Egg white
|
|
The egg white is a clear liquid (also called albumen or ovalbumin) contained within a bird's egg. It is the cytoplasm of the egg, which until fertilization is a single cell. It consists mainly of about 10% proteins dissolved in water. Its primary purpose is to protect the egg yolk and also to provide additional nutrition for the growth of the embryonic chick, as it is rich in proteins and is of high nutritional value. Unlike the egg yolk, it contains little fat.
It is often used and separated for cooking (for glairs, meringues, soufflés, and some omelettes), which is where it derives its name from: when egg white is beaten it turns white.
In schools it is often used to teach pupils how to test for protein using Biurets reagent.
| Contents |
Composition
The main constituent of egg white is a protein termed ovalbumin, which is structurally a serpin (a class of proteins), although it does not have a known function in inhibiting other proteins.
Denaturation
All proteins, including those in egg white, are made of long chains of amino acids which might be considered as like beads on a string. In a raw egg, these strings are ravelled up in a tangled compact mass. There are bonds between the amino acids within each protein that stop the ball from unravelling. As the egg cooks the heat causes the bonds within the proteins to break. Each ball of protein unfolds and tangles up with the other protein balls. Bonds form between the amino acids on different proteins setting the albumen into a rubbery gel. When boiling eggs in water, some people add salt to the water for no good reason [1] (http://www.princeton.edu/~lehmann/BadChemistry.html#Solutions).
What happens to an egg white as it is beaten
The protein partially unravels and forms a good foaming agent. A foam is formed by the protein forming a stable film around the included air. Studies show that the best foam forms when the unraveling of the protein is only partial. Overbeating egg white destabilises the foam by completely unravelling the protein molecules. The protein is elastic, so when the egg white is cooked, and the air expands, the white stretches then sets in the expanded position.
Use of a copper bowl
Many cooks recommend using a copper bowl to beat egg white. This has the effect of making it take longer to form the foam, but leads to a much more stable foam. This probably because the copper atoms form a complex with the conalbumin protein which makes it difficult to unravel, which means that it takes around twice the time to get a good foam, but very much more difficult to overbeat. (The danger of overbeating may be the reason many cooks recommend beating by hand rather than use an electric whisk)
The one possible drawback of using a copper bowl is the toxicity of copper. However, the amount incorporated into the white is likely to be far too small to cause ill effect.
Cream of Tartar
Cream of tartar (potassium bitartrate) is an acidic salt can be used to change the pH of the egg white to an acidic range of around 3-4. This has the effect of stabilising the foam, and is therefore a good alternative to using a copper bowl. Cream of tartar should not be used if a copper bowl is used, because it can react with the copper and force more of it into solution (acids react with metals, in general), thus increasing the toxic effect.