Egg whites and egg yolks are rich in protein. Protein, which is held together by weak hydrogen bonds, can be easily changed by heat.
Boiling water maintains a steady temperature of 212 degrees Fahrenheit (100 degrees Celsius), which is hot enough to change the physical form of proteins. It breaks down the weak hydrogen bonds.
The proteins in eggs occur naturally as loose balls, held together by those hydrogen bonds. Boiling causes the balls to unravel, then solidify in their longer form. That makes the consistency more solid, though it is in reality a firm gel.
The longer the duration of boiling, the more denaturing will occur. A three-minute egg ("soft boiled") is runny, because much of its proteins have not been denatured. A hard-boiled egg with a hard yolk is completely denatured.
An egg white contains mostly protein (and water), while a yolk contains protein, cholesterol, carbohydrates and fat (and water). This is why the two behave differently, and the white of an egg solidifies before the yolk.
No one has yet "unfried" an egg, which would reveal the complex chemical nature of proteins. Viruses such as the common cold are also proteins, which is part of the reason that viruses are difficult to manage, chemically.
- "Understanding Primary Science"; Martin Wenham; 2004
- US Department of Energy, "Ask a Scientist"
- Photo Credit Image by Flickr.com, courtesy of Rick Audet
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