Structure, interface emulsions

P. Perrin and F. Lafuma Low Hydrophobically Modified Poly (Acryhc Acid) Stabilizing Macroemulsions Relationship Between Copolymer Structure and Emulsions Properties. J. Colloid Interface Sci. 197, 317 (1998). 

Figure 10. This series of photographs from a bottle test shows the emulsion separating over time as evidenced by the steady increase in the upper oil phase. The lower water phase contains most of the solids but does not change in volume significantly. The interface emulsion in between the oil and water steadily decreases (destabilizes) in volume and resolves into the oil and water phases. As shown in Figure J, these interface emulsions can have a complex morphology or structure.

Interactions between proteins and polysaccharides give rise to various textures in food. Protein-stabilized emulsions can be made more stable by the addition of a polysaccharide. A complex of whey protein isolate and carboxymethylcellulose was found to possess superior emulsifying properties compared to those of the protein alone (Girard et al., 2002). The structure of emulsion interfaces formed by complexes of proteins and carbohydrates can be manipulated by the conditions of the preparation. The sequence of the addition of the biopolymers can alter the interfacial composition of emulsions. The ability to alter interfacial structure of emulsions is a lever which can be used to tailor the delivery of food components and nutrients (Dickinson, 2008). Polysaccharides can be used to control protein adsorption at an air-water interface (Ganzevles et al., 2006). The interface of simultaneously adsorbed films (from mixtures of proteins and polysaccharides) and sequentially adsorbed films (where the protein layer is adsorbed prior to addition of the polysaccharide) are different. The presence of the polysaccharide at the start of the adsorption process hinders the formation of a dense primary interfacial layer (Ganzelves et al., 2008). These observations demonstrate how the order of addition of components can influence interfacial structure. This has implications for foaming and emulsifying applications. 

A growing use of nanostructured emulsions is the delivery of bioactive lyophilized components (McClements et ah, 2007). The size of the emulsion droplets and the nature of the interface have a significant impact on the properties of the emulsions. For example, one would expect the rate of reactions that occur at an interface such as oxidation and lipolysis to increase with decreasing droplet size. However, there are conflicting reports in the literature on the effects of droplet size on these reactions. For example, there was no effect of droplet size on oxidation of structured lipid emulsions (Osborn and Akoh, 2004), no clear effects of heat treatment and droplet size on oxidation (Kiokias et ah, 2007), and a slower oxidation rate in methyl linoleate emulsions with smaller droplet size (Imai et ah, 2008). This shows that the effects of droplet size on the properties of emulsions cannot be considered in isolation from the nature of the interface. 

Perrin P, Lafuma F. Low hydrophobically modified poly(acrylic acid) stabilizing macroemulsions relationship between copolymer structure and emulsion properties. J Colloid Interface Sci 1998 197 317-326. 

Serfert Y, Schroder J, Mescher A et al. (2013) Characterisation of the spray-drying behavior of emulsions containing oil drop-lets with a structured interface. Journal of Microencapsulation 30 325-334. 

Food emulsions and foams are complex dispersions that constitute a major part of the food products that are consumed daily. The correct elaboration of such colloidal systems determines the functional properties of the final product such as texture or long-term stability. Accordingly, the optimization of the food product depends fundamentally on the comprehension of the structural characteristics of its components. Foams and emulsions are dispersions of air and liquid in another immiscible liquid, respectively. Formation of these dispersions is subject to the presence of amphiphilic molecules, which tend to place themselves at the air-water interface (foams) or the oil-water interface (emulsion). Thus, they constitute molecular barriers that stabilize the dispersion. The composition and structure of these molecular barriers determine ultimately the behavior of foams and emulsions. 

Food typically is a complicated system with diverse interfaces. Stable air-water or oil-water interfaces are essential for the production of food foams and emulsions. Interface phenomena, therefore, attract great interest in the food industry. AFM provides enough resolution to visualize the interface structures, but it cannot be directly applied on air-liquid or liquid-liquid interfaces. Fortunately, the interface structure can be captured and transferred onto a freshly cleaved mica substrate using Langmuir-Blodgett techniques for AFM scan. Images are normally captured under butanol to reduce adhesion between the probe and the sample. Then, sample distortion or damage can be avoided (Morris et al, 1999). 

Phospholipids are amphiphilic compoimds with high surface activity. They can significantly influence the physical properties of emulsions and foams used in the food industry. Rodriguez Patino et al. (2007) investigated structural, morphological, and surface rheology of dipalmitoylpho-sphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers at air-water interface. DPPC monolayers showed structural polymorphisms at the air-water interface as a function of surface pressure and the pH of the aqueous phase (Fig. 6.18). DOPC monolayers showed a... 

The surface characteristics of these species are determined by the particulates and stress transfer across the membrane will tend to be low, reducing internal circulation within the drop. The structure of the interface surrounding the drop plays a significant role in determining the characteristics of the droplet behaviour. We can begin our consideration of emulsion systems by looking at the role of this layer in determining linear viscoelastic properties. This was undertaken by... 

A.T. Florence, T.K. Law, and T.L. Wateley Nonaqueous Foam Structures from Osmotically Swollen W/OAV Emulsion Droplets. J. Colloid Interface Sci. 107,584 (1982). 

This broad class of hydrolases constitutes a special category of enzymes which bind to and conduct their catalytic functions at the interface between the aqueous solution and the surface of membranes, vesicles, or emulsions. In order to explain the kinetics of lipolysis, one must determine the rates and affinities that govern enzyme adsorption to the interface of insoluble lipid structures -. One must also account for the special properties of the lipid surface as well as for the ability of enzymes to scooC along the lipid surface. See specific enzyme Micelle Interfacial Catalysis... 

Hence, from the previously described light-scattering study of caseinate self-assembly in solution, we can postulate that heating/cooling not only alters the nature and strength of the physical (hydrophobic) interactions between emulsion droplets covered by caseinate. It most likely also transforms the nanoscale structural characteristics of the protein network in the bulk and at the interface, thereby affecting the viscoelastic and microstructural properties of the emulsions. 

The elasticity of the protein layer structure is supposed to act against the tendency of an emulsion or foam to collapse because it allows the stretching of the interface. This behaviour is most commonly observed for globular proteins, which adsorb, partially unfold, and then develop attractive protein-protein interactions (Dickinson, 1999a Wilde, 2000 Wilde et al., 2004). The strength of such an adsorbed layer, reflected in the value of the elastic modulus, and the stress at which the structure breaks down, can be successfully correlated with stability of protein-based emulsions and (more especially) protein-based foams (Hailing, 1981 Mitchell, 1986 Izmailova et al., 1999 Dickinson, 1999a). 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

Emulsifiers. The chemical structures of emulsifiers, or surfactants, enable these materials to reduce the surface tension at the interface of two immiscible surfaces, thus allowing the surfaces to mix and form an emulsion. An emulsifier consists of a polar group, which is attracted to aqueous substances, and a hydrocarbon chain, which is attracted to lipids. Emulsifiers include mono- and diglycerides, lecithin, propylene glycol esters, luctylated esters, sorbttan and sorbitol esters, polysorbate.x, and sucrose esters. 

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