Science: Molecule of the Month: Temptation clad in a flimsy wrapper: Casanova loved it and its chemistry still has the power to seduce. John Emsley on the ingredient that makes chocolate irresistible

More chocolate is eaten at Christmas than at any other time of year, and in Britain we annually spend pounds 3bn to satisfy our craving. Women account for 40 per cent of sales, while children buy 35 per cent and men 25 per cent.

Chocolate tops the list of difficult-to-resist foods, and accounts for more than half of all food cravings. Some people even claim to be 'chocoholics' and say they find it impossible to resist chocolate - and a few women claim to feel the urge for chocolate irresistible as their periods approach.

Chantal Coady, author of Chocolate (Pavilion, pounds 9.99), questions whether there really are chocoholics: 'Although chocolate contains many active chemicals, some of which mimic natural hormones, none of these is addictive.' She believes 'women turn to chocolate for consolation and it is its intense sweetness that they seek'. And she should know - Ms Coady sells more than 15 tonnes of hand-made chocolates a year from her shop, Rococo, in Chelsea.

The link between chocolate and sex has always been strong, and is still played on by advertisers. The Mayas of South America discovered chocolate and supplied the nobles of the Aztec court, who drank it as an aphrodisiac. When it came to Europe its reputation sailed with it. Casanova proclaimed it as his preferred drink.

The reason could lie with one of the 300 chemicals that have been detected in chocolate, which consists chiefly of fats and sugars. There is a little caffeine, but the main stimulant is a similar chemical, theobromine, named after the cocoa tree, whose botanical name Theobroma cacoa means 'food of the gods'. The substance which may explain the mystery of chocolate, however, is phenylethylamine (PEA), of which there can be up to 700 milligrams in a 100 gram bar (0.7 per cent). In the pure state, PEA is an oily liquid with the chemical formula C8 H11 N and a fish-like smell; it can be made in the laboratory from ammonia and has the curious property of absorbing carbon dioxide from the air.

For some people, PEA may be the chemical that provides the boost they seek. It has been shown to raise blood pressure and blood glucose levels, and to produce a feeling of well-being and alertness when injected. According to Viv Cockcroft, writing in the Biochemist, PEA may have an effect on nerve centres in the brain, boosting the effects of dopamine and behaving like amphetamines to which it is chemically closely related.

Detectable amounts of PEA are produced naturally in our bodies and it is formed from the amino acid, phenylalanine. PEA increases in our body at times of stress and is also higher than normal in schizophrenics and hyperactive children. Normally our body is well equipped to remove PEA; it is done by an enzyme, monoamine oxidase. However, not everyone can cope with a sudden influx of PEA, a known vasopressor which restricts the walls of certain blood vessels in the brain. This may be one of the reasons it can trigger a migraine. People allergic to chocolate this way may well have difficulty making enough of the enzyme to neutralise the PEA.

Cocoa beans are harvested from the cacoa tree which grows best in warm, moist climates and within 200 degrees latitude of the equator. The world production of beans is 2 million tonnes, harvested in Brazil and Mexico for the North American market and in West Africa for the European market. Britain imports 150,000 tonnes of cocoa a year and we make about 500,000 tonnes of chocolate from it.

After cocoa pods are harvested, the beans are removed and left in the sun to ferment. This turns them brown and converts some of their sugars first into alcohol and then to acetic acid. This kills the shoot and releases other flavour molecules. The PEA probably forms during this stage. The beans are then roasted, which removes most of the acetic acid, and milled. At this stage the cocoa fat becomes molten and the residual bitterness also evaporates; the extent of the grinding process determines the different grades of chocolate.

The fat, called cocoa butter, is mainly saturated fat, and far more saturated than animal fats. Despite this, Herve Robert, author of the book Les Vertus Therapeutiques du Chocolat, reports that chocolate fat has no effect on blood cholesterol levels. Normal fats are a mixture of chemicals, but cocoa butter consists predominantly of only one kind and this melts sharply at 35C, just below body heat; chocolate literally melts in the mouth.

However, cocoa butter can solidify in different ways and each melts at a different temperature. Only the so-called type VI is right for chocolate and this explains why chocolate-making is still regarded as more of an art than a science, and why careful cooling of the molten chocolate is necessary to ensure that only VI solidifies.

In Europe, chocolate was considered a drink until, in 1847, J S Fry & Sons introduced a more palatable form to be eaten as a sweet. They made this by pressing molten chocolate to squeeze out the cocoa butter, and then adding this to more molten chocolate which then was cooled to become Fry's chocolate. The pressed cake from which the cocoa butter had been squeezed was sold as cocoa powder. Milk chocolate was first made in 1876 by Henri Nestle, who added condensed milk, which made it lighter, less bitter and the kind we shall be eating most of this Christmas.

And when you open that last present of chocolates and find them covered with a greasy white bloom, don't throw them away. It's only another of the crystal forms of cocoa butter. Console yourself and eat it; its PEA is still all there]

Dr John Emsley is science writer in residence in the department of chemistry, Imperial College, London.

(Photograph omitted)