Normally boiling something softens it, so why do eggs go hard-boiled

A.� If you know anything about nutrition and food groups then you'll know that eggs are packed with protein. It's the egg's protein content that causes this apparent reverse of states and makes eggs go hard in boiling water.

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Q.� How can protein harden an egg

A.� Protein is basically a string of amino acids, which fold in a particular way. The folding pattern of the amino acid strings determines the proteins biological and chemical properties.

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Q.� And boiling an egg changes the folding pattern of the strings

A.� That's right, or to give it its official term, it 'denatures' the protein. Weak bonds hold the proteins that become broken or denatured when heat is applied.

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Q.� How does boiling an egg break the bonds

A.� The proteins gain energy as the rising temperature of the water heats the egg, making them shake themselves free and unfold.

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Q.� Breaking bonds sounds as if it should make the egg even runnier, not set it hard

A.� Well once the weak bonds have been broken the proteins are then free, and sufficiently energised by the heat, to form new, stronger bonds, with other proteins. At the same time water, which surrounded the weakly linked proteins, is forced out by this new relationship and the egg becomes set hard.

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Q.� Is heat the only way to denature a protein

A.� No you can also do it chemically although the results may not be so palatable as with boiling.

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Q.� When whisking egg whites to form firm peaks, is this breaking the bonds too

A.� Yes, but this is caused by mechanical energy breaking the bonds and allowing the proteins to make new stronger ones.

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Q.� Is the process reversible

A.� No, once the bonds have been broken and reformed the new network of strong bonds is permanent.

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Q.� What causes the green ring between the yolk and white of a hard boiled egg

A.� This is the result of a chemical reaction between the iron in the yolk and the hydrogen sulphide in the white, which forms a ring of iron sulphide. Try using a low heat to hard-boil your egg to void this unsightly, though harmless ring.

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Puzzled about why things react differently to the same forces Ask The AnswerBank to explain things.

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by Lisa Cardy