Gel forming property of proteins
Gel is an intermediate phase between a solid and a liquid. Technically, it is defined as essentially diluted system which exhibits no steady state flow. It is made up of Amino Acids Cross-linked via either covalent or non covalent bonds to form a network that is capable to bind with water and other low molecular weight substance (Fennema, 1996).
According to Meyer (1960), gel formation is a very important process in food chemistry. Not only do the properties of living cells, both animal and vegetable, depend on the gel structure, but in food preparation the stiffening which occurs during meat and flour cookery, the rigidity of pectin and starch gels, the high viscosity of many plant juices, the changes that occur in egg cookery and many other processing operations are a function of the gel.
Proteins are able to bind by hydration approximately 1 g of water per 5 g of dry protein. Proteins which form gels readily have structures with a high degree of asymmetry. These long proteinaceous fibers form a three-dimensional matrix primarily by establishment of inter protein hydrogen bonds, and this crosslinked structure is sufficiently well developed to hold water in an immobilized (non flowable) state. If the attractive forces of the protein are increased, e.g., by changing the pH to a value closer to the isoelectric point of the protein, the gel would tend to shrink. This shrinkage would expel some of the immobilized water, a process called “syneresis”. By decreasing the attractive forces between the protein molecules, however, such as by adjusting the pH away from the isoelectric point of the protein, the gel can be made to immobilize a larger amount of water (Fennema, 1996).
Proteins can form gels by acid coagulation, action of enzymes, heat and storage. A gel is a protein network that immobilizes a large amount of water. The network is formed by protein-protein interactions. Gels are characterized by having relatively high non- Newtonian viscosity, elasticity, and plasticity. Gelatin gels are produced when a heated solution of a gel is cooled. This sol-gel transformation is reversible. The nature of gels depends on a variety of covalent and non covalent interactions involving disulfide bonds, hydrogen bonds, ionic and hydrophobic interactions, or combination of these (Deman, 1999).
Foaming property of proteins
Foams consist of an aqueous continuous phase and a gaseous (air) dispersed phase. Semi processed foods are foam-type products. Generally, protein-stabilized foams are formed by bubbling, whipping, or shaking a prepared solution. The foaming property of a protein refers to its ability to form a thin tenacious fibrous gas-liquid interface so that large quantities of gas bubbles can be incorporated and stabilized.
As stated by Bennion (1980), foams are similar to emulsions except that the dispersed foam droplets are gaseous. Gas is always the discontinuous phase in a foam. The foaming agent lowers the surface tension of the liquid phase and allows expansion on its surface area.
Subsequently, Cumper (1953),explained the stabilization of the protein foams in terms of three successive processes: adsorption of the protein at the gas-liquid surface, surface denaturation, and finally coagulation of the protein. The polar groups of a protein surfactant cause the molecules to spread and denature at the surface.
Protein foams are an integral component of many foods. They play an important role in determining the quality of a product. Egg white protein (EWP) has been historically used as a foaming agent because of its ability to form foams with high volume–overrun–and stability. Foams form as a result of rapid diffusion of protein to the air-water interface, which reduces the surface tension, necessary for foam formation. Then the protein partially unfolds, which encapsulates air bubbles and creates the association of protein molecules, leading to an intermolecular cohesive film with a certain degree of elasticity. When egg white is beaten, it becomes foamy, increases 6 to 8 times in volume and stands in peaks (Miller, 1998).
Stability of the foam increases with beating time up to a point. Excessive beating creates an unstable foam due to the breakage of the coagulated egg-protein films. An overbeaten egg-white foam curdles – liquid drains from the structure and the air pockets coalesce. The rate, time and configuration of the beater or mixer also influence foaming. Temperature also affects the results. Low temperature increases egg white viscosity, and increases the time required to incorporate air. Room temperature whipping increases volume and creates a finer air cell. (DeMan, 1976)
Foams act as part of the leavening system in baked goods – heat expands the air pockets, then solidifies the protein-based structure.
Effect of heat on proteins
Denaturation is a process of change in the structure without breaking of covalent bonds. The process is peculiar to proteins and affects different proteins to different degrees. De Mann (1976) stated that, denaturation can be brought about by a variety of agents of which the most important are heat, pH, salts and surface effects. Denaturation usually involves loss of biological activity and important changes in some physical or functional properties, e.g., solubility. Heat denaturation is sometimes desirable, as in the denaturation of whey proteins for the production of milk powder used in baking.
Moreever, the proteins of egg white are readily denatured by heat and by surface forces when egg white is whipped to foam. Meat proteins are denatured in the temperature range 57-75oC and this has a profound effect on texture, water holding capacity and shrinkage.
Denaturation may sometimes result in the flocculation of globular proteins but may also lead to the formation of gels. Many food proteins undergo protein destablization on freezing. Fish proteins are particularly susceptible. After freezing, fish may become tough and rubbery and lose moisture. The caseinated micelles of milk which are quite stable to heat may be destablized by freezing. On frozen storage of milk, there is a progressive decrease in the stability of the caseinate and this may lead to complete coagulation.
Protein denaturation and coagulation are aspects of heat stability which can be related to the amino acid composition and sequence of the protein. Denaturation can be defined as “A major change in the native structure which does not involve alteration of the amino acid sequence.” The effect of heat usually involves a change in the tertiary structure leading to a less ordered arrangement of the polypeptide chains. The temperature range in which denaturation of most proteins take place is 55-75oC. There are some notable exceptions to this general pattern. Casein and gelatine are examples of proteins which can be boiled without apparent change in stability. The exceptional stability of casein makes it possible to boil, sterilize and concentrate milk, without coagulation. In the first place, restricted formation of disulfide bonds due to low content of cystine and cysteine results in increased stability. Peptides which are low in these particular amino acids are less likely to become involved in the type of sulphydryl agglomeration. Casein, with its extremely low content of sulphur amino acids, exemplifies this behaviour (De Mann, 1976).
Coagulation of Proteins
Coagulation is a term used to describe this entire process, which results in a loss of solubility or a change from fluid to a more solid state. The term gelation, meaning the formation of gel, also is used to designate the loss of fluidity of egg white and egg yolk. The coagulation of egg is responsible for the thickening effect that eggs have in products such as custards. Egg white begins to thicken as the temperature reaches 62° C, and 65C it will not flow. At 70°C the mass is fairly firm. Yolk thickens at 65°C and looses its fluidity at about 70°C. Coagulation does not occur instantaneously but gradually over a period of time. The reaction proceeds more rapidly as the temperature of heating is increased. Since heat is absorbed during coagulation of egg proteins, the reaction is endothermic. This means that as custards are heated, the temperature will remain the same or even fall as the custard is thickening (Campbell, et.al, 1979).
Opalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals (Fennema, 1996).
There are different degrees of opalescent behavior. One can still see through a slightly opalescent phase. The more particles and the bigger the particles are, the stronger the scattering arising from them and the cloudier the particular phase will look. At a certain concentration the scattering is so strong that all light passing through is scattered, so that it is not transparent any more (Fennema, 1996).
Effect of pH on the Hydration of Meat Proteins
Water is essential constituent of foods. The rheological and textural of food depend on the interaction of water with other food constituents, especially with macromolecules such proteins and polysaccharides. Water modifies the physico-chemical properties of proteins. For example, the plasticizing effect of water on amorphous and semicrystalline food proteins changes their glass transition. The glass transition temperature refers to the conversion of a brittle amorphous solid (glass) to a flexible rubbery state, whereas the melting temperature refers to transition of a crystalline solid to a disordered structure (Deman, 1976).
In low and intermediate moisture foods, such as comminuted meat products, the ability of proteins to bind water is critical to the acceptability of these foods. The ability of a protein to exhibit a proper balance of protein and protein water interactions is critical to their thermal gelation (Deman, 1976).
The water binding capacity of proteins is defined as grams of water bound per gram of protein when a dry protein powder is equilibrated with water vapor at 90-95% relative humidity. Water binding capacities (also called hydration capacity) of various polar and nonpolar groups of proteins (Fennema, 1996).
Proteins exhibit the least hydration at their isoelectric pH, where enhanced protein-protein interactions result in minimal interaction with water. Above and below isoelectric pH, because of the increase in the net charge and repulsive forces, proteins swell and bind more water. The water binding capacity of proteins is greater at pH 9-10 than at any other pH. This is due to ionization of sulfhydryl and tyrosine residues. Above pH 10, the loss of positively charged amino groups of lysyl residue results in reduced water binding (Fennema, 1996).
Salt increases the water binding capacity of proteins. This is because hydrated salt ions bind to charged groups on proteins. At this low concentration, binding of ions to proteins does not affect the hydration shell of the charged groups on the proteins, and increase in the water binding essentially comes from water associated with bond ions. However, at high salt concentrations, much of the existing water is bound to salt ions, and this result in dehydration of proteins (Fennema, 1996).