Complex interfaces in food: Structure and mechanical properties

Trends in Food Science & Technology 37 (2014) 59e71

Review

Complex interfaces in food: Structure and mechanical properties Leonard M.C. Sagis* and Elke Scholten Food Physics Group, Department of Agrotechnology and Food Sciences, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands (Tel.: D31 317 485023; e-mails: [email protected], [email protected]) Multiphase food systems (emulsions, foam) often have interfaces with a complex microstructure, formed by interfacial self-assembly of proteins, lipids, or colloidal particles. The response of these interfaces to deformations tends to be highly nonlinear and far more complex than the response of interfaces stabilized by simple low molecular weight surfactants. In this review we present an overview of various types of complex interfaces encountered in food products, and discuss their microstructure and mechanical properties. We also discuss how to properly characterize the nonlinear behavior of these interfaces, using surface rheological techniques, droplet deformation studies, and structural characterization methods.

Introduction Many food products are (or at least partially consist of) multiphase systems with a complex microstructure. Examples are emulsion based products such as salad dressings, sandwich spreads, or coffee creamers, or foam based (or foam containing) products such as mousses, bread, or beer. Many of these systems have interfaces stabilized by (mixtures of) proteins (and other surface active components), and these may form highly elastic adsorption layers with either gel-like or glass-like behavior. Functional foods

* Corresponding author. 0924-2244/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tifs.2014.02.009

such as sports drinks may contain nano- or microcapsules for encapsulation and controlled delivery purposes, and the shells of these capsules generally also have complex microstructures, consisting for example of mixtures of proteins and polysaccharides, or colloidal particles. The interfaces in these examples tend to have mechanical properties (surface shear and dilatational moduli, bending rigidities) significantly higher than interfaces stabilized by simple low molecular weight surfactants. The impact of the interfacial properties on stability and macroscopic dynamic behavior of products with complex interfaces is therefore very important, and often even dominates this behavior. For a targeted design and effective innovation of complex multiphase food products, a proper understanding of their dynamic behavior is essential, and for understanding this behavior, detailed knowledge of their interfacial properties is a fundamental requirement. Characterizing the microstructure and mechanical properties of interfaces is complicated by the fact that interfacial regions are in general very thin, of the order of one to at most a few tens of nanometers. We will briefly review some of the most commonly used methods to characterize interfacial structure at the end of this paper. Accurate methods to probe the mechanical properties of interfaces are currently available (see below). But most studies on interfacial rheology focus on the small deformation limit, where the relation between applied deformation and surface stress is still linear (Sagis, 2011). Most complex interfaces found in food systems have a highly nonlinear stress response to deformations, even at very small deformations (0.1e1%). Particularly in dilatational rheological measurements using bubble pressure or profile analysis methods, a linear response regime often cannot even be observed. For food systems which during production or consumption are exposed to high deformation rates, a characterization of the large deformation limit is far more relevant to understand the behavior of these systems, than data in the small deformation limit. Most commercially available surface rheological tools analyze data sets with models appropriate for simple interfaces. Models appropriate for the analysis of data in the nonlinear regime are only scarcely available (Sagis, 2011). Recently theoretical frameworks based on nonequilibrium thermodynamics (NET) have shown to be a very promising tool to construct nonlinear constitutive models for complex interfaces A detailed discussion of the basic principles of some of these

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frameworks, and the results that can be obtained with them, can be found elsewhere (Sagis, 2011). Computational methods on both micro- and macroscopic scales can also provide important tools to explore the microstructure and properties of complex interfaces, and their effect on stability and overall dynamics of a product. Since the microstructure of interfaces is often difficult to assess using experimental techniques, microscopic simulations based on Monte Carlo and Nonequilibrium Molecular Dynamics (NEMD) schemes can provide important information on interfacial microstructure and evolution of that structure, which may be helpful in the interpretation of surface rheological experiments. On colloidal and macroscopic length scales, schemes like the lattice Boltzmann method can for example be used to simulate the behavior of single or even multiple droplets, with complex interfaces (Kr€ uger, Frijters, G€ unther, Kaoui, & Harting, 2013). Recent reviews on computational methods for multiphase systems were presented by Gross and Reusken (2013) and Kr€uger et al. (2013). We believe that an approach that integrates experimental surface rheological and structural characterization methods, with theoretical and computational modeling, is ultimately the most promising approach to construct structureefunction relationships for complex multiphase products, that can link the properties of its ingredients on molecular length scales, to the stability and dynamic behavior of the product on macroscopic length scales. But significant efforts in each of these fields are needed to reach this target, and at the end of this review we will identify some of the future research needs. This review considers only experimental methods to analyze complex interfaces, and is structured as follows: We first start with an overview of some of the most relevant types of complex interfaces encountered in food, focusing primarily on their microstructure and mechanical properties. We then discuss experimental techniques to characterize these microstructures and mechanical properties. We finalize with a discussion on future trends in this field. Complex interfaces in food In this review we will define an interface to be a simple interface, if the surface active species adsorbed at the interface do not self-assemble into microstructures, and the interface has liquid-like behavior. Such interfaces typically have very low surface shear and dilatational viscosities (<10
8 N s/m), and surface tension tends to be the only surface parameter that affects the macroscopic dynamics of products that contain these interfaces. In contrast, an interface is considered complex, if the materials that adsorb at the interface self-assemble into microstructures, which give the interface significant (visco) elasticity. For complex interfaces mechanical properties like the surface shear and dilatational moduli, and the bending rigidity have a significant effect on overall dynamics of a product. A wide range of microstructures can

be encountered in complex interfaces, and some of these are two-dimensional (2d) gels (interfaces stabilized by proteins, proteinepolysaccharide complexes), 2d glass phases (proteins, colloidal particles), 2d emulsions (mixtures of immiscible lipids or surface active polymers), or 2d liquid crystalline phases (protein fibrils). In this section we will review the most relevant types of interfaces encountered in multiphase food products, and discuss their mechanical properties. Interfaces stabilized by colloidal particles When interfaces in a system are stabilized by colloidal particles, we refer to it as a Pickering stabilized system (Pickering, 1907). So far this form of stabilization has been applied mainly in non-food emulsions and foam (stabilized with latex, silica, or clay particles), but recently also food-grade Pickering stabilized systems have been developed (Dickinson, 2010; 2012). These are stabilized with, for example, colloidal starch particles (Timgren, Rayner, Sj€o€o, & Dejmek, 2011), microcrystalline cellulose (Kargar, Fayazmanesh, Alavi, Spyropoulos, & Norton, 2012), spray-dried soy-protein particles (Paunov et al., 2007), zein particles (de Folter, van Ruijven, & Velikov, 2012), or solid lipid particles (Gupta & Rousseau, 2012). The propensity of colloidal particles to stabilize emulsions was discovered early in the 20th century, and since then a substantial body of work has been published on the microstructure of Pickering stabilized interfaces, and the stability of Pickering stabilized emulsions and foam (Binks, 2002). Particle stabilized emulsions and foam have a much higher stability, than similar systems stabilized by low molecular weight surfactants. The colloidal particles tend to adsorb almost irreversibly at the interfaces (Binks, 2002), especially if the contact angle of the particle with the oil/water or air/water interface is close to 90 . At high surface coverage the particles can form various types of microstructures at an interface, depending on the balance of attractive and repulsive interactions between them. When attractive interactions (usually of a capillary nature) dominate, the particles tend to form 2d particle gels. When particles interact mainly through excluded volume interactions, they may form 2d glass phases. At high (electrostatic) repulsive interactions between the particles, they show a tendency to form 2d crystalline structures. These types of structures tend to have surface shear and dilatational moduli much higher than those of simple interfaces, and this contributes to a higher resistance to coalescence (Binks, 2002). Studies on surface rheological properties of interfaces stabilized by mixtures of surfactants with either spherical (Masschaele, Fransaer, & Vermant, 2009; Ravera, Santini, Loglio, Ferrari, & Liggieri, 2006), or anisotropic particles (Madivala, Fransaer, & Vermant, 2009) confirm the increase in surface rheological properties, when particles adsorb at the interface, and also show that ellipsoidal and rod-like particles tend to increase the surface rheological properties more than spherical ones

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(Basavaraj, Fuller, Fransaer, & Vermant, 2006). The mechanical properties of interfaces with anisotropic particles tend to have a more complex dependence on surface loading than those of interfaces stabilized with spherical particles. This is related to the fact that adsorbed anisotropic particles have a more complex phase diagram. Apart from the phases mentioned above (2d gels, glasses, or crystalline phases), these particles can also form liquid crystalline phases, in which the length axes of all particles are aligned in the same direction, either parallel or perpendicular to the interface. The significant increase in stability colloidal particles can give an emulsion or foam, compared to more traditional food-grade emulsifiers, will likely lead to an increase of the use of Pickering stabilization in food products, particularly when more food-grade particles, either (modified) protein, polysaccharide or lipid based, suitable for this purpose, come onto the market. The characterization of the mechanical properties of interfaces stabilized by such particles is a nontrivial issue, particularly for the dilatational properties. We will address this issue below, in the section on tensiometry methods. Interfaces stabilized by proteins Proteins are widely used in the food industry to stabilize emulsions and foam, either in their native state, or in the form of aggregates. When proteins adsorb at an interface, they tend to form dense adsorption layers, with values for surface shear and dilatational moduli, much higher than those observed for simple interfaces. This increased mechanical strength of the interfaces is thought to decrease coalescence and disproportionation in emulsions and foam stabilized with proteins. Proteins may (at least partially) denature after adsorption at an interface, and the resulting microstructure of the interface is affected (besides by other factors, such as surface concentration, pH, and ionic strength of the aqueous sub phase) by the degree of unfolding. When proteins denature significantly, and attractive interactions between them dominate the repulsive interactions, 2d particle gels can be formed. When the interfacial denaturation exposes free thiol groups, covalent sulfur bonds can be formed between the proteins, which can impart significant elasticity to this 2d gel structure. Proteins which show only a minor degree of unfolding, and retain most of their globular structure after adsorption, are more likely to form 2d soft glass phases (Cicuta, Stancik, & Fuller, 2003). Hence they tend to show strain hardening upon compression, and strain thinning upon extension of the interface. But even in such systems covalent crosslinks can be created, either by post-adsorption heat treatment (Xu, Dickinson, & Murray, 2008), or enzymatic crosslinking with transglutaminase (Faergemand & Murray, 1998). When proteins are cross-linked in this way, dilatational and shear moduli of the interface often show a significant increase, and foam and emulsions treated

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in this way therefore tend to show increased stability. However, if the enzyme favors intra- over intermolecular bond formation, the structure of the protein can become even more compact, reinforcing the glass-like behavior, and even reducing the surface moduli (Ercili-Cura et al., 2012). Interfaces stabilized by native proteins typically have dilatational storage moduli in the range of 20e80 mN/m (Bos & van Vliet, 2001). Recent studies have shown that fibrillar aggregates of proteins impart higher surface rheological moduli to interfaces than native proteins, in the range of 60e140 mN/m for dilatational storage moduli, and 80e200 mN/m for surface shear storage moduli (Humblet-Hua, van der Linden, & Sagis, 2013; R€ uhs, Scheuble, Windhab, & Fischer, 2013). Jordens et al. have shown that interfaces stabilized by b-lactoglobulin fibrils, at sufficiently high surface density, show coexistence of isotropic regions, in which the (in-plane) fibril orientation is random, and nematic regions in which fibrils are aligned parallel to each other (Jordens, Isa, Usov, & Mezzenga, 2013). For increasing surface coverage the fraction of nematic regions increases. The higher moduli of interfaces stabilized by protein fibrils, compared to those of interfaces stabilized by native protein, suggest that fibrils may have potential as emulsifiers or foam stabilizers. Random protein aggregates have also been shown to give interfaces with increased surface rheological properties, compared to native protein, for example for ovalbumin (Kudryashova, Visser, & de Jongh, 2005), or heat treated soy protein (Wang et al., 2012). When the aggregation is irreversible, i.e. the aggregates do not disassemble after adsorption, interfacial layers tend to be much thicker than those formed by native proteins, which may explain the increased mechanical properties of the interface. Aggregation prior to adsorption may not always lead to increased stability. When aggregates grow too large, such that they start to precipitate from solution, they may no longer be able to adsorb at an interface. Interfaces stabilized by proteinepolysaccharide mixtures The mechanical properties of complex interfaces can also be altered by co-adsorption of proteins with polysaccharides. As most polysaccharides are not surface active by themselves, associative interactions between proteins and polysaccharides are required for adsorption. These interactions are often of a physical nature (attractive electrostatic interaction), to give proteinepolysaccharide complexes or coacervates. Complexes are formed in a limited pH range and for relatively low protein:polysaccharide ratios. Their size is relatively small (roughly 50e100 nm) and they are formed due to complexation of one polysaccharide with multiple proteins (Schmitt & Turgeon, 2011; Schmitt et al., 2005). Coacervates are found for higher protein:polysaccharide ratios, and gives much larger aggregates that may become insoluble, and separate out of solution (Schmitt & Turgeon, 2011). Their specific

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size and solubility are mainly dependent on the solvent conditions (pH, ionic strength) (Dickinson, 2007), and may lead to inhibition of interface adsorption. The co-adsorptions of proteins and polysaccharides can be accomplished by two different methods: (i) formation of complexes in a separate solution and subsequent adsorption at the interface during emulsification (also denoted as “mixed layers”), and (ii) formation of a secondary layer of polysaccharides onto a previously formed proteincoated interface (“bi-layer”). As the size of the complexes is much larger than that of single proteins, the resulting mechanical properties of the final interface are very different from conventional protein-stabilized interfaces. Complexes and coacervates can change their conformation and orientation at the interface, thereby increasing the number of segments directly adsorbed at the interface. Similar to protein aggregation, additional 2d gel formation of the polysaccharides can occur due to chemical crosslinking or physical interactions. Due to the larger interfacial thickness of the complexes (and coacervates), these interfaces show different interfacial elasticity and viscosity compared to interfaces stabilized by proteins only. In general, an increase in the dilatational elasticity is found, as for b-lactoglobulin/acacia gum complexes (1.5-fold) (Schmitt et al., 2005), sodium caseinate/sodium dextran sulfate, complexes (2-fold) (Jourdain, Schmitt, Leser, Murray, & Dickinson, 2009), and soy-protein isolate/high methoxyl pectin complexes (4-fold) (Piazza, Durr-Auster, Gigli, Windhab, & Ficsher, 2009). However, also examples exist where complexation leads to a decrease in dilatational elasticity, such as for blac/pullulan complexes (Ganzevles, Kosters, van Vliet, Cohen Stuart, & de Jongh, 2007), and for sodium caseinate/chitosan complexes no differences in dilatational properties were observed between protein and protein complex stabilized interfaces (Zinoviadou, Scholten, Moschakis, & Biliaderis, 2012). Often a gradual increase in the elasticity over time can be observed, indicating that rearrangements of the complexes take place, leading to more rigid interfacial layers. The protein:polysaccharide ratio (and accompanying charge density) determines to a large extent the degree of rearrangements that can take place. Due to electrostatic repulsion, highly charged complexes may lead to a much less denser packing than for neutral complexes, and as a result they can exhibit lower dilatational moduli due to higher compressibility (Ganzevles et al., 2007). The degree of rearrangement may also depend on the initial protein content. For high content of proteins, no conformational changes may occur due to jamming of the systems, compared to systems with lower protein content. As a result, a less ordered and less compact structure is formed that requires less pressure upon compression, and hence has a lower modulus (Ganzevles, Zinoviadou, van Vliet, Cohen Stuart, & de Jongh, 2006). The formation of a dense rigid layer of complexes was also confirmed by shear rheology, and yielding

(or fracture) of the dense network can even be observed, expressed as a maximum in the shear modulus. Ganzevles et al. have shown that the layers prepared with both protein and polysaccharides always exist out of two sub-layers: a dense primary layer existing mainly out of proteins with a size of roughly the size of the proteins (4 nm), and a second more diffuse layer, for which the density depends on the charge density of the complexes (Ganzevles, Fokkink, van Vliet, Cohen Stuart, & de Jongh, 2008). Results indicate that the proteins present in the complexes diffuse to the primary protein layer, thereby increasing its density, and may also influence the interfacial properties over time. Therefore, the method of the coadsorption has an influence on the structure of the interface and its concomitant rearrangements. When proteins are already present at the interface before the polysaccharides are added (bilayer formation), the primary layer is already dense and no protein diffusion will take place after the polysaccharides are adsorbed. The more gradual adsorption of the polysaccharides may be characterized by a slower increase of the interfacial properties, and by rearrangements of the polysaccharides at the surface to allow strong complexation of the secondary layer with the primary layer. When pre-formed complexes are used, the complexation of the secondary layer has already occurred, but rearrangements of the complexes may densify the primary layer over time. Although Ganzevles et al. (2006, 2008) did not see this in experiments within the first 20 h, Jourdain et al. (2009) show that the shear viscosity and elasticity of both the mixed and bilayer systems became similar after 2e3 days. Before this equilibrium situation was obtained, both preparation methods show different gradual changes in the interfacial properties, depending on the polymeric composition and the environmental conditions used. Interfaces stabilized by composite multilayers The electrostatically driven bilayer formation described in the previous section can also be used to create interfaces with a multilayer composite structure. By using two oppositely charged polyelectrolytes, with different degrees of flexibility, interfaces can be created with a structure similar to 3d synthetic fiber-reinforced composite materials. Using layer-by-layer (LbL) adsorption of semi-flexible protein fibrils, and flexible polysaccharides (or flexible proteinepolysaccharide complexes), on oil droplet templates, microcapsules with highly elastic shells can be created. The process is illustrated in Fig. 1. The first step in the process is the production of the initial template emulsion, with either the fibrils or the flexible proteinepolysaccharide complexes as the emulsifier. Subsequent layers are adsorbed on the template droplet using a sequence of centrifugation and redispersion steps. Examples of microcapsules produced in this manner are those produced from b-lactoglobulin fibrils and High Methoxy (HM) pectin (Sagis et al., 2008), ovalbumin fibrils and HM pectin

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(Humblet-Hua, Scheltens, van der Linden, & Sagis, 2011), and lysozyme fibrils and lysozyme/HM pectin complexes (Humblet-Hua, van der Linden, & Sagis, 2012). This type of multilayer composite microcapsules has also been produced using combinations of whey protein isolate, HM pectin, and spherical silica particles (Rossier-Miranda, Schro€en, & Boom, 2012). The mechanical strength of microcapsules with a fibril reinforced shells is superior to those with shells consisting purely of flexible polyelectrolytes. Transmission electron microscopy images show that when for the latter capsules the template particle is removed, either by dissolution (in case of solid particles) or freeze or critical point drying (in case of oil droplet templates), the remaining shells are unable to retain a spherical shape, and show a complete collapse when adsorbed at a solid surface. In contrast, with only as little as 4e8 layers, fibril reinforced composite shells remain spherical after removal of the template (Humblet-Hua et al. 2011, 2012; Sagis et al., 2008). The mechanical strength and shell permeability of the capsules can be controlled by varying the number of alternating layers, the fibril surface concentration, the length of the fibrils, and their rigidity. For example, Humblet-Hua et al. (2012) showed that shells composed of up to 5 layers of long and rigid lysozyme and HM/lysozyme complexes, were more brittle and less strong than shells composed of an equal number of layers of short and semi-flexible ovalbumin fibrils and ovalbumin/HM complexes. Only for shells with more than 5 layers the longer more rigid lysozyme fibrils produced capsules with mechanical strength superior to those produced with ovalbumin fibrils. The more flexible and shorter ovalbumin fibrils produce a shell structure which is more homogeneous, with fewer defects, and is also more stretchable. Only when the layer number is sufficiently high, the positive effect of the higher bending rigidity of the lysozyme on mechanical strength of the shell can be observed. Although the tuneable mechanical strength and permeability of fibril reinforced composite capsules make them promising candidates for use as delivery vehicles for function ingredients, their production is rather involved, and therefore expensive. Since profit margins on food products are generally low it is uncertain if such systems will find widespread application in food products. Alternatively, such systems could be used in pharmaceutical applications, where production volumes tend to be much smaller and profit margins tend to be much higher.

Fig. 1. Schematic representation of the assembly of fibril reinforced multicomposite interfaces, by the layer-by-layer deposition technique: (A) a template emulsion droplet is stabilized with a negatively charged proteinepolysaccharide complex; (B) a layer of positively charge protein fibrils is adsorbed on the primary layer by dispersing the primary emulsion in a solution of protein fibrils; (C) and (D) this process is repeated until a multilayer composite structure is obtained.

Interfaces stabilized by (mixtures of) lipids Lipids and lipid conjugated molecules are a widely used class of stabilizers in the food industry. Lipids adsorbed at an interface can have a rich two-dimensional phase behavior, with phases ranging from simple “gas” and “liquid” phases, to liquid condensed, and (liquid) crystalline phases (Kaganer, Mohwald, & Dutta, 1999). Most of the liquid-like phases tend to have low values for their

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surface rheological properties. They stabilize foam and emulsions through the GibbseMarangoni mechanism, rather than through increasing mechanical strength of the interface. When two incompatible lipid species are adsorbed at the interface, phase separation may occur, which can lead to the formation of a structure consisting of disklike patches of one of the species, dispersed in a twodimensional continuous phase of the other lipid. When the patches are solid, the structure obtained is a two-dimensional suspension; when they are in a liquid state, the structure is a two-dimensional emulsion. The latter can be stabilized by adding a third surface active component which adsorbs preferentially at the boundary between the patches and continuous phase. Such a component is often referred to as a lineactant, and is the one-dimensional equivalent of a surfactant (Bernardini, Stoyanov, Arnaudov, & Cohen Stuart, 2013). For these systems the interfacial rheology is strongly dependent on the area fraction of patches. At low area fractions the behavior will be viscous, with low values for the surface viscosities. At high area fractions, viscosities may increase significantly, and particularly in large amplitude dilatational deformations the interface may become jammed in the compression part of the cycle, and show glass-like behavior. In twodimensional emulsions the patches are deformable, and at high area fractions these could form the two-dimensional equivalent of a High Internal Phase Emulsion (HIPE). Two-dimensional phase separation is an important process in certain disease patterns, such as sickle cell anemia, which involve the formation of cholesterol rich patches in the cell membrane (Deuticke, 1968). In food systems it has mainly been studied for protein lipid and lipidelipid systems (see for example Rodrıguez Patino, Rodrıguez Ni~no, and Carrera Sanchez, 2003, and references therein). Although it has not been used widely yet in food, the process has potential for designing encapsulation systems with patterned surfaces, an application which is currently already used for the production of non-food nanoparticles with patterned surfaces (Jackson, Myerson, & Stellacci, 2004). Interfaces in water-in-water emulsions Far less studied than the interfaces we discussed above, are interfaces found in water-in-water emulsions. These interfaces arise spontaneously as a result of incompatibility of dissolved components in aqueous media, and separate two distinct aqueous bulk phases. This incompatibility can occur between different proteins, polysaccharides, or between proteins and polysaccharides, and already occurs at relatively low concentrations. As a result, both bulk phases exist roughly out of 95% of water with very similar properties (such as density). Unlike conventional interfaces, they do not have a well-defined thickness due to accumulation of surface active components, but rather have a diffuse nature. The interface is defined as the region in which the concentration of the two separated components differs from the

bulk phases, which may lead to thicknesses many times the size of the components. Especially for samples with a composition close to the critical point in the phase diagram, the bulk phases become so similar that the interfacial thickness increases towards infinity. The interfacial tension of these interfaces is reported to be very low (of the order of 1 mN/m), which is much smaller than those found for conventional oilewater or watereair interfaces (of the order of 10 mN/m). Creating these interfaces therefore does not require large energy input (they actually appear spontaneously due to density fluctuations in the mixture), but stabilization of these interfaces is very challenging. When both bulk phases are liquid, the phase separation process tends to proceed until complete demixing, thereby minimizing the interfacial area. Very recently, however, research has shown that the demixing process can be slowed down and emulsion stability can be increased by a Pickering mechanism due to accumulation of sub-micrometer colloidal particles, such as polystyrene latex and protein particles, at the waterewater interface (Firoozmand, Murray, & Dickinson, 2009; Nguyen, Nicolai, & Benyahia, 2013). A more common strategy to limit the demixing process is by gelling one of the phases, thereby halting the phase separation process, and specific morphologies of two co-existing phases can be “frozen” (Turgeon, Beaulieu, Schmitt, & Sanchez, 2003). This phase separation method can be used to create isotropic or anisotropic protein micro-particulates. The dynamics of the phase separation determines the final morphology, and is dependent on the interfacial tension, interfacial permeability, and for small length scales, also the bending rigidity of the interface, as discussed by Scholten et al. (Scholten, Sagis, & van der Linden, 2005; Scholten, Sagis, & van der Linden, 2006a). Characterization of interfacial structure and mechanical properties Bubble pressure and profile analysis based tensiometry Dilatational rheology probes the resistance of an interface against isotropic compression or extension. Hence the surface area of the interface is changed, while maintaining its shape. For simple interfaces, the main property determined in dilatational rheology is the surface dilatational modulus, defined as Ed ¼ A

vg vA

ð1Þ

where A is the surface area of the interface, and g is the surface tension. The dilatational modulus is the inverse of the isothermal compressibility of the interface, and basically the two-dimensional equivalent of the bulk modulus. It can be determined using tensiometry, employing either a Langmuir trough combined with a Wilhelmy plate, or droplet based tensiometry methods, such as bubble pressure or droplet profile analysis (Javadi et al., 2013).

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In Langmuir troughs the interface is deformed by moving the barriers of the trough in an oscillatory fashion. In this method the deformation of the interface is not purely isotropic, and as a result the response of the interface is not purely dilatational, and includes contributions from the surface shear properties (Sagis, 2011). For simple interfaces those contributions tend to be negligible, but for complex interfaces these contributions are often significant and must be accounted for. In droplet based tensiometry methods, a droplet is formed at the tip of a capillary, and the interface is deformed by pumping fluid in and out of the droplet in an oscillatory manner, either using a piezo element, or a motor driven syringe. In bubble pressure methods the surface tension is determined by measuring the pressure inside the droplet, and using the YoungeLaplace equation to calculate g (Javadi et al., 2013). In profile analysis tensiometry the surface tension is determined by capturing video images of the droplet profile, and fitting the YoungeLaplace equation to that profile (Javadi et al., 2013). For both techniques, the YoungeLaplace equation used is of the form Pi
Po ¼ 2Hg

ð2Þ

where Pi is the pressure inside the droplet, Po is the pressure outside the droplet, and H is the mean curvature of interface (equal to 1/R for a spherical interface with radius R). For sinusoidal area changes in the linear response regime, a sinusoidal response for g is obtained from equation (2), which can then be used to extract Ed using equation (1). This is typically done using a Fourier transform of the surface tension signal. Although for simple interfaces this approach gives reliable data for the dilatational modulus, for complex interfaces this analysis is often not adequate. For complex interfaces a generalized version of the YoungeLaplace equation must be used, which contains additional terms describing the effects of interfacial bending rigidity, in-plane viscous (or viscoelastic) stresses, and inertial and viscous stresses exerted on the interface by the adjoining bulk phases (Sagis, 2013a). It can be shown that when a complex interface is subjected to an oscillatory area deformation, and stresses exerted by the adjoining bulk phases are negligible, the effective surface tension extracted from such an experiment takes the form (Sagis, 2013b) geff ¼ g


kC0
trðss Þ R

ð3Þ

where k is the bending rigidity of the interface, C0 its spontaneous curvature, and ss is the surface stress tensor (tr(ss) denotes the trace of this tensor). To establish which of these terms contributes to the effective surface tension a protocol is needed which involves droplet radius, deformation amplitude, and deformation frequency variations. From the scaling behavior of the effective surface tension with these parameters the dominant contributions can be identified (Sagis, 2013b).

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When the microstructure of complex interfaces is affected significantly by the applied deformation, the response of the interface will be nonlinear, and the extracted effective surface tension signals will no longer be purely sinusoidal. Most commercially available droplet based tensiometry instruments have a lower limit for the deformation which can still be applied with sufficient accuracy, of around 1%. For many complex interfaces this is already beyond the linear response limit (Humblet-Hua et al., 2013; R€uhs et al., 2013). Instruments extract data for the dilatational moduli by Fourier transforming the surface tension signal, and then taking the intensity and phase of the first harmonic. Such first harmonic moduli are meaningless when the response is highly nonlinear. An alternative method to analyze nonlinear dilatational data is to plot the data in the form of a Lissajous plot of surface pressure versus deformation. Fig. 2 shows some typical examples of these plots for a linear purely elastic, a linear purely viscous, a linear viscoelastic, and a nonlinear viscoelastic response (on the vertical axis the surface pressure geg0 is plotted, and on the horizontal axis DA/A). The nonlinear response shows strain thinning in extension (upper part of the curve, from left to right), and strain hardening in compression (lower part of the curve, from right to left), and is a typical response we would expect from an interface with soft glass-like behavior. We see that this Lissajous plot is highly asymmetrical, a typical feature for interfaces with a complex microstructure (R€uhs et al., 2013; Van Kempen, Schols, van der Linden, & Sagis, 2013). From these Lissajous plots moduli at minimal and maximum deformation can be extracted, similarly to a scheme recently introduced by Ewoldt, Hosoi, and McKinley (2008), for the analysis of nonlinear shear rheology of complex bulk fluids. This procedure is illustrated in Fig. 2E and F, and for an asymmetrical response like the one presented here, up to four distinct dilatational moduli can be extracted: the modulus at minimum extension, EdEM (the slope of the extension part of the curve at DA/A ¼ 0), the modulus at large extension, EdEL, the modulus at minimum compression, EdCM, and the modulus at large compression, EdCL. One would perhaps expect that EdEM ¼ EdCM, since both of these are determined at DA/ A ¼ 0. But for viscoelastic interfaces the moduli depend on the history of deformation, and in general these two moduli will not be equal. It is easy to see (for example in Fig. 2B or C) that these four moduli reduce to one single value for linear responses. To characterize the degree of nonlinearity of the dilatational response two additional parameters can be defined: SE ¼

EdEL
EdEM EdEL

ð4Þ

SC ¼

EdCL
EdCM EdCL

ð5Þ

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Fig. 2. Lissajous plots of surface pressure geg0 (vertical axis) versus deformation DA/A (horizontal axis). (A) Linear purely elastic response; (B) linear purely viscous response; (C) linear viscoelastic response; (D) nonlinear viscoelastic response, with strain hardening in compression, and strain thinning in extension; (E) extraction of dilatational moduli at Minimum (M) and Large (L) extension; (F) determination of dilatational moduli at minimum and large compression.

These two parameters are equal to zero if the response is linear, <0 if the response is strain thinning, and >0 when the response is strain hardening. The four moduli presented here are a far more meaningful characterization of the dilatational properties of a complex interface than the first harmonic moduli typically extracted by most commercial tensiometers. Stain sweeps, a common type of test in bulk rheology, are still rarely performed in dilatational rheology studies (Sagis, 2011). For a proper characterization of complex interfaces this type of test should be a standard ingredient in any tensiometry protocol.

Surface shear rheology In surface shear deformations the shape of a surface element is changed, while the interfacial area is kept constant. The relevant mechanical properties determined with this technique are the surface shear storage modulus, the surface shear loss modulus, the surface loss tangent, and (a spectrum of) surface shear relaxation times. The most widely used methods currently in use to measure these are (in decreasing order of sensitivity) the bi-cone, the Double Wall Ring (DWR), and the magnetically driven needle rheometer (Fig. 3). Other types of surface shear rheometers are available (based on for example the Du No€uy ring method, or particle image velocimetry in channels) but are less frequently used (for a review see Slattery, Sagis, & Oh, 2007, Chapter 5).

In the bi-cone method the edge of a thin bi-conical disc is placed in the interface between oil and water, or air and water (Fig. 3A). A drawback of this geometry is that the disk has a substantial contact area with the adjoining bulk phases, so the measured torque signals may include a substantial bulk contribution, depending on the viscosity of these phases. This limits the usefulness of this geometry to interfaces with substantial elasticity, such as those stabilized by protein films, proteinepolysaccharide complexes, protein fibrils, or colloidal particles (Humblet-Hua et al., 2013; R€uhs et al., 2013). The DWR geometry (Fig. 3B) has a much smaller contact area with the adjoining bulk phases, and this somewhat alleviates the sensitivity issues of the bi-cone. The ring of the geometry is placed at the fluidefluid interface in a circular channel, formed by two concentric cylinders. For both the bi-cone and DWR a rather extensive calculation is needed to extract interfacial shear properties from the torque signals of the rheometers (Erni et al., 2003; Vandebril, Franck, Fuller, Moldenaers, & Vermant, 2010). By far the most sensitive surface shear rheometer currently available is the needle rheometer (Fig. 3C), in which a magnetized needle is placed at the interface of a Langmuir trough, and moved back and forth in an oscillatory fashion, driven by two magnetic coils (Brooks, Fuller, Frank, & Robertson, 1999). The lower detection limit of this device for surface viscosities is about two orders of magnitude lower than the lower limit of the bi-

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Fig. 3. Commonly used geometries for the determination of surface shear properties: (A) a bi-cone geometry; (B) a double wall ring geometry; and (C) a magnetically driven needle rheometer.

cone geometry. It is however quite involved to extract data from measurements with this device, and it is limited to relatively low applied shear rates (Reynaert, Brooks, Moldenaers, Vermant, & Fuller, 2008), which has so far limited its use. Droplet shape deformation-relaxation methods Methods to determine interfacial properties such as the pendant or sessile drop methods derive values for these

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properties from the balance between surface and gravitational forces. Although they are very valuable for most simple interfaces, they are not suitable for systems with very low interfacial tensions, as they are not sensitive enough. For interfaces with low interfacial tensions, techniques that rely on droplet deformation, either static or dynamic, are more often used. In these techniques a droplet is deformed in either shear or elongational flow, and the deformation of the droplet is determined as a function of the applied flow rate (static measurements) or time (dynamic measurements). There are various modes in which these measurements can be performed. For droplets in a shear flow, at high _ 0 =g (where he values of the Capillary number, Ca ¼ he gR is the viscosity of the external phase, g_ is the applied shear rate, R0 is the radius of the non-deformed droplet, and g is the surface tension) elongation of a larger single droplet results in rupture into smaller droplets. Although not often used, this dynamic breakup process can be used to deduce interfacial tensions (De Hoog & Lekkerkerker, 2001). For lower values of Ca, a steady-state droplet deformation is obtained which is usually expressed as a deformation parameter. In steady-state conditions, the interfacial forces are balanced by the viscous forces, from which the interfacial tension can be determined. An alternative method that is based on the deformation of a single droplet is deformation relaxation after cessation of flow. When droplets are subjected to a shear flow, the droplets deform into ellipsoidal shapes. Upon cessation of flow the droplets retract to a spherical shape, which is characterized by a relaxation time. This method relies on accurate measurement of the droplet shape in time, which requires optical microscopy and high precision flow cells. Deformation is obtained by placing a single droplet between two counter-rotating glass plates and applying a shear flow. For accurate analysis of the interfacial properties, the gap width between the plates needs to be roughly 5 times larger than the initial size of the droplet to avoid wall effects (Guido, 2011). Instead of using a shear flow, deformation can also be induced by centrifugal forces, which is used in the spinning drop technique. Although this technique is often used to deduce low interfacial tensions, contributions of other interfacial parameters, such as bending rigidity and interfacial permeability are often neglected. Scholten et al. (2005, 2006a), Scholten, Sagis, & van der Linden (2006b) and Scholten, Sprakel, Sagis, & van der Linden (2006) have shown that for waterewater interfaces, bending rigidity and especially the permeability play an important role, and for permeable interfaces the spinning drop technique is less suitable, as a steady-state deformation is never reached due to dissolution of the dispersed phase into the continuous phase (Scholten et al., 2006b). As air is also partly soluble in water, this method will also be less suitable to measure airewater interfaces. The method would be suitable for oilewater interfaces, but due to the more complicated experimental sample handling, methods such as the pendant drop are often more preferred.

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These authors also showed that when taking into account a second mechanism of shape relaxation (i.e. mass transfer through the interface), besides relaxation driven by interfacial forces, droplet relaxation measurements can be used to deduce the interfacial tension and the interfacial permeability simultaneously (Scholten et al., 2006; Sagis, 2008). Without taking into account permeability effects, the interfacial tension is overestimated. Therefore, the droplet relaxation method is preferred for interfaces that are characterized by a low interfacial tension and high permeability. Recently, deformation of droplets in confined geometries has also received increasing attention, due to the growing interest in microfluidics. In microfluidics the gap width between the walls becomes comparable to the size of the droplets, and wall effects start to enhance droplet deformation and stretching of the droplets (Guido & Preziosi, 2010). Due to the confinement and the high fluid velocity near the wall, formation of tips and tails are likely to occur, and shape transitions into bullet- and rocketshaped deformation have already been observed (Mulligan & Rothstein, 2011). The difference in curvature of the leading and trailing edge may lead to Marangoni stresses and surfactant accumulation. Already for simple surfactant-covered oilewater interfaces, this confinement leads to phenomena such as jet formation, formation of Haines jumps and eddies, snap-off, end-pinching, tip and tail streaming (ejection of tiny fragments) and crossstreaming (Guido & Preziosi, 2010). Some of these phenomena have also recently been observed for more complex interfaces covered by starch or silica particles (Desse, Mitchell, Wolf, & Budtova, 2011; Mulligan & Rothstein, 2011). Since particlecovered interfaces are more elastic/solid-like and displacement of particles from the interface is unlikely, droplet deformation is more resisted. Reduction of interfacial area due to relaxation drives particles into a jammed state and complex interfaces are therefore more prone to buckling and crumpling (Mulligan & Rothstein, 2011). When the droplets are still liquid like, particle rearrangements at the interface might lead to a phenomenon known as tanktreading, where the particles move around the interface following the shear flow. This has been observed for vesicles such as red blood cells (Oishi, Utsubo, Kinoshita, Fujii, & Oshima, 2012). When the droplet interfaces become solid-like, phenomena such as tumbling can also be seen (Subramaniam, Abkarian, Mahadevan, & Stone, 2006). Due to the complex nature of these interfaces, adequate descriptions of these phenomena are not available, and large discrepancies between theory and experiments still exist. Some progress has already been made by taking into account the viscoelastic nature of these interfaces and characterizing the systems with an elasto-capillary number, rather than a viscosity based capillary number. This parameter has been used successfully to describe breakup processes of

droplets in dispersions with enhanced viscoelastic stresses (Lee, Walker, & Anna, 2011), and to describe oscillating deformation phenomena observed for droplets stabilized with proteins (Erni, Fischer, & Windhab, 2007). Colloidal probe atomic force microscopy Microcapsules such as those produced with the aforementioned LbL technique, or those produced by precipitation or polymerization of macromolecules on a spherical template, may have interfaces with a substantial thickness, and the properties of these interfaces cannot easily be determined using surface shear and dilatational methods. As an alternative to these methods, colloidal probe atomic force microscopy can be used to examine their interfacial stress-deformation behavior. The technique is illustrated in Fig. 4, and can be considered a micro-compression experiment, in which a microcapsule is deformed using a hard colloidal probe, attached to the cantilever of an Atomic Force Microscope (AFM). The probe is displaced in the vertical direction (dz), at a fixed rate, and the force needed to generate this displacement (F ) is determined from the deflection of the cantilever (which has a known

Fig. 4. Schematic representation of a micro-compression experiment on a single microcapsule using colloidal probe atomic force microscopy. A probe particle attached to the cantilever of the AFM is used to deform the microcapsule. From the deflection of the cantilever (which has a known force constant) a forceedisplacement curve can be obtained.

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force constant). A typical forceedisplacement curve is displayed in Fig. 4B. When the microcapsule deforms affinely, the Young’s modulus of the interface can be calculated from the slope of the force curve at small displacements, using so-called shell models (Elsner et al., 2006). When the microcapsules display nonaffine deformations the extraction of mechanical shell parameters is more complicated. Buckling of the interface may significantly increase the contact area of the probe with the capsule shell. For such systems the AFM measurement needs to be combined with for example optical microscopy with microinterferometry capabilities (Dubreuil, Elsner, & Fery, 2003), or confocal laser microscopy, to resolve the shape of the deformed capsule. Osmotic pressure methods Micro-compression of microcapsules can also be achieved by inducing osmotic pressure differences between the interior and the outer area of the capsules. This technique leads to deformation of the entire capsule surface, and is therefore less sensitive to variations in surface properties. When the osmotic pressure of the surrounding phase is higher (by added polymers) than the inner phase of the microcapsule, water is extracted through the permeable shell of the capsule, and the capsule shrinks. At a certain critical pressure, sufficient shrinkage leads to shape changes from spherical to non-spherical geometries (concave cup-like), which can be visualized by confocal laser microscopy. This critical pressure for shape deformation depends on the elasticity of the capsule shell, the thickness of the shell and the size of the microcapsules. The elastic modulus of the shell can be deduced from the slope of the critical pressure versus the shell thickness or microcapsule size (Gao, Donath, Moya, Dudnik, & M€ ohwald, 2001). Structural characterization methods As we discussed earlier in this review, when complex interfaces are deformed, the applied deformation causes changes in their microstructure, which lead to nonlinear stress-deformation behavior. Linking this behavior to the actual structural changes in the interface is a difficult task, when only rheological data are available. Additional information can be obtained by combining surface rheological methods with structural characterizations, either of the equilibrium structure, but preferably obtained simultaneously with the rheological data, in the deformed state of the interface. The simultaneous determination of rheological properties and microstructure is often referred to as rheo-optics, a field which has not yet found wide application in food science (Van der Linden, Sagis, & Venema, 2003). Techniques commonly used to characterize interfacial structure are light microscopy (Basavaraj et al., 2006; Masschaele et al., 2009; Madivala et al., 2009), atomic force microscopy, transmission or scanning electron microscopy, confocal scanning laser microscopy, ellipsometry

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(Piazza et al., 2009; Grigoriev, Derkatch, Kr€agel, & Miller, 2007), brewster angle microscopy (Van der Linden et al., 2003; Xu et al., 2008), or (neutron or X-ray) reflectivity measurements (Kaganer et al., 1999). Characterization of the interface is generally performed in the equilibrium state of the interface, and not in a deformed state.

Future trends In this paper we have reviewed several types of microstructures observable in complex fluidefluid interfaces in food systems. We have discussed some of the dynamic properties of these interfaces, and reviewed experimental methods which can be used to explore the often highly nonlinear response of these interfaces to deformation, temperature, and concentration gradients. We believe that the most efficient approach to develop and characterize highly stable emulsions, foam, or encapsulation systems, is a multiscale approach, that closely integrates experimental, theoretical, and computational methods. In all of these fields further developments are needed before we can treat real food systems, but current developments in these fields are promising, and a comprehensive modeling of complex interfaces on all of their relevant length scales is within reach. In the experimental field there is a particular need for advances in structural evaluation methods, which can be combined with (preferably simultaneous) surface shear or dilatational measurements (2d rheo-optic). In the field of theoretical constitutive modeling a considerable effort is needed to generate new nonlinear constitutive models, capturing the link between time evolution of an interface’s microstructure and its rheological response. Currently still very few models of that nature are available (Sagis, 2011). Finally, in the field of multiphase computational methods there is a definite need for solvers which can handle the aforementioned nonlinear constitutive models, on arbitrarily shaped and arbitrarily deforming interfaces.

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