Interface emulsion

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). 

Whey protein concentrates (WPC), which are relatively new forms of milk protein products available for emulsification uses, have also been studied (4,28,29). WPC products prepared by gel filtration, ultrafiltration, metaphosphate precipitation and carboxymethyl cellulose precipitation all exhibited inferior emulsification properties compared to caseinate, both in model systems and in a simulated whipped topping formulation (2. However, additional work is proceeding on this topic and it is expected that WPC will be found to be capable of providing reasonable functionality in the emulsification area, especially if proper processing conditions are followed to minimize protein denaturation during their production. Such adverse effects on the functionality of WPC are undoubtedly due to their Irreversible interaction during heating processes which impair their ability to dissociate and unfold at the emulsion interface in order to function as an emulsifier (22). 

It is essential to consider the physico-chemical properties of each WPC and casein product in order to effectively evaluate their emulsification properties. Otherwise, results merely indicate the previous processing conditions rather than the inherent functional properties for these various products. Those processing treatments that promote protein denaturatlon, protein-protein Interaction via disulfide interchange, enzymatic modification and other basic alterations in the physico-chemical properties of the proteins will often result in protein products with unsatisfactory emulsification properties, since they would lack the ability to unfold at the emulsion interface and thus would be unable to function. It is recommended that those factors normally considered for production of protein products to be used in foam formation and foam stabilization be considered also, since both phenomena possess similar physico-chemical and functionality requirements (30,31). 

These emulsions are liquid-liquid systems comprising immiscible polymer solutions in nonpolar solvents and BG copolymer emulsifiers. The emulsifying power of BG copolymers has been attributed (12) to coalescence barriers formed by accumulation of the BG copolymers in the emulsion interface. This interface apparently has the structure of a double layer consisting of the different subchains of the BG copolymers which are solvated by the organic solvent. The chemically different sequences in BG copolymers are separated in different layers in the interface because polymer chains of different chemical structures are usually incompatible (3, 4), particularly in nonpolar solvents. 

For the cloud-emulsion mass transfer, the mass balance over the cloud-emulsion interface gives ANa,c... 

More commonly, demulsifiers are surface-active substances (surfactants) that have the ability to destabilize emulsions. This involves reducing the interfacial tension at the emulsion interface, often by neutralizing the effect of other surfactants that are stabilizing the emulsion. An example is antagonistic action - the addition of an O/W promoter to break a W/O emulsion (see sensitization in Section 5.4). Mikula... 

There are also process-specific considerations. For example, a long-residence-time settling vessel might allow the use of a demulsifier that diffuses to the interface slowly, but which produces a very well resolved (separated) emulsion, whereas a centrifuge process (having a short residence time) may require a demulsifier that diffuses to the emulsion interface rapidly, but is slightly less effective at emulsion breaking [68],... 

The process of homogenization can be followed by different methods, e.g., by turbidity, by conductivity, and by surface tension measurements. With increasing time of ultrasound, the droplet size decreases and therefore the entire oil/water interface increases. Since a constant amount of surfactant has now to be distributed onto a larger interface, the interfacial tension as well as the surface tension at the air/emulsion interface increases since the droplets are not fully covered by surfactant molecules. The surface tension can reach a value... 

Figure 7. A schematic of probable distribution of hydrophilic and hydrophobic antioxidants in bulk oil (oil-air interface) and oil-in-water emulsion interface [adapted from (46)].

With direct observation, the sample must be kept cold in the electron microscope, and care is required to prevent sample damage in the beam and to prevent microscope contamination. In addition, these frozen samples are often difficult to image because of charging effects that distort the image. The benefit of this extra care in sample handling, however, is that electron beam interactions with the sample produce characteristic X-ray signals that allow identification of components of the emulsion being observed. This technique has been refined to the point where, in special cases, chemical compositional differences at the emulsion interface can be identified, as well as the composition of the dispersed and continuous phases 109, 110),... 

Changes in process control procedures for the demulsification operation may be required for oil-in-water emulsions. Interface detection instruments must be able to detect the difference in water and an oil-in-water emulsion. Adjustment of control levels in separation vessels may be required for proper operation. 

An estimate of the time available for growth, Tp, may be obtained from the dense-phase tangential velocity Uijj (equal to r, dijjfdd, with ijj the angle measured from the vertical) at the bubble-emulsion interface ... 

Tangential velocity along the bubble-emulsion interface Time-averaged interstitial velocity of liquid Interstitial mean liquid velocity, Eq. (4-15) Ascending velocity of bubbles... 

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. 

The amine-based Henry reaction catalyst was encapsulated via the interfacial polymerization of oil-in-oil emulsions. PEI was encapsulated by dispersing a methanolic PEI solution into a continuous cyclohexane phase. Upon emulsification, 2,4-tolylene diisocyanate (TDI) was added to initiate crosslinking at the emulsion interface, forming polyurea shells that contain free chains of PEI. The microcapsules crenate when dry and swell when placed in solvents such as methanol and dimethylformamide, suggesting a hollow capsule rather than a solid sphere formation. The catalyst loading was determined to be 1.6 mmol g . ... 

The value of k b (the overall coefficient at the bubble-emulsion interface) can be calculated by an elaborate method described by Miyauchi and Marooka (1969) (see also Doraiswamy and Sharma, 1984), giving a value of 0.819 cm/s. Thus,... 

The presence of liquid-crystalline material at the emulsion interface has been shown by electron microscopy using the freeze-etching technique 18). Typical liquid-crystalline structures are shown in Figure 16. These liquid-crystalline compositions are viscous, and the lamellar phase displays pseudoplastic rheology. The lamellar phase is the most important of all liquid-crystalline phases for emulsion stability. The presence of a liquid-crystalline phase causes a reduction of the available London-van der Waals forces for coalescence 16). As a consequence of the reduction of the influence of these dispersion forces and the high viscosity of the liquid-crystal layer, the time for coalescence is increased dramatically. 

Many food items contain emulsions and foams, which are often stabilised by proteins forming a protective membrane at the interface. By preparing the food, adsorption of the available proteins, - by virtue of their surface activity -, is performed at the liquid/air (foams) and/or at the liquid/liquid (emulsions) interface. One way to study the interfacial behaviour of food proteins at those interfaces is to follow the interfacial tension decay accomplished by the adsorption of the proteins. 

Whey proteins rapidly adsorb on the emulsion interface, where they self-aggregate and form continuous and homogeneous membranes around oil droplets Various procedures can yield microcapsules from whey proteins ... 

An attempt was made to elucidate the rote of oleic acid in the stabilizing process of a diazepam submicron emulsion using a new approach of emulsion interface characterization. The study provided evidence that the close-packed film resulting from molecular interactions of the various film-forming components at... 

A few polysaccharides do appear to interact with emulsion droplets. Of these, the best known example is carrageenan. It is established from studies in milk and with isolated components from milk that A -car-rageenan can interact with /c-casein (143). Presumably, therefore, emulsions which contain car rageenans and caseins should show this interaction, and because the casein is likely to be found on the droplet interface, it is likely that the carrageenan will be found there as well. This is likely to result in a highly stable particle (144,145), which will have very strong steric stabilization because the carrageenan molecules may protrude far into the solution from the emulsion interface (146). 

Several scientists have tried to correlate the nature of the oil phase and its volume fraction to the stability of the double emulsions. No unusual or surprising findings were observed. Double-emulsion interfaces behave very much like simple emulsions except for the severe limitations on sizes of the droplets and the internal distribution of the emulsifiers. 

Some of these studies indicate that HLBis not the only property of the chemical which determines the demulsifier power. Cooper et al. (285) indicated that water reduetion was dependent on the chemical structure of the surfaetant when two surfactants with similar HLBs gave opposite results. The effects of the interaction of the chemical stmcture with emulsion interfaces are the more important factors in demulsification, as these influence the film rheology of the system. 

After defining the exchange coefficients for bubble-cloud and cloud-emulsion interfaces (Equations 22.30 and 22.31), and assuming a first-order reaction, we get the following equation ... 

FIGURE 11.4. In a manner similar to stabilization by cxrlloidal particles, surfactant Uquid crystals may adsorb at the emulsion interface and provide mechanical, steric, and/or electrostatic stabilization. 

If this is the valid transfer mechanism, it remains to explain how protein can ultimately be released in vivo. In light of these results, we now believe that demulsification must be taking place, but on a much slower time scale than that under which these experiments were performed. Alternatively, the presentation of the protein at the emulsion interface may be sufficient to induce the measured biological response. Establishing the exact rate of demulsification, and clarifying the structure of proteins at these interfaces, will therefore be the next thrust for this research. 

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