Proteins interaction with emulsifiers

Figure 8 Air bubbles in ice cream (a). Interface (arrows) between a large air bubble (A) and water phase (W) in an ice cream sample without emulsifier. There is very little adsorption of fat globules to the air-water interface, which is stabilized by a thin protein film only, (b) Corresponding structures in an ice cream with emulsifier (saturated mono-diglycerides). Fat globules interact strongly with the air-water interface. Reprinted from reference 23, p 242, courtesy of Marcel Dekker Inc.

The interaction of para-casein-covered emulsified fat particles (which may be considered as pseudo-protein particles) with the casein matrix of the cheese, and... 

Fats are first emulsified in the stomach by its churning action and stabilized by interaction with luminal lecithin and protein fragments. The lingual and gastric lipases do not require bile salts or cofactors to function they have a pH optimum of 3 to 6 and their action produces 1,2-diacylglycerols and fatty acids. These products further... 

The most important functional properties of proteins are solubility, water absorption and binding, rheology modification, emulsifying activity and emulsion stabilization, gel formation, foam formation and stabilization, and fat absorption [1-6]. They reflect the inherent properties of proteins as well as the manner of interaction with other components of the system under investigation. 

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... 

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. 

---