formed by protein-catalyzed crosslinking of the alkoxysilane in albumin/TES-PDMS-stabilized emulsions, are involved in stabilizing the emulsion. The nature of the stabilization of the interface is discussed.     

The heat stability of milk protein-stabilized oil-in-water emulsions: A review

The knowledge on the factors affecting the heat-induced physicochemical changes of milk proteins and milk protein stabilized oil-in-water emulsions has been advanced for the last decade. Most of the studies have emphasized on the understanding of how milk-protein-stabilized droplets and the non-adsorbed proteins determine the physicochemical and rheological properties of protein-concentrated dairy colloids. The physical stability of concentrated protein-stabilized emulsions (i.e., against creaming or phase separation/gelation after heat treatment) can be modulated by carefully controlling the colloidal properties of the protein-stabilized droplets and the non-adsorbed proteins in the aqueous phase. This article focusses on the review of the physical stability of concentrated milk protein-stabilized oil-in-water emulsions as influenced by physicochemical factors, interparticle interactions (i.e., protein–protein, and droplet–droplet interactions) and processing conditions. Emphasis has been given to the recent advances in the formation, structure and physical stability of oil-in-water emulsions prepared with all types of milk proteins, reviewing in particular the impact of pre- and post-homogenization heat treatments. In addition, the importance of common components found in the continuous phase of heat-treated nutritional emulsions that can promote aggregation (polymers, sugars, minerals) will be highlighted. Finally, the routes of manipulating the steric stabilization of these emulsions to control heat-induced aggregation—through protein–surfactant, protein–protein, protein–polysaccharide interactions and through the incorporation of protein based colloidal particles—are reviewed.     

Microstructure of beta-lactoglobulin-stabilized emulsions containing non-ionic surfactant and excess free protein: influence of heating

The influence of the non-ionic surfactant Tween 20 on the microstructure of beta-lactoglobulin-stabilized emulsions with substantial excess free protein present was investigated via confocal microscopy. The separate distributions of oil droplets and protein were determined using two different fluorescent dyes. In the emulsion at ambient temperature the excess protein and protein-coated oil droplets were associated together in a reversibly flocculated state. The pore-size distribution of the initial flocculated emulsion was found to depend on the surfactant/protein ratio R, and at higher values of R the system became more inhomogeneous due to areas of local phase separation. Evidence for competitive displacement of protein from the oil-water interface by surfactant was obtained only on heating (from 25 to 85 degrees C) during the process of formation of a heat-set emulsion gel. By measuring fluorescence intensities of the protein dye inside and outside of the oil-droplet-rich areas, we have been able to quantify the evolving protein distribution during the thermal processing. The results are discussed in relation to previous work on the competitive adsorption of proteins and surfactants in emulsions and the effect of emulsion droplets on the rheology of heat-set protein gels.     

Phase transformation on heating of an aged cement paste

The standard cement paste (C-43-St) was studied previously by static heating, SH, immediately after 1 month hydration at w/c = 0.4 [J. Therm. Anal. Calorim. 69 (2002) 187]. This paste after 5-year ageing (unprotected from contact with air) was subject to thermal analysis in air and in argon (DTA, DTG and TG), to XRD at various temperatures, T, in a high temperature chamber, to mass spectroscopy (MS) and to IR spectroscopy. The aim of this study was to compare the results of SH (fresh paste) and of TG (the aged one), to verify the assumptions made on SH interpretation and to check the change in hydration products with ageing as measured by phase transformation on heating (ΔM versus the final mass). The sorbed water (EV), escaping at 110 °C from the fresh paste, was bound on ageing with a higher energy and escaped at higher temperatures. The joint water content of hydrates and of C-S-H gel increased on ageing by 1–2% in the dense paste C-43-St and did not change in the less compact one C-43-I. C-S-H gel transformed on heating above 600 °C into C₂S and C₃S. Portlandite content did not change on ageing. In the air atmosphere it became partly carbonated, which was accompanied by an increase in mass between 500 and 600 °C. Carbon dioxide and/or carbonate ions to form carbonates, were sorbed during ageing and were present in the aged paste in some form undetectable by XRD (amorphous or crypto-crystalline). Sensitivity to carbonation ΔM(700–800 °C) increased highly with ageing.     

Interface composition of multiple emulsions: rheology as a probe

We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.     

Thermally induced gelling of oil-in-water emulsions comprising partially crystallized droplets: the impact of interfacial crystals

We produced triglyceride-in-water emulsions comprising partially crystallized droplets, stabilized by a mixture of protein and low molecular weight surfactant. The emulsions were emulsified in the melted state of the oil phase and stored at low temperature (4 degrees C) right after fabrication to induce oil crystallization. The systems were then warmed to room temperature for a short period of time and cooled again to 4 degrees C. Owing to this treatment referred to as temperature cycling or "tempering", the initially fluid emulsions turned into hard gels. We followed the bulk rheological properties of the materials during and after tempering. The storage modulus, G', exhibited a dramatic increase when tempering was applied. We showed that the systems evolved following two distinct regimes that depend on the average droplet size and on the surfactant-to-protein molar ratio. Gelling may involve partial coalescence of the droplets, i.e., film rupturing with no further shape relaxation because of the solid nature of the droplets. Alternatively, gelling may occur without film rupturing, and is reminiscent of a jamming transition induced by surface roughness. We discussed the origin of these two mechanisms in terms of the properties (size and protuberance) of the interfacial oil crystals.