Influence of interfacial mechanisms on the rheology of creaming emulsions

A set of protein-stabilised emulsions at pH 7.0, pH 6.0, and pH 5.0, and their counterpart surfactant emulsions, was designed with near-identical droplet size distribution and phase volume to study the specific contribution of hydrodynamic and pair potential interactions to the interfacial mechanisms of these emulsions systems. In this way, further the interfacial layer of these creaming emulsions to enhance perceived fat content could be manipulated. Creaming behaviour, surface shear, and bulk rheological measurements were performed. This work reflects the great importance of local pair potential in the formation of a highly viscoelastic interfacial film, which could be manipulated changing the surface charge of the protein to develop a well-packed cream layer in the protein-stabilised emulsions.     

Investigating the effect of surfactants on lipase interfacial behaviour in the presence of bile salts

It is known that non-ionic surfactants and phospholipids provide large protection in emulsions against lipase-induced destabilization as compared to proteins, even in the presence of bile salts. In relation to this, the aim of this study is to probe the ability of two surfactants of industrial interest, poloxamer Pluronic F68 (non-ionic) and Epikuron 145V (phospholipid), to modify the adsorption of lipases at an oil–water interface under the physiological conditions existing in the duodenum. We have designed an experimental procedure by means of a pendant drop film balance equipped with a subphase exchange technique, which allows sequential adsorption of the compounds. This allows the investigation of the interfacial behaviour of lipase in the presence and absence of surfactant. According to this experimental approach, the lipase is added directly into the subphase only after the surfactant has been adsorbed onto the oil–water interface. We have used interfacial dilatational and shear rheology techniques to characterise the interfacial layers. The results suggest that Pluronic F68 reduces the interfacial activity of lipase more efficiently than Epikuron 145V. Furthermore, it seems that Pluronic F68 affects the accessibility of the lipase to the oil–water interface, even in the presence of the bile salts. These results may have applications in the development of novel strategies to rationally control lipid digestion in the diet.     

Interfacial behaviour of crayfish protein isolate

The interfacial behaviour of a crayfish protein isolate (CFPI) produced as by-product from the food industry has been studied at the air–water surface at two pH values (2 and 8). An analysis of the surface pressure and interfacial shear behaviour has been carried out as a function of CFPI content for the two mentioned pH values. The results indicate that CFPI shows higher surface pressure, smaller aggregates and higher interfacial viscoelasticity at pH 8 than at pH 2, being highly dependent on CFPI content. In fact, a transition from fluid-like behaviour to a gel-like behaviour have been obtained by increasing the protein content. These results are in agreement with the higher solubility and z-potential values and the longer emulsion stability found under alkaline conditions. Furthermore, usage of highly sensitive interfacial shear techniques, such as the magnetic rod interfacial stress rheometer or the double wall ring, both of them providing rather coincident results, has proven to be highly useful to study the viscoelastic properties of CFPI interfacial films at levels that cannot be detected by other methods.     

Interfacial and emulsifying behaviour of crayfish protein isolate

The interfacial behaviour of adsorbed protein films constituted with a crayfish protein derivate that is typically produced as by-product from the food industry, has been studied at the air-water and oil-water interfaces. An analysis of the surface pressure under compression-expansion cycles of this protein was carried out as a function of time, concentration and pH (2 and 8). Besides, interfacial tension and adsorption kinetics also were determined as a function of time at different concentrations and pH values. Interfacial rheological properties were studied under dilatational deformations applied to a single droplet, either at the initial step of film formation or once the interfacial tension was at equilibrium and the film was completely formed. The contribution of the interfacial properties to the behaviour of oil-in-water emulsions stabilised with this protein derivative were also analysed. Finally, droplet size distributions obtained for concentrated emulsions stabilised by crayfish protein were analysed and related to the interfacial tension behaviour. We have demonstrated that crayfish proteins at pH 8 show higher solubility, smaller aggregates and better interfacial activity (higher surface pressure and lower interfacial tension) with higher interfacial viscoelasticity, than at pH 2. A two-dimensional model of the results showed that oil-water and air-water interfaces are clearly related to the improved stability of emulsion made with crayfish proteins at pH 8.     

Interfacial and emulsifying behaviour of rice protein concentrate

Interfacial and emulsifying properties of rice protein concentrate (RPC) have been studied in order to evaluate its potential application to stabilize O–W emulsions. The interfacial behaviour of adsorbed proteins films constituted with RPC has been studied at the air–water and oil–water interfaces at two pH values (2 and 8). The type and the amount of soluble proteins have been determined in aqueous dispersions and results put forward the presence of most frequent rice protein profile and a significant degree of protein denaturation with a very low solubility. Air–water and oil–water interfacial properties have been determined as a function of time, concentration and pH: air–water by surface pressure under compression–expansion cycles and oil–water by interfacial tension. Interfacial rheology has been studied under dilatational deformations, either at the initial step of film formation or once the interfacial tension was at equilibrium (the film was completely formed). RPC-stabilised O–W emulsions has been also analysed by Droplet Size Distribution (DSD) measurements and interfacial protein concentration. Both interfacial and bulk emulsion properties reveal that RPC showed an enhanced potential as emulsifier at low pH. Globally; results indicate clearly important differences in the structural characteristics of rice protein films between pH 2 and 8 that impact on emulsifying properties.     

Stabilization of foams and emulsions by mixtures of surface active food-grade particles and proteins

Surface active biopolymers such as proteins can form films with particularly high interfacial elasticities and viscosities and these molecules are widely exploited as foaming and emulsifying agents in foods. Solid particles of the correct size and wetting characteristics can also be extremely effective stabilizers of foams and emulsions, although the underlying mechanism of stabilization is somewhat different. Relatively little is known about what happens when both surface active polymers and surface active particles are present together. This work presents recent findings on the effects of mixtures of proteins plus novel food-compatible surface active particles. The proteins include caseins and whey proteins. The surface active particles prepared include cellulose + ethyl cellulose complexes, hydrophobically-modified starch granule particles and stable (non-spreading) protein-stabilized oil droplets. Interfacial shear rheology of adsorbed films was measured via a biconical bob apparatus and interfacial dilatational rheology was measured via a Langmuir trough type apparatus. The corresponding stability of bubbles to coalescence and disproportionation was assessed in separate experiments. Stability of oil-in-water emulsions was assessed via measurement of particle size distributions as function of time and visual assessment of the tendency to creaming and oiling off. In general, it is shown that the surface active particles on their own exhibit much lower measures of interfacial elasticity and viscosity than the proteins, but in combination with the proteins they appear to enhance the interfacial viscoelasticity considerably, with concomitant increases in bubble and emulsion droplet stability. There is little evidence of attractive interactions between the particles and the proteins, so a possible explanation of the increased stability is that the proteins increase the accumulation of particles at the interface, giving rise to increased jamming of particles at the interface.     

Behavior of mixed pea-whey protein at interfaces and in bulk oil-in-water emulsions

Due to the growing interest in creating mixed plant-dairy protein emulsion-based foods, it is important to understand how to include plant-dairy protein mixes in existing and new formulations. The aim of this work was to study the dynamics occurring at the oil-water interface in systems containing mixtures of pea and whey protein isolates. By using drop tensiometry combined with bulk-phase exchange, we compared the interfacial dynamics and viscoelastic properties of plant-dairy protein interfaces formed through simultaneous or sequential adsorption. Differences in the adsorption isotherm and interfacial behaviors suggest different interfacial structures for simultaneous and sequential addition. These results were supported by changes in the interfacial composition observed in bulk emulsions. Bulk emulsions prepared with simultaneous adsorption of whey and pea protein isolate showed the lowest creaming stability. The results show how different emulsion properties can be achieved using the same protein mixtures by fine tuning their interfacial composition to reach maximum product functionality.     

Rheology of siloxane-stabilized water in silicone emulsions

Using silicone copolymers in personal care products can improve the aesthetic performance of formulations. During their manufacture, distribution and topical application they are subject to various mechanical stresses. In this study rheology was used to measure their effects. A number of water in silicone (w/Si) emulsions were prepared in which the oil phase consisted of cyclomethicone. The surfactant used was a branched type silicone copolymer. Both viscoelastic and viscometry measurement were performed on model systems and on commercial products. Experimental data were obtained using a Bohlin rheometer. The measurements were taken applying shear rates in the range of 0.46-58 l s-1 and for the strain sweep frequencies of 0.1 Hz, 1 Hz and 10 Hz were applied. Oscillation tests were performed in the 0.1 Hz to 10 Hz range. All measurements were taken at 35deg;C, representing the approximate temperature encountered during topical application. The effect of surfactant concentration on viscoelastic properties was examined. It was shown that with increasing surfactant concentration the elastic moduli G' and the viscous moduli G" increased. Furthermore, the emulsions showed a transition from a predominantly elastic to a predominantly viscous response as the surfactant concentration increased. The effect of varying the water phase volume fraction on viscometry and viscoelastic measurements was also examined. With increasing water phase volume fraction the viscosity of the emulsions, as well as the yield stress, increased. The Cross and Sisko models were applied. From the Dougherty and Krieger equation phieff was calculated. It was found the the data derived from the Sisko model gave more reliable results. Results obtained from commercial samples showed a high proportion of elasticity; oscillation tests and viscometry experiments suggested that tumbling had the biggest impact on the theological profiles; viscosity, eta, shear stress, sigma, elastic module, G', and dynamic viscosity, eta', dropped to a minimum in these samples. Results from the two commercial samples were compared and it was observed that, although both were w/o emulsions, different rheological behaviour could be observed.     

Influence of pH on linear viscoelasticity and droplet size distribution of highly concentrated O/W crayfish flour-based emulsions

In the present work, the influence of pH on stability of oil-in-water (O/W; 75 wt%/25 wt%) emulsions stabilized by crayfish flour (CF) has been studied. CF (containing ca. 65 wt%) showed poor functionality in a wade range of pH, which inhibits formation of stable emulsions. An indirect procedure has been developed in order to obtain emulsion at a broad pH range, including isoelectric point (pI) value. The emulsions have been characterized by means of linear dynamic viscoelasticity and droplet size distribution (DSD) analysis. These emulsions present a behaviour characteristic of highly concentrated emulsions with a well-developed plateau region. The most unfavourable DSD and linear viscoelastic results correspond to the pI. A significant improvement takes place as the pH value departs from the pI. An exception for linear viscoelasticity results have been found at pH 10, which is close to the pI for rod segments of myofibrillar proteins. Stability of these emulsions has been studied by following the evolution of DSD and linear viscoelastic parameters along time. The poorer results in emulsion stability correspond to pH values close to the pI, at which the increase in droplet size or uniformity parameter as well as the decrease in the plateau modulus become more pronounced. Conversely, the stability significantly improves as the pH departs from pI.     

Tannic acid enhanced the emulsion stability,rheology and interface characteristics of Clanis Bilineata Tingtauica Mell protein stabilised oil-in-water emulsion

Clanis Bilineata Tingtauica Mell Protein (CBTMP) was one of the new natural insect protein resources. However, due to its strong surface hydrophobicity and poor emulsion stability, its application has greatly been limited. In this study, Pickering emulsions were prepared by CBTMP–Tannic acid (CBTMP–TA) particles and soybean oil (φ = 50%). The presence of TA significantly increased the zeta-potential (from −23.5 ± 0.73 to −29.23 ± 0.58 mV) and three-phase contact angle (from 129.9 ± 4.53° to 87.27 ± 3.20°) of complex, especially the concentration of TA was 0.4 mm . Besides, the stability of emulsions was significantly improved according to the results of the droplet size and static multiple light scattering. Furthermore, rheological properties including viscosity, thixotropy and creep recovery were significantly enhanced, which indicated the formation of a more stable network structure. Interfacial behaviour (Interfacial tension, interfacial film viscoelasticity and diffusion rate) of emulsions was significantly changed, forming a more stable interfacial layer as TA content increased. The CBTMP–TA complex was adsorbed at the interface and the adsorption rate was also mainly affected by the content of TA. The study indicated CBTMP–TA compositions to enhance stability, rheological behaviours and interfacial characteristics, and demonstrated the potential applications of insect protein resources in food emulsion systems.     

Delaying lipid digestion through steric surfactant Pluronic F68: A novel in vitro approach

The objective of this study was to investigate and compare the lipolysis rate of emulsions stabilized with two surfactants of a markedly different nature: an ionic mixture of phospholipids and the almost purely steric surfactant Pluronic F68. We have studied emulsions stabilized with only one of them, and also with mixtures with different proportions of these two emulsifiers. The impact on lipolysis of the addition of different amounts of β-casein, which is capable of forming thick interfacial layers, was also investigated. To carry out this study, we propose an alternative method to analyze the digestion of emulsions in complex media, which relies on the changes in backscattered light monitored in time by means of a Turbiscan.Our results show that the steric emulsifier is considerably more effective on delaying the digestion of the emulsions than the ionic one. Besides, it is possible to speed up or slow down the lipolysis by varying the proportion between steric and ionic emulsifiers, and therefore to influence the absorption of lipids and/or hydrophobic compounds dissolved on them. β-casein is not able to delay lipolysis under duodenal conditions, since it is easily cleaved by proteases and displaced by endogenous bile salts.     

Formation and stability of emulsions stabilised by Yucca saponin extract

Yucca saponin extract containing steroidal saponins was examined for its interfacial and emulsifying properties. For this purpose, we produced oil‐in‐water emulsions and tested their stability against environmental stresses (pH, ionic strength, heating and freeze‐thawing). Yucca saponin extract was highly surface‐active and formed negatively charged submicron‐sized emulsions at low surfactant‐to‐oil ratio. The emulsions were stable at pH 5–9 and showed only minor increase in droplet size when heated up to 90 °C. Our results indicate Yucca saponin extract as a new potential natural surfactant that may be used to replace synthetic surfactants in the food and beverage industry for selected applications.     

INFLUENCE OF GEL NETWORKS IN CONTROLLING CONSISTENCY OF O/W EMULSIONS STABILISED BY MIXED EMULSIFIERS

A coherent explanation is presented of the mechanisms by which mixed emulsifiers of the surfactant/fatty alcohol type both stabilise and control the consistencies of oil-in-water emulsions. Original and published data are used to develop a theory in which the semisolid and other flow properties of emulsions are related to the nature, strength and extent of viscoelastic networks present in the continuous phases. These networks form from the interaction of surfactant solution with fatty alcohol and have similar properties to ternary system gels formed when the mixed emulsifiers disperse in water, in the absence of an oil. The theory is discussed with particular reference to liquid paraffin-in-water emulsions prepared with the surfactants anionic sodium dodecyl sulphate, cationic alkyltrimethylammonium bromides or non-ionic cetomacrogol and fatty alcohols cetyl and stearyl and their mixtures, especially cetostearyl. It may be used also to explain the properties of emulsions prepared with a variety of oils, surfactants and amphiphiles. The effects of variation in storage time, surfactant type (ionic or non-ionic), temperature, mixed emulsifier concentration, and alcohol and surfactant chain lengths on the rheological and microscopical properties of ternary systems and emulsions support this theory. The importance of small strain experiments (creep and oscillatory) in providing a true measure of consistency is emphasized. Continuous shear experiments, although theoretically less satisfactory, provide useful additional information when correlated with the small strain data.     

蛋白界面膜及其评价方法研究进展

食品中乳化类产品的稳定性决定了其价值和顾客满意度,天然蛋白的乳化特性多年来受到食品胶体和界面科学领域广泛关注。蛋白吸附到水油界面形成的黏弹性薄膜具有降低界面张力、维持乳液稳定的特性。研究蛋白界面膜的物化性质有助于了解蛋白大分子在水油界面的吸附规律,能够用于评价、表征和预测蛋白乳液稳定性,并为优化蛋白乳液稳定性提供理论基础。基于此,该文对水包油乳液蛋白界面膜结构和影响因素进行探讨,系统综述了目前用于评价蛋白质界面特性的手段和方法,包括微观成像技术、热力学技术、光谱技术和界面流变学技术等,以研究蛋白类乳化液性能在宏观和微观层面的联系,为蛋白质界面膜在乳化食品中的应用提供参考。     

Effectiveness of encapsulating biopolymers to produce sub-micron emulsions by high energy emulsification techniques

In this study, different emulsifying ingredients were used to produce sub-micron emulsions for encapsulation purposes. Maltodextrin combined with a surface-active biopolymer (modified starch, or whey protein concentrate), or a small molecule surfactant (Tween 20) were used as the continuous phase, while d-limonene was the dispersed phase. Results showed that biopolymers are not efficient ingredients to produce very small emulsion droplets compared with small molecule surfactants because of their slow adsorption kinetics. The main problem with surfactants also is instability of the resulted emulsions due to “depletion and bridging flocculation” caused by free biopolymers and competition between surfactant and surface-active biopolymers. In general, it was not possible to produce a fairly stable microfluidized emulsion with surfactants for encapsulation purposes.     

Application and evaluation of mesquite gum and its fractions as interfacial film formers and emulsifiers of orange peel-oil

Mesquite gum was fractionated using hydrophobic interaction chromatography, yielding three fractions (F₁, F₂, F₃) whose average molecular masses ranged from 1.81 × 10⁴ to 5.23 × 10⁵ Da; F₁ had 90% polysaccharide and 1% protein contents, while F₂ and F₃ contained 16 and 46% of protein, respectively. Fractions' ability to form oil–water interfacial films and to stabilize orange peel–oil emulsions was evaluated. The highest interfacial viscosity (321 m Nm⁻¹) and highest instantaneous elastic modulus (E₀) = 0.113 × 10⁻⁴ m Nm⁻¹ were exhibited by F₂ and these values were significantly higher than those exhibited by the whole mesquite gum. F₁ did not exhibit viscoelastic properties. Emulsions made with F₂, F₃, and the whole mesquite gum had coalescence rates of the order of 10⁻⁸ s⁻¹, indicating that these emulsions were very stable. Nevertheless, emulsions made with F₂ were significantly more stable than those made with F₃ and whole mesquite gum, and emulsions made with F₁ broke after 1 day aging. These results indicate that there is a close correlation between emulsion stability, interfacial rheological properties, and an adequate relatively high protein/high polysaccharide balance in the fractions.     

Interfacial and emulsifying properties of sucrose ester in coconut milk emulsions in comparison with Tween

In this study, sucrose esters were presented as a promising alternative to petrochemically synthesized Tweens for application in coconut milk emulsions. The interfacial and emulsifier properties of sucrose ester (SE), mainly sucrose monostearate, had been investigated in comparison with Tween 60 (TW), an ethoxylate surfactant. The interfacial tension measurement showed that SE had a slightly better ability to lower the interfacial tension at coconut oil–water interface. These surfactants (0.25 wt%) were applied in coconut milk emulsions with 5 wt% fat content. The effects of changes in pH, salt concentration, and temperature on emulsion stability were analyzed from visual appearance, optical micrograph, droplet charges, particle size distributions, and creaming index. Oil droplets in both SE and TW coconut milk emulsions extensively flocculated at pH 4, or around the pI of the coconut proteins. Salt addition induced flocculation in both emulsions. The pH and salt dependence indicated polyelectrolyte nature of proteins, suggesting that the proteins on the surface of oil droplets were not completely displaced by either added nonionic SE or TW. TW coconut milk emulsions appeared to be thermally unstable with some coalesced oil drops after heating and some oil layers separated on top after freeze thawing. The change in temperature had much lesser influence on stability of SE coconut milk emulsions and, especially, it was found that SE emulsions were remarkably stable after the freeze thawing.     

Impact of surfactants on the lipase digestibility of gum arabic-stabilized O/W emulsions

The bioavailability of lipids from an emulsion can be controlled and regulated by the property of the stabilizing interfacial layer. Here we evaluate how low-molecular weight surfactants including hexadecyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Tween 80 (T80) influence the interfacial behavior of lipase and bile extract on the surface of lipid droplets stabilized with gum arabic (GA). The lipolysis behavior was influenced by surfactant type and concentration. The results showed that anionic SDS could completely displace GA from droplet surface. Cationic CTAB might either adsorb onto existing GA layers or displace GA, whereas non-ionic T80 could co-adsorb with GA on the interface. When the concentration of surfactants was much higher than the critical micelle concentration (CMC), all the surfactants would form a dense adsorption layer on the droplet interface to prevent lipase from the direct contact with lipids. A considerable amount of surfactant in the aqueous phase may also compete with the bile salt and lipase, thus leading to suppressed digestion of lipids. Ionic surfactants would denature the lipase resulting in reduced enzyme activity, and T80 micelles may interact with the lipase, hindering their adsorption onto the droplet interface as well. These results were confirmed both by the digestion model and interfacial techniques. The results provided guidance for the development of emulsion-based delivery systems for functional lipid foods.     

Influence of Droplet Charge on the Chemical Stability of Citral in Oil-in-Water Emulsions

Abstract: The chemical stability of citral, a flavor component widely used in beverage, food, and fragrance products, in oil-in-water emulsions stabilized by surfactants with different charge characteristics was investigated. Emulsions were prepared using cationic (lauryl alginate, LAE), non-ionic (polyoxyethylene (23) lauryl ether, Brij 35), and anionic (sodium dodecyl sulfate, SDS) surfactants at pH 3.5. The citral concentration decreased over time in all the emulsions, but the rate of decrease depended on surfactant type. After 7 d storage, the citral concentrations remaining in the emulsions were around 60% for LAE- or Brij 35-stabilized emulsions and 10% for SDS-stabilized emulsions. An increase in the local proton (H⁺) concentration around negatively charged droplet surfaces may account for the more rapid citral degradation observed in SDS-stabilized emulsions. A strong metal ion chelator (EDTA), which has previously been shown to be effective at increasing the oxidative stability of labile components, had no effect on citral stability in LAE- or Brij 35-stabilized emulsions, but it slightly decreased the initial rate of citral degradation in SDS-stabilized emulsions. These results suggest the surfactant type used to prepare emulsions should be controlled to improve the chemical stability of citral in emulsion systems.     

Structure and rheology of semisolid o/w creams containing cetyl alcohol/non-ionic surfactant mixed emulsifier and different polymers

Oil-in-water (o/w) emulsions for cosmetic use, such as lotions and creams, are complex multiple-phase systems, which may contain a number of interacting surfactants, fatty amphiphiles, polymers and other excipients. This study investigates the influence of two synthetic cationic polymers, Polyquaternium-7 and Polyquaternium-11, and the natural anionic polymer, gum of acacia, on the rheology and microstructure of creams prepared with a non-ionic mixed emulsifier (cetyl stearyl alcohol-12EO/cetyl alcohol) using rheology (continuous shear, and viscoelastic creep and oscillation), microscopy and differential scanning calorimetry (DSC). A control cream containing no polymer was also investigated. The semisolid control cream was structured by a swollen lamellar gel network phase formed from the interaction of cetyl alcohol and the POE surfactant, in excess of that required to stabilize oil droplets, with continuous phase water. Endothermic transitions between 25 and 100 degrees C were identified as components of this phase. Incorporation of cationic polymer into the formulation caused significant loss of structure to produce a mobile semisolid containing larger oil droplets. The microscopical and thermal data implied that the cationic polymer caused the swollen lamellar gel network phase to transform into non-swollen crystals of cetyl alcohol. In contrast, incorporation of gum of acacia produced a thicker cream than the control, with smaller droplet sizes and little evidence of the gel network. Microscopical and thermal data implied that although there were also interactions between gum of acacia and both the surfactant and the swollen gel network phase, the semisolid properties were probably because of the ability of the gum of acacia to stabilize and thicken the emulsion in the absence of the swollen lamellar network.