Small-molecule emulsifiers

Therefore, to understand the behavior of food emulsions, we need to know as much as possible about these types of emulsifiers, because fliey may not behave exactly similarly to classical small-molecule emulsifiers. For example, phospholipid molecules can interact with each other to form lamellar phases or vesicles they may interact with neutral lipids to form a mono- or multi-layer around the lipid droplets, or they may interact with proteins which are either adsorbed or free in solution. Any or all of these interactions may occur in one food emulsion. The properties of the emulsion system depend on which behavior pattern predominates. Unfortunately for those who have to formulate food emulsions, it is rarely possible to consider the emulsion simply as oil coated with one or a mixture of surfactants. Almost always there are other components whose properties need to be considered along with those of the emulsion droplets themselves. For example, various metal salts may be included in the formulation (e.g. Ca " is nearly always present in food products derived from milk ingredients), and there may also be hydrocolloids present to increase the viscosity or yield stress of the continuous phase to delay or prevent creaming of the emulsion. In addition, it is very often the case, in emulsions formulated using proteins, that some of the protein is free in solution, having either not adsorbed at all or been displaced by other surfactants. Any of these materials (especially the metal salts and the proteins) may interact with the molecules... 

The structures of the interfacial layers in emulsion droplets might be expected to be simple when small-molecule emulsifiers are used, but this is not necessarily the case, especially when not one but a mixture of surfactant molecules is present. Although simple inter-facial layers may be formed where the hydrophobic moieties of the surfactants are dissolved in the oil phase, and the hydrophilic head groups are dissolved in the aqueous phase, it is also possible to form multilayers and liquid crystals close to the interface (78). These, of coiuse, depend on the natiue and the concentrations of the different siufactants. Interactions between surfactants generally enhance the stability of the emulsion droplets, because more rigid and structured layers tend to inhibit coalescence. Also, mixtiues of different surfactants having different HLB numbers appears to provide structured interfacial layers, presumably because of the different affinities of the siufactants for the oil-water interface (79). 

Obviously, emulsions made using proteins have different properties from those made using small-molecule emulsifiers because their siufaces are very different. It is interesting to speculate on the manner in which the protein is replaced by increasing amounts of surfactant. It appears... 

It is apparent that real food emulsions are likely to behave in a more complex way than are simple model systems studied in the laboratory. This may be especially important when lecithins are present in the formulation. Although these molecules are indeed surfactants, they do not behave like other small-molecule emulsifiers. For example, they do not appear to displace proteins efficiently from the interface, even though the lecithins may themselves become adsorbed (123). They certainly have the capability to alter the conformation of adsorbed layers of caseins, although the way in which they do this is not fully clear it is possibly because they can fill in gaps between adsorbed protein molecules (124). In actual food emulsions, the lecithins in many cases contain impurities, and the role of these (which may also be surfactants) may confuse the way that lecithin acts (125). It is possible also for the phospholipids to interact with the protein present to form vesicles composed of protein and lecithin, independently of the oil droplets in the emulsion. The existence of such vesicles has been demonstrated (126), but their functional properties await elucidation. 

The possibility of the formation of nonisotropic surfaces on some types of emulsion droplets has been recently demonstrated. On interfaces of protein which have been treated with small-molecule emulsifiers, the protein is displaced. However, when insuffi dent emulsifier is added to cause desorption of all of the protein, there is a tendency for the different surfactants to form regions (i.e., to phase separate on the interface) (122). Clearly, such an interface offers the opportunity for directed aggregation because of the anisotropy of the surface. However, it depends on the presence of at least two surfactants. 

Because of the importance of ice cream as a product, much has been written on its stmcture and for mation (171), and the process can only be summarized here. In toppings and ice cream (and indeed simply in whipping cream), it is first necessary to produce a stable emulsion. Ice-cream mix is a complex mixtiue, but the initial emulsion is basically homogenized milk, containing an admixture of small-molecule siufactants as well in whipped toppings, the emulsion is made with oil and a surfactant mixture, which may or may not contain protein and in cream, the natural membrane of phospholipid and protein surrounds the milk fat. In all of these, it is necessary to have some small-molecule emulsifiers so as to exchange with, and weaken the rigidity of, the adsorbed layer of protein (118). The second essential is fliat the fat or oil in the formulation is partly crystalline neiflier completely liquid nor completely solid oil will perform optimally. If the oil is partly crystalline, then the emulsion droplets may not be truly spherical but may have protrusions of crystals on their surfaces. 

Feed—constituent interactions further affect retention (28,29). Dispersing agents and emulsifiers are partially retained because they attach to the dispersed phase. Small molecules may similarly adsorb onto larger particles. 

Milks of most species contain more water than any other constituent. Certainly this is true of the milks consumed by humans. The other constituents are dissolved, colloidally dispersed, and emulsified in water. The dissolved solutes in bovine milk aggregate about 0.3 M and depress the freezing point by about 0.54°C (see Chapter 8). The activity of water in milk, aw, which is the ratio of its vapor pressure to that of air saturated with water, is about 0.993. A small amount of the water of milk is bound , o tightly by proteins and by the fat globule membrane that it does not function as a solvent for small molecules and ions. Water content is usually determined as loss in weight upon drying under conditions that minimize decomposition of organic constituents, e.g., 3 hr at 98-100°C (Horwitz 1980). 

In the pharmaceutical industry, it is common to immediately suspend a portion of sample in solutions of a small-molecule surfactant. The surfactant is expected to rapidly adsorb at incompletely covered droplet surfaces to prevent droplet coalescence between sample withdrawal and analysis of droplet size or concentration. However, the addition of small surfactant molecules can result in a displacement of the original emulsifier from the droplet interface and profoundly alter droplet-droplet interactions. Changes in system composition may therefore lead to greater errors than those generated by the lag between sample withdrawal and analysis (see Background Information, discussion ofOstwald ripening). 

Encapsulation of enzymes in LMs offers further improvements for immobilization of complex enzyme systems, as the enzymes / cofactors, etc. are situated in aqueous droplets surrounded by a stable liquid hydrocarbon film (Figure 1). Instead of the physical pores present in microcapsules, the HC barrier, which has a diffusion thickness of about 0.1-1.0 p, effectively blocks all molecules except those which are oil-soluble or transportable by the selected carriers. Encapsulation of enzymes in LMs is accomplished simply by emulsifying aqueous enzyme solutions. Hence, LMs offer many advantages over other systems used for separation and eirzyme immobilization they are inexpensive and easy to prepare they promote rapid mass transport they are selective for various chemical species they can be disrupted (demulsified) for recovery of internal aqueous solutions gradients of pH and concentration (even of small molecules) can be maintained across the HC barrier multiple enzyme / cofactor systems can be coencapsulated and enzymatic reaction and separation can be combined. Some of the potential disadvantages of LMs for enzyme encapsulation have been discussed earlier. 

Although finely divided insoluble solid particles constitute an important class of emulsifying agents [44-46], the preparation of liquid-liquid dispersions traditionally involves the use of ionic and nonionic small-molecule surface-active agents. Mixtures of surfactants can also be used to achieve a desirable viscosity of emulsions [12] and to enhance the stabilization properties compared to the effect of one of the emulsifiers [47-49], although evidence of synergistic effects are not always found. 

Although there is much to be optimistic about the future of protein pharmaceuticals, there are still many unique problems with their development, production, and delivery. Among the more obvious problems with protein drugs is the fact that they are much more delicate than small-molecule drugs. Proteins such as hormones, antibodies, and enzymes cannot normally be compounded or pressed into dry pills or emulsified or concentrated into tinctures. This type of conventional pharmaceutical manufacturing and formulation would destroy the activity of most protein pharmaceuticals. Similarly most peptide hormones, antibodies, and enzymes cannot be stored indefinitely at room temperatures in nonsterile containers instead they must be kept in a cool, dark, aqueous, sterile environment for no more than a few weeks. These limitations to protein preparation and formulation have created a significant challenge to pharmaceutical chemists. Potential solutions to these problems are discussed in Chapter 4 of this book. 

Bread staling can shorten the shelf-life of bread and generate economic loss. This encourages more researchers to develop new techniques to control or retard the staling process. These techniques include, but are not limited to, the addition of glucose oxidase, xylanase, whey protein, soy protein, pentosans, arabinoxylans, hydroxypropylmethylcellulose and emulsifiers [19,26]. Small molecule materials, such as ribose, xylose, maltose and fructose, are also considered as antistaling agents due to their ability for the transformation of bread crystalline patterns [27]. 

Miiny important systems, however, do not follow Smith-Ewart Case 2 kinetics n can be less than 0.5 if free radicals can diffuse from the particles into the aqueous phase. This radical transport is believed to follow chain transfer reactions to small molecules such as monomers, solvent, added chain transfer agents and even emulsifier. The resulting radicals are sufficiently mobile so that a fraction of them can diffuse out of the particles thus causing n to be less than 0.5. 

Proteins, on the other end of the scale of molecular complexity, act as emulsifiers but behave differently from the small molecules, because of their individual molecular structures, and, indeed, it is the particular proteins present which give many food emulsions their characteristic properties. Most, if not all, proteins in their native states possess specific three-dimensional structures which are maintained in solution, unless they are subjected to dismptive influence such as heating (6). When they adsorb to an oil-water interface, it is unlikely that the peptide chains of proteins dissolve significantly in the oil phase, as they are quite hydro-philic as a result of the presence of carboxyl or amido groups it is more likely that the major entities penetrating the interface are the side chains of the amino acids (Table 1). It is possible, for example, for an a-helical portion of a protein to have a hydrophobic side, created by the hydrophobic side chains which lie outside the peptide core of the helix. However, even proteins lacking such regular structures possess amino acids with hydrophobic side chains which will adsorb to the oil-water interface. When a protein is adsorbed, the structure of the protein itself will... 

The authors of this review (46) have used BSA along with monomeric emulsifiers, both in the inner and the outer interfaces (in low concentrations of up to 0.2 wt %), and found significant improvement both in the stability and in the release of markers as compared to the use of the protein in the external phase only (Fig. 13). It was postulated that while the BSA has no stability effect at the inner phase it has strong effect on the release of the markers (mechanical film barrier). On the other hand, BSA together with small amounts of monomeric emulsifiers (or hydrocolloids) serve as good steric stabilizers, improve stability and shelf-life, and slow down the release of the markers. The BSA plays, therefore, a double role in the emulsions as film former and barrier to the release of small molecules at the internal interface, and as steric stabilizer at the external interface. The release mechanisms involving reverse micellar trans-... 

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