Food interfacial layers

Another characteristic property of many biopolymers (proteins, modified starch, chitosan, etc.) which is useful for the encapsulation of bioactive molecules is their ability to adsorb at the oil-water interface and to form adsorbed layers that are capable of stabilizing oil-in-water (OAV) emulsions against coalescence (see Table 2.2). It is worthwhile to note here that the formation of an emulsion is one of the key steps in the encapsulation of hydrophobic nutraceuticals by the most common technique used nowadays in the food industry (spray-drying). The adsorption of amphiphilic biopolymers at the oil-water interface involves the attachment of their hydrophobic groups to the surface of the oil phase (or even their slight penetration into it), whilst their hydrophilic parts protrude into the aqueous phase providing a bulky interfacial layer. 

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

Martinet, V., Valentini, C., et al. (2005). Composition of interfacial layers in complex food emulsions before and after aeration effect of egg to milk protein ratio. J. Dairy Sci. 88, 30-39. 

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. 

In food systems, the interfacial layer often comprises both proteins and LMWE. Proteins and LMWE typically coexist in the interfacial layer, sometimes imassociated with each other, but also in association, with specific functions in the processing and properties of the final product (Hassenhuette and Hartel, 1997 Whitaker et al., 1998 Patino et al., 2003). 

The structure of the interfacial layers in food colloids can be quite complex as these are usually composed of mixtures of a variety of surfactants all are probably... 

Nevertheless, much is known about the structure of adsorbed 6-casein, certainly more flian is known for any other food protein, and various techniques have been used to study the adsorbed protein. The first evidence from DLS showed that 6-casein adsorbed to a polystyrene latex caused an increase in the radius of the particle by 10 to 15 nm (84). Later studies using small-angle X-ray scattering confirmed this and showed, in addition, that the bulk of the mass of the protein was close to the interface, so the interfacial layer was not of uniform density throughout (85). Neutron-reflectance studies also showed that most of the mass of protein was close to the interface (86). Only a relatively small portion of the mass of the adsorbed protein extends from the tightly packed interface into the solution, but it is this part which determines the hydrodynamics of the particle and which is almost certainly the soiuce of the steric stabilization which the 6-casein affords to emulsion droplets (84). It is to be noted that all of the studies just described were performed on latex particles or on planar interfaces however, it has also been demonstrated that the inter-facial structiues of 6-casein adsorbed to emulsion dro plets resemble those of the model particles (39, 85). Although detailed control of emulsion droplets dining their... 

In many food emulsions, more than one surfactant is present, so that mixtures of proteins, small-molecule surfactants (oil soluble and water soluble), and lecithins may be present. The result of this is that the interfacial layer will contain more than one type of molecule. The properties of the emulsion (the sizes of the droplets and the functionality) will, in turn, depend on which of the molecules in the formulation is actually on the interface. 

Therefore, in a real food emulsion, the composition of the interface may be exceedingly complex. Probably all of the types of surfactant present will be adsorbed to some extent, but it is at present impossible to do more than broadly predict what the composition of the interfacial layer will be, espe dally when the emulsion may be subjected to a vari ety of environmental changes (e.g., changes in pH and various sterilization procedures). Likewise, the prediction of stability or otherwise, and other functional properties of the emulsion, which depend on the composition and structure of the adsorbed layer, will become extremely complex. 

Even though emulsions generally need to be as stable as possible, there are several food produets where the opposite effeet is desired, so a shelf-stable emulsion ean be eontrol-lably destabilized when required. Sueh emulsions are the basis for produets such as whipped toppings and ice cream and generally depend for their effect on the processes of partial coalescence (168-170) and destabilization by whipping air bubbles into the mixture, at which time the interfacial layer of the emulsions may be mechanically broken and liquid oil spread aroimd the air-solution interface. 

The non-equilibrium properties of interfacial layers have a large impact on various technologies, comprising food processing, coating, oil recovery, and in particular the formation and stabiUsation of foams and emulsions in widespread fields of application. 

Establishing reliable criteria for selecting the most efficient antioxidant, AO, for a particular application is a major unsolved problem in food emulsions and dispersions and one of general importance in nutrition and health. Recent reviews " point to multiple factors that affect activities of antioxidants including the properties and reactions of the antioxidant and the polyunsaturated lipids being oxidized the locations or distributions of the antioxidant within emulsified food the effect of other components on antioxidant activity and the relevance of the model system to real food. Frankel and Meyer s summarized the antioxidant distribution problem highlighthing the crucial role of the antioxidant distributiOTi into emulsifier-rich interfacial layers in hetero-phasic food emulsions[9, 17],... 

Emulsions are thermodynamically unstable structures given a degree of kinetic stability by an adsorbed interfacial layer of amphiphilic emulsifiers. The emulsifiers serve to lower the interfacial tension and provide some inter-droplet repulsive forces to stabilize the dispersions (e.g., steric and electrostatic). The interfacial layer is typically between about 1 and lOnm thick for food grade emulsifiers, such as surfactants, phospholipids, proteins, or polysaccharides, and the interfacial concentration is in the order of a few mg per square meter of surface (McClements, 2005 McClements and Decker, 2000). 

Pavlovskaya, G., Semenova, M., Tsapkina, E., Tolstoguzov V. (1993). The influence of dextran on the interfacial pressure of adsorbing layers of 1 IS globulin Viciafaba at the planar w-decane/aqueous solution interface. Food Hydrocolloids, 7, 1-10. 

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). 

Klinkesom, U., Sophanodora, P., Chinaclioti, P., Decker, E.A., and McClements, D.J. (2006). Characterization of spray-dried tuna oil emulsified in two-layered interfacial membranes prepared using electrostatic layer-by-layer deposition. Food Res. Int. 39,449-457. 

---