How much albumin is theoretically present in an egg

Differences in gel-forming ability among globular proteins generally reflect the variety of degrees of protein-protein interactions and the number and extension of interactive sites available within the opened molecule Phillips et al.

The intermolecular disulphide linkages increase the stability of the gel matrix. The increased size of polypeptide chains can delay the rupture of non-covalent interactions, and favor the gel network stability. Consequently, the reactive thiol groups are exposed and can yield intermolecular disulphide bond Schmidt et al. The reactions of intermolecular changes of thiol-disulphide groups can promote an increase of crossed disulphide linkages within the gel matrix, according to the reaction scheme.

The dimerous molecule formed by this reaction can keep on reacting with sulphydrylic groups in other protein molecules, producing the necessary cross-linking to gelation Phillips et al. In addition to these steps, a third one has been proposed, involving denaturation followed by soluble aggregation and by interaction of aggregates within the network, evidences supported by electronic microscopy Beveridge et al. Ferry introduced the concept that gelation occurs in two steps; he emphasized the importance of the relation between denaturation and aggregation.

If the attractive strengths are broken by denaturation, this should quickly promote the chain association. This would result in accumulation of free denatured proteins, as an intermediary phase.

In these conditions, a thin layer of gel should be formed. A rough gel should be formed when increasing attractive strengths lead to gelation before the accumulation of many free chains. With increasing attractive strengths, only aggregates should be formed. The relationship between denaturation and association affects the kind of formed network in either mechanism: two steps or one of three steps Oakenfull, The coagulation reaction in endothermic, i.

Therefore, the velocity of this reaction is affected by temperature. The denaturation temperatures of conalbumin, globulin, ovalbumin and lysozyme are There are two main foam classes: the spherical and the polyedric foams. Protein-based foams are formed by air bubbles.

Each droplet is involved by a thin and continuous film of protein molecules and each bubble is separated by a lamella. Air bubbles are originally spherical high internal pressure , and the lamella is dense thick , containing large amounts of fluid. Liquid in the polyedric foam is distributed among lamellar channels where the films are located. The drainage of lamellar fluids is the main destabilization strength, causing air bubbles to get closer and take on polyedric shape Adamson, During the formation of protein-based foam, a sequence of reactions occurs.

The rearrangement of polypeptides occurs in the interface and is oriented by polar mobility, which is directed to water, and the nonpolar segments preferably lead to air particles.

This process occurs through the non-covalent interactions of the polypeptides, and is the base of a cohesive, continuous film Phillips, The structural components and the attractive electrostatic interactions strengths that enable intermolecular associations improve foaming properties; exceedingly repulsive electrostatic interactions lessen foaming ability.

The extension of molecular interactions of proteins in the air-water interface and the properties of the interfacial film depend on the kind of protein and the dominant conditions of the solution, which determine foam formation and stabilization Phillips et al. The foaming properties of proteins are fundamentally related to their properties of forming films in the air-water interface. Proteins that can be opened and quickly adsorbed present better foaming properties than those slowly adsorbed and whose structures are more difficult to be opened in the interface.

The same strengths that determine the structure and flexibility of a protein, such as the electrostatic and hydrophobic interactions, and the disulphide linkages also determine the interfacial behavior and the protein properties Phillips et al.

In homogenous systems, the main attractive strengths are hydrogen bounds, hydrophobic interactions, electrostatic and van der Waals forces Kinsella, The strength magnitude, which keeps the native protein structure either in solution or at the interface, is important in the foaming properties Adamson, Barmore apud Halling , showed the stability decrease of foam in the super-beating of the eggwhite, and the drainage increase correlated to the decrease of viscosity of the liquid drained off.

These changes in viscosity proved to be an important feature of the super-beating. The viscosity decrease does not sufficiently explain the increase in the drainage rate in long periods of eggwhite beating. Conceivably a specific protein component also performs an important role in the eggwhite super-beating Halling, Attractive electrostatic force: essential to the protein network formation and film cohesion, in excess may cause coagulation of polypeptides.

On the other hand, the role of electronegativity in foam stabilization has been attributed to the electrostatic repulsion in adjacent films. Important features to the formation of optimum films in a simple system can delay the film formation and cause foam destabilization. For instance, some rheological properties that improve film stability are maximized in the isoelectric point range for many proteins; at this point solubility tends to a minimum.

To determine foaming properties of a protein, it is necessary to know the balance between hydrophobic and ionic components, although the precise location of these components in three-dimensional molecular form is unknown Phillips et al.

The nonpolar residues intensively contribute to the interactive forces at the hydrophobic interface. The foam stability depends on the ability of surface activities of proteins improving the film elasticity. The formation process is related to the mixing velocity, the beater geometry and the surface properties of the material to be foamed Phillips et al.

The maximum levels of air incorporation during the beating reflect in a better actual dynamic equilibrium between mechanical strength and the bubble destruction. This provides an actual measure of foam stability. The foam stability is measured by the time required by a certain amount of liquid to be drained from the foam.

The extension of protein film formation is related to the ability of the protein to diminish the surface tension between the air droplet and the protein solution. The foam stability depends on the film nature, which reflects the extension of interactions within the film matrix Phillips et al. The structural characteristics of proteins leading to fast foam formation are low molecular weight and amphipathic molecules. To form a protein capsule that can hold an air bubble, the protein components should present non-covalent interactions, such as electrostatic and hydrophobic strengths, hydrogen bounds and disulphide linkages.

The critical balance of non-covalent interactions leads to the formation of a cohesive and viscous film, required to stabilize the foam. The electrostatic repulsion can reduce foam stability and delay the film formation. Disulphide linkages reduce the protein flexibility. The change of a thiol group to a disulphide has great importance to functional properties. The formation of disulphide linkages during the foam formation and the foaming properties of ovalbumin were studied by Doi et al.

The formation of disulphide bindings at the air-water interface can improve foam stability. The protein concentration, the film thickness, the ionic strength, pH, temperature, and the presence of other components in the food systems, in addition to physical-chemical properties of proteins, affect foaming properties.

For instance, the increase of the protein concentration generally causes the formation of a thick lamellar film, which yields more stable foam Phillips et al. Meringues and eggwhite cakes can be made out of ovomucins and ovoglobulins alone. When the ovalbumin is alone in an Angel cake mixture, longer beating period is required for foaming and the end product will have a thick texture MacDonnel et al.

Eggwhites from which globulin and ovomucin are removed, require longer beating periods, and when these mixtures are used in cakes they cause a volume reduction. If the ovoglobulins are returned in the cake mixture, there is an increase in the volume, but the beating period is still long. When the ovomucin is added replacing ovoglobulin, the beating period is diminished, but the volume of cakes is not improved. When both ovoglobulin and ovomucin are added, an increase in the cake quality is observed.

The addition of a great amount of ovomucin produces an increase in the velocity of foam formation and excessive coagulation of proteins in the bubble surface, plus reduced elasticity of the film surrounding the bubble. Therefore, the volume of a cake baked from this mixture is smaller. The addition of a larger quantity of ovoglobulin in the eggwhite causes a weak foaming property and increases the cake volume.

The stability in these cases is obtained only when the quantity of ovomucin is regular MacDonnel et al. The volume of drained liquid increased and the foam stability decreased with increasing s-ovalbumin contents. The super-beating of the eggwhite makes the ovomucin insoluble, causing losses in the elasticity of the film involving the bubbles.

Each successive beating of the drained foam liquid requires a longer period of beating and yields less stable products MacDonnel et al. Large quantities of ovomucin are removed in the first beating, and the drained liquid that is collected of eggwhite foam is identical to that from fresh eggwhite. Ovoglobulins contribute to high viscosity and to diminish the drainage of liquids from the foam.

They also diminish the surface tensions which are especially helpful in the foam initial stage. The loss of surface tension promotes the formation of small air bubbles and causes a smooth texture.

The use of protein mixtures produces a mixture of opposed charges that can improve the stabilization of food foams, since the possible electrostatic interactions that may occur are known. Most globular proteins have their own internal structural disposition, as a result of different covalent and non-covalent bindings between and inside the protein molecule, and also between protein groups and the solvent. Physical-chemical features involved in the technological process such as concentration, drying or dilution are known to induce the denaturation process, and change the tertiary and quaternary structures of a protein.

In the food industry, proteins are exposed to processes that include purification, high pressure, and heat, among others.

Consequently, positive and negative effects can be observed in the development of textural properties, such as gelation, foam formation, and stability. The thermal behavior of proteins in specific chemical conditions can be evaluated by monitoring the structure-function relations. This could enable maximizing the beneficial and minimizing the negative effects.

All of these reactions are exacerbated by extremes of pH, temperature and relative humidity. What is clear, however, is that sugars not bound to the protein chain will react with the protein, forming conjugated double bonds which are chromophoric. Sugars which have been shown to participate in Maillard browning are hexoses notably glucose and pentoses notably ribose.

Albumen has a free glucose content of 0. However, the total amount of sugar able to drive the Maillard reaction in an albumen binder layer is largely dependant on the amount and type of processing the albumen has undergone prior to coating. If a fermentation step has been employed, the sugar content of the albumen will be substantially lowered due to microorganism activity. Removing sugar in this manner been found to be effective in reducing the amount of albumen yellowing Reilly As indicated by the elemental breakdown of the primary albumen proteins see page 4 , there is an appreciable amount of sulfur contained in hen's egg white..

Overall, albumen contains 0. Sulfur can be chemically bound to proteins in various ways. These configurations are both associated with the amino acid cysteine. Sulfur is also known to react with the silver image material in albumen prints which causes image fading and detail loss.

More relevant to the discoloration of the albumen binder, however, are reactions of sulfur with the chemically bound, non-image silver. It was found that this initially invisible silver, was irreversibly bound to the albumen proteins. This non-image silver called silver albuminate or argento-albumen could not be solubilized by the usual fixing baths of sodium thiosulfate; nor could it e removed by any other chemical means that would not destroy the image.

Proteins, in fact, have a great capacity to bind with metal ions. Complexes formed with the transition metals, copper, zinc, mercury and silver, tend to be very strong chemically and can occur in conjunction with several amino acid functional groups.

Silver is very strongly bound to the protein chain by sulfhydryl groups R-SH Klotz They are also formed during denaturation and fermentation when the disulfide bridge between two cysteines is broken. This tendency is in keeping with its biological function of sequestering metallic contaminates in the egg Powrie Due to substantially decreased reactivity, silver ions sequestered by conalbumin would tend not to engage in degradation reactions. The Effects of Light.

The absorption of ultraviolet light by proteins results in several known degradation reactions Doty and Geidushek Types of photodecomposition include the breakdown of hydrogen bonds and the destruction of disulfide bonds creating two reactive -SH groups. These amino acids are all present in the egg white proteins in moderate or high proportions.

Furthermore, it is known that amino acids can become separated from the protein chain by the action of light, which implies that the peptide bond itself is responsible for ultraviolet light absorption.

The main goal for individual photographers and manufacturers was to create an albumen binder with the best working properties possible. Based on the preceding discussion of the involved protein chemistry, it is apparent that the range of variables in processing can have a significant impact on the composition and complexity of the finished photograph. Frothing, straining, drying, combined with the other assorted processing steps, ultimately have an impact on the deterioration characteristics of individual albumen prints.

Specific methods of manufacture and past display conditions for individual photographs are usually impossible to determine. As a result, generalizations about the composition, reactions and degradation properties of the albumen binders must often be tentative ;and qualified. Therefore, conservation research must consider, and attempt to compensate for, the range of albumen processing techniques and the diversity of possible degradation reactions.

Ovalbumin can also be classed as a phosphoglycoprotein since phosphate and glycogen are integrated into the protein chain. An important egg white protein which does not have a globular conformation is ovomucin. Ovomucin has a "fibrous" conformation which means that the polypeptide chain is not highly intertwined as in globular proteins but is arranged along a single axis.

It is ovomucin that is responsible for the foaming of egg white when it is shaken. Creighton, T. New York, New York: W. Freedman and Company. Dickerson, R. The Structure and Action of Proteins. Menlo Park, California: W. Benjamin Inc. Doty, P. Optical properties of the proteins. Neurath and -K. Volume 1, Part A: Hardy, P. London; Chapman Hall Ltd. Klotz, I. Protein interactions. Neurath and K. Volume 1, Part B: Lehninger, A.

MacDonnell, LR. Feeney, H. A simple form of protein, egg albumin protein presents several unique characteristics that make it extremely useful to pet food manufacturers. First, as we mentioned previously, it offers thermal coagulation. Upon first cracking an egg, you can observe the translucency and soft, liquid-like nature of the egg white.

When cooked, the heat causes denaturation, hardening the albumen and coloring it opaque white. The changes in viscosity and color reflect the denaturation of the egg albumin protein. Next, albumin is water-soluble. This characteristic assists with the incorporation and dispersion of the protein when making pet foods or treats. When a protein offers high solubility, its range of potential applications may expand.