Hervé This is on a mission. 'By the time I die, I want cooking to have changed,' he says. In his native France, Hervé This (pronounced 'Tiss') is a star, the country's most famous chemist. He appears on TV and radio, his books are bestsellers, government ministers ring him for advice and there is a travelling exhibition based on his work.

In this country, one of Hervé's biggest fans is Heston Blumenthal, with whom Hervé, 51, has worked on several experiments. 'So much of cooking is mystique,' Blumenthal says. 'That way, it makes chefs look like geniuses. But the more we can explain, the more approachable it becomes. That's how food will move forward and Hervé has turned many things on their head.'

His specialism is the science of cooking. For him, every foodstuff is 'a chemical mixture'. 'When aromatic compounds are formed on the surface of a roast,they are the result of a chemical reaction. When mushrooms turn black after being chopped, it is the fruit of a chemical reaction.'

Over the years, his musings on chemical reactions have led to a number of discoveries. He has worked out how to uncook an egg. He has calculated that you can produce 24 litres of mayonnaise with a single yolk. He has invented a Béarnaise sauce by replacing butter with melted chocolate, as well as 'chocolate chantilly' (a form of whipped chocolate prepared in the same way as crème chantilly). He's baked an egg for an hour at 55°C, managing somehow to leave the yolk 'exceptionally smooth and tender'.

Almost 10 years ago, Hervé investigated colloids (substances that are neither completely solid nor liquid, such as emulsions, mousses and gels) and devised a system of formulae based on the dispersion of gases, liquids and solids within each other.

'It can be used to invent an infinite number of dishes,' he claims. He told Pierre Gagnaire, one of France's leading chefs, about his find, and the chef came up with a new recipe called Saint-Jacques 'Faraday' - an emulsion created with scallops, orange-flavoured oil, smoked tea and gelatine. Every month Hervé sends Gagnaire an idea from his lab which the chef incorporates into a recipe. (A collection of them is posted on Gagnaire's website.)

Hervé has taught at the universities of Tours and Montpellier, and has his own laboratory at the venerable Collège de France, which, today, is also the base for all four of France's living Nobel prize winners. One of them, Jean-Marie Lehn, invited Hervé to join his department in 1995. In Hervé's laboratory, there is a cupboard full of chemical compounds such as sulphuric acid, sodium carbonate and Mercurochrome, while the shelves are stacked with things you'd normally associate with the kitchen: bottles of balsamic vinegar and olive oil, pots of mustard and cinnamon, bags of salt and shallots, and a bar of chocolate. There are also piles of notebooks, in which he notes down his experiments, as well as literary sayings and advice he comes across.

'I go through ancient recipe books, preferably from the 18th and 19th centuries, jot down what they have to say and test it,' he says. He says that Madame de Saint-Ange (who wrote the bible of bourgeoise cuisine in 1927) is a mine of sayings, but that much of her advice is incorrect. More exact are Le Livre de Cuisine by Jules Gouffé (first published in 1867) and Auguste Escoffier's Guide Culinaire. 'Some of Escoffier's advice is startlingly sound,' says Hervé. He recommended, for example, that pepper be added to a stock only eight minutes before it is taken off the heat. The explanation, Hervé has discovered, is that further cooking means the more desirable, spicy aromatic molecules volatilise and are replaced by bitter tannins.

'There is no limit to Hervé's curiosity,' says the three-Michelin-starred chef, Antoine Westermann. He arrives at his lab at 7:30am and works non-stop. Once, during his summer holidays, he wrote three books, taking breaks only to play ping-pong with his two sons. For those in the profession, however, he makes himself far more available. 'He doesn't work for financial gain,' says Gagnaire. 'Simply to transmit his knowledge.' According to American Vogue's food critic, Jeffrey Steingarten, Hervé has the tools to do so: 'He's an exciting lecturer. He talks fast and loud, cracks jokes and is quite a showman. He has the audience in the palm of his hand.'

On the morning of our second meeting, Hervé measures the pressure inside a chip. He's aiming to disprove a famous scientific paper which states that oil infiltrates chips during cooking. He heats oil to 168°C, measures a chip, hooks it up to a manometer and plunges it in the oil. Unfortunately, the experiment goes wrong and ends up with the chip falling to the bottom of the pan. He is unconcerned. 'In order to come up with conclusive results, I'll have to carry it out 100 times.'

He then shows me several other experiments. There is a jar full of pears and a few bits of tin. There is a quail's egg that has been standing in vinegar for three years. The acid has attacked its shell and made it translucent. By osmosis, it has swelled up to about four times its original size. Hervé says that it has the same consistency as a hard-boiled egg and could be eaten in a salad.

But what really intrigues me, of course, is exactly how he managed to unboil an egg. He explains that when an egg is cooked, the protein molecules unroll themselves, link up and enclose the water molecules. In order to 'uncook' the egg, you need to detach the protein molecules from each other. By adding a product like sodium borohydride, the egg becomes liquid within three hours. For those who want to try it at home, vitamin C also does the trick.

While the discovery may be fun, it does not have many practical applications (most of us prefer our eggs both cooked and flavoured with salt rather than strange chemicals). Many of his other results, however, can be applied to everyday cooking. He says that adding oil to water while cooking spaghetti does not prevent the different strands from sticking together - unless you use very little water and litres of oil. He advises that, in order for a poached or fried egg white to be evenly cooked, you should sprinkle a little salt around the yolk (it speeds up the cooking of a protein found there, which coagulates less easily than the other proteins in the egg white). The discovery he is proudest of, however, concerns meat stews. 'For many centuries, so much rubbish was written about them.' One tradition said that you should start off cooking the meat in cold water. Another claimed the opposite. According to Hervé, it makes no difference. The meat loses the same amount of weight in both cases.

Hervé, who has written seven bestselling books, grew up in a family of gourmets. Whenever they went skiing in the Alps, they would stop to eat at Paul Bocuse. On a boating holiday one summer, he invented 27 different recipes for mackerel. At the age of six, he was given his first chemistry set. 'I was always doing strange experiments at home,' he says. 'My brother still recalls with despair how I once distilled violets for Mother's Day.' Later, he went on to study at the prestigious Ecole de Physique-Chimie in Paris and also managed to do a French literature degree at the same time. On graduation in 1980, he joined Pour La Science (the French edition of Scientific American). He would stay there for 20 years as a scientific adviser.

His experiments in food began in the spring of 1980 when he tried to make a Roquefort soufflé from a recipe in Elle, which stated that you had to add the eggs two by two. Hervé decided to ignore the advice and put all the eggs in at the same time. His soufflé was a flop. The following week, he had another go, this time adding the eggs one by one. The result was better, but not as good as when he made yet another soufflé, this time following the recipe to the letter. It got him thinking about the scientific reasons for adding the eggs two by two.

Over the years, he says, his sons have eaten some 400 soufflés. I join him and his wife, Pascale, a cancer doctor, for dinner, at their house near Versailles, for a meal full of his inventions. The starter is tomate royale, a sort of barely-set quiche. And the main course, salmon en croûte is accompanied by an unidentifiable pile of vegetables. 'It's an emulsion of chestnuts', says Hervé The food is hugely variable. The salmon scrumptious, the dessert disgusting.

Hervé believes his originality lies in the fact that he applies his knowledge of chemistry to the food of ordinary people, while other scientists work in the food industry. 'Food science,' he says, 'has never dealt with cooking. It doesn't give a damn about soufflés and stews.'

Still, he's had precursors, notably the Hungarian-born, Oxford-based physicist Nicholas Kurti, who in 1969 gave a lecture at the Royal Institute called The Physicist in the Kitchen. Hervé first met Kurti in 1988. It was, he says, 'a friendly case of love at first sight'. They ended up working on experiments together.

By his own admission, Hervé's relations with chefs are 'very friendly, but very complicated'. Philippe Conticini has elaborated a menu for Petrossian in Paris based around Hervé's work on 'frothy emulsions'. There is his regular monthly link-up with Pierre Gagnaire; Alain Passard says he has one of Hervé's books constantly on his desk. 'It's a real education,' he says. 'He teaches us things we could never have suspected.'

Even the government has been won over. Former French Minister of Education, Jack Lang, introduced La Casserole des Enfants into schools and Hervé came up with a list of 10 different experiments that could be carried out by primary- school children.

Blumenthal believes Hervé is a true radical. 'If anyone is going to change the way we cook, it will be him,' he says.