Heat is one strategy for protein denaturation. When a protein is denatured by heat, the heat energy added to the mixture of proteins makes the molecules move
around. Add enough energy and the proteins move and groove enough to break all the non‐covalent interactionsholding the protein together, and the 3‐dimensional globularprotein structure unravels. This is the type of denaturation and coagulation we see in the cooking of an egg.
When proteins denature, the exposed hydrophobic parts of the protein join
together with exposed hydrophobic parts of other proteins, clumping together in a process called coagulation. We observe this process in the cooking of an egg – the clear, runny white becomes a white solid mass of denatured and coagulated protein
The red and blue colors are not significant – they are just there to help you visualize the different protein chains
When we add other liquids and/or molecules like starch, sugar and/or fat (from milk, butter, cheese etc) to the eggs, we dilute the protein mixture and raise the temperatureat which the proteins begin to coagulate. Dilutionof the proteins surrounds the protein molecules with many more water (and/or sugar or fat) molecules, which makes it harder for the proteins to find one another and stick their hydrophobic parts together…so we have to raise the temperature, which makes the molecules move around that much more rapidly and find each other.
When the coagulated protein network does finally form in these diluted mixtures, the solid matrix of coagulated protein is tender and fragile – the large networks of coagulated proteins are filled with water, sugar and/or fat molecules – as in a custard.
The setting is caused by the protein in the egg yolks denaturing and coagulating
The red and blue colors are not significant – they are just there to help you visualize the different protein chains
When we add other liquids and/or molecules like starch, sugar and/or fat (from milk, butter, cheese etc) to the eggs, we dilute the protein mixture and raise the temperatureat which the proteins begin to coagulate. Dilutionof the proteins surrounds the protein molecules with many more water (and/or sugar or fat) molecules, which makes it harder for the proteins to find one another and stick their hydrophobic parts together…so we have to raise the temperature, which makes the molecules move around that much more rapidly and find each other.
When the coagulated protein network does finally form in these diluted mixtures, the solid matrix of coagulated protein is tender and fragile – the large networks of coagulated proteins are filled with water, sugar and/or fat molecules – as in a custard.
Denaturinga protein uses some kind of energy to break the hydrogen bondsholding the protein together and subsequently unravel its globular structure. The unraveled protein now has exposed hydrophobicregions that can stick together (coagulate) with exposed hydrophobicregions of other proteins.
Agitationcan also denature proteins – specifically the kind of aggressive agitation that introduces large amounts of air into the protein mixture, creating a foam.
When cooks use a whisk or wire beater to beat egg white, the protein will (over time) stiffen into a white foamy semisolid. This stiffening and change in color from clear to white, mirrors what we see in heat denaturationof egg white protein.
When egg white protein is denaturedby whipping or beating air into the mixture, the denaturedproteins coagulateand trap air bubbles in addition to water. The denaturedproteins cluster around and stick their exposed hydrophobic portions into the air bubbles. The large numbers of trapped air bubbles give the coagulated protein matrix a soft, semisolid texture. The semisolid foam of denaturedprotein and air bubbles can be mixed with other ingredients and then heated.
Meringues like the one shown here are a classic examples of a cooked egg white foam. The structure of the meringue is created almost entirely by egg white protein.
The expansion of the trapped air bubbles upon heating is what causes the rise. The
“setting” of the proteins with heat is what maintains the structure of the cooked souffle
Ovalbumin, which is about 54% of the white’s protein content, coagulates when heated, forming a solid framework around entrapped air that enables the wall structure to resist collapse. Ovalbumin does not unfold much when the egg is beaten, but without it, the foam would collapse when baked. Simply, ovalbumin allows a liquid foam to solidify. The highly elastic conalbumin, which is about 12% of the white’s protein content, together with two other proteins — ovomucin and globulins — both present in quite small amounts, stabilizes the foam at room temperature.
A number of variables can impact the stability of egg white foams. For starters, it is possible to overbeat or over whip egg white foam. When this happens, the foam dries out and sets, which prevents the air bubbles from further expanding during the baking process. If egg white foam stands for more than five minutes, air starts to escape and it quickly returns to its liquid state. A little bit of a recipe’s sugar
created is extremely tight, as the sulfur molecules are prevented from reacting with any other material. Today, a more common approach to stabilizing egg white foam is to add cream of tartar, known chemically as potassium acid tartrate. This acidic salt lowers the pH of egg white, which in turns increases the number of free‐floating hydrogen ions and helps stabilize the foam, much like copper.
(Well, when a cook beats egg whites in a copper bowl – the ovotransferrin binds the copper from the bowl! Copper and Iron are both metals after all…and ovotransferrin will bind copper as easily as iron. When ovotransferrin (a.k.a. conalbumin) binds copper it becomes more stableand therefore denatures at a higher temperature. The result is that the egg white foam is more elastic and stable.)
As we saw with custards – addition of other molecules dilutes the proteins and makes it more difficult to form the coagulated protein network that stiffens, thickens or solidifies. In particular, when making an egg white foam, fat molecules are particularly destructive.
http://www.decodingdelicious.com/egg‐foams/
http://www.jbc.org/content/250/15/6026.full.pdf ‐Denaturation temperatures of conalbumin (ovotransferrin) with and without iron.
Apo ovotransferrin is 1ryx
Iron bound ovotransferrin is 1N04
http://www.decodingdelicious.com/egg‐foams/
S‐ovalbumin is 1uhg Regular ovalbumin is 1ova
I compared them as was done in this paper: http://www.jbc.org/content/278/37/35524.full
