Proteins as emulsifiers

A protein is a linear chain of amino acids that assumes a three-dimensional shape dictated by the primary sequence of the amino acids in the chain [2]. The side chains of the amino acids play an important role in directing the way in which the protein folds in solution. The hydrophobic (non-polar) side chains avoid interaction with water, while the hydrophilic (polar) side chains seek such interaction. This results in a folded globular structure with the hydrophobic side chains inside and the hydrophilic side chains outside [9]. The final shape of the protein (helix, planar or random coil ) is a product of many interactions, which form a delicate balance [10, 11]. These interactions and structural organisations are briefly discussed below [11]. 

Proteins are classified according to the secondary structures a-proteins with a-helix only (e.g. myoglobein), j8-proteins mainly with 8-sheets (e.g. immunoglobin), a + j8 proteins with a-helix and j8-sheet regions that exist apart in the sequence (e.g. lysozome). 

The protein structure is stabilized by covalent disulphide bonds and a complexity of non-covalent forces, e.g. electrostatic interactions, hydrogen bonds, hydrophobic interactions and van der Waals forces. Both the average hydrophobicity and the charge frequency (parameter of hydrophobicity) are important in determining 

Protein denaturation can be defined as the change in the native conformation (i.e. in the region of secondary, tertiary and quaternary structure) that takes place without change of the primary structure, i.e. without splitting of the peptide bonds. Complete denaturation may correspond to totally unfolded protein. 

When the protein is formed, the structure produced adopts the conformation with the least energy. This structure is referred to as the native or naturated form of the protein. Modification of the amino side chains or their hydrolysis may lead to different conformations. Similarly, addition of molecules that interact with the amino acids may cause conformational changes (denaturation of the protein). Proteins can be denaturated by adsorption at interfaces, as a result of hydrophobic interaction between the internal hydrophobic core and the non-polar surfaces. 

From the above discussion, it is clear that proteins act as stabilizers for emulsions by different mechanisms depending on their state at the interface. If the protein molecules unfold and form loops and tails, they provide stabilization in a similar way to synthetic macromolecules. On the other hand, if the protein molecules form globular structures, they may provide a mechanical barrier that prevents coalescence. Finally, precipitated protein particles that are located at the oil/water interface provide stability as a result of the unfavorable increase in the wetting energy on their displacement. It is clear that in all cases the rheological behavior of the film plays an important role in the stability of the emulsions. 

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The progress made using the methods described in this report has opened up a number of opportunities. There are consumer and health pressures to reduce the consumption of synthetic emulsifiers used in processed foods. Therefore, a need exists to identify alternative natural replacement emulsifiers. One approach is to develop natural , biodegradable emulsifiers through biosynthetic routes using enzymes. Alternatively, more widespread use of proteins as emulsifiers would be an option if their functional properties were more... 

The effects of heat on emulsions depend primarily on the type of protein which is present. However, other factors are important for example, the pH and the presence in solution of specific minerals. Thus, an emulsion which is stable to heating at one pH value may not be stable at another (153). It is, for example, difficult to produce heat-stable emulsions at pH values of about 4, using milk proteins as emulsifying agents other emulsifiers have to be used to offset the tendeney of the proteins to aggregate when heated in this pH region. However, added emulsifiers themselves have an effeet on the gel properties of the destabilized emul sions (155, 156). 

Bloomberg, G. (1991) Designing proteins as emulsifiers. Lebensmittel-Technologie, 24, 130-131. 

Ice-cream is a product which has been developed since mechanical refrigeration became available. Ice-cream mixes comprise fats (not always dairy), milk protein, sugar and additives such as emulsifiers, stabilizers, colourings, together with extra items such as fruit, nuts, pieces of chocolate, etc., according to the particular type and flavour. The presence of this mixture of constituents means that the freezing... 

GA is well recognized as emulsifier used in essential oil and flavor industries. Randall et al., 1998, reported that the AGP complex is the main component responsible for GA ability to stabilize emulsions, by the association of the AGP amphiphilic protein component with the surface of oil droplets, while the hydrophilic carbohydrate fraction is oriented toward the aqueous phase, preventing aggregation of the droplets by electrostatic repulsion. However, only 1-2% of the gum is absorbed into the oil-water interface and participates in the emulsification thus, over 12% of GA content is required to stabilize emulsions with 20%... 

Once the product is allowed to emerge from the extruder the water flashes into steam, expanding the product to a foam. Any protein and emulsifiers present help stabilise the bubbles, which set as the product cools. As the water flashed to steam the latent heat of the steam is lost to the product thus cooling it rapidly. 

Milk powder contains several useful components, namely protein and lactose. Lactose is a reducing sugar that undergoes the Maillard reaction to produce flavour and colour. The proteins as well as participating in the Maillard reaction have useful emulsifying abilities. These benefits are only obtained if the lactose is dissolved and the proteins dissolved or dispersed. 

Ice cream serves as a wonderful (and tasty) example of a complex, dynamically heterogeneous food system. A typical ice cream mix contains milk or cream (water, lactose, casein and whey proteins, lipids, vitamins, and minerals), sucrose, stabilizers and emulsifiers, and some type of flavor (e.g., vanilla). After the ingredients are combined, the mix is pasteurized and homogenized. Homogenization creates an oil-in-water emulsion, consisting of millions of tiny droplets of milk fat dispersed in the water phase, each surrounded by a layer of proteins and emulsifiers. The sucrose is dissolved in... 

The digestion and absorption of fat is considerably more complex than that of carbohydrate or protein because it is insoluble in water, whereas almost aU enzymes catalyse reactions in an aqueous medium. In such media, fat can form small droplets, an emulsion, which is stable in this medium. Formation of an emulsion is aided by the presence of detergents these possess hydrophobic and hydrophilic groups, so that they associate with both the fat and the aqueous phases. Such compounds are known as emulsifying agents and those involved in digestion are mainly the bile salts and phospholipids. 

Figure 6. Emulsifying activity index of soy proteins as a function of pH. Intact soy protein (O) soy protein proteolyzed with pronase E for 1 h (V) and 3 h ( ) deamidated soy protein ( ).

Non-sulfonated lignins find utility as emulsifiers and stabilizers in water-based asphalt emulsions, as coreactants in phenolic binder applications, as negative plate expanders in lead acid storage batteries, as protein coagulants in fat rendering, and as flocculants in waste water systems. 

Akhtar, M., Dickinson, E. (2007). Whey protein-maltodextrin conjugates as emulsifying agents an alternative to gum arabic. FoodHydrocolloids, 21, 607-616. 

Table 7.1 Effect of xanthan (XG) and NaCl on oil-in-water emulsions (50 vol% com oil) made at pH = 7.0 with legume seed protein isolate (LSPI) as emulsifying agent total protein adsorbed (Tt), average droplet size c/32, and amount of LSPI adsorbed per unit area of surface (Ts). Data from Makri et al. (2005) with permission.

Milk protein products. As indicated in Table 1, the food industry is placing major emphasis on the production and utilization of milk protein products in a wide variety of formulated food products (20,21,22). Although nonfat dry milk (NFDM) and whey powder are major milk protein ingredients in formulated foods, casein and whey protein concentrates, which contain their proteins in a more highly concentrated and functional form, are essential for certain food product applications, such as those products that require the proteins as an emulsifier agent. Additional details on the processing methods and conditions used to produce the various milk protein products are available (23). 

The following factors appear to control the emulsification properties of milk proteins in food product applications 1) the physico-chemical state of the proteins as influenced by pH, Ca and other polyvalent ions, denaturation, aggregation, enzyme modification, and conditions used to produce the emulsion 2) composition and processing conditions with respect to lipid-protein ratio, chemical emulsifiers, physical state of the fat phase, ionic activities, pH, and viscosity of the dispersion phase surrounding the fat globules and 3) the sequence and process for incorporating the respective components of the emulsion and for forming the emulsion. 

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