Chlorophyll goes on the menu as top chefs lead the food science trend.

Take out your science books, class, because chlorophyll is the hot ingredient setting cutting-edge restaurants apart from the merely gourmet venues this autumn.

Take out your science books, class, because chlorophyll is the hot ingredient setting cutting-edge restaurants apart from the merely gourmet venues this autumn.

Guru of the new breed of chemistry-minded chefs is food scientist Harold McGee, whose book On Food and Cooking, The Science and Law of the Kitchen (Simon & Schuster, £14.99), analyses what happens in the kitchen - yet gives no recipes.

Head boys of the new food science trend are Heston Blumenthal of The Fat Duck in Bray and John Campbell from Lords of the Manor in Upper Slaughter. Confirming their status as culinary prefects, the just-published The Good Food Guide 2001 has made Campbell's one of two restaurants of the year in England and named Blumenthal chef of the year. Swotty Heston, who failed his physics A-level, is now teacher's pet, conducting food and taste experiments for fun with Dr Peter Barham, head of physics at Bristol University. Campbell, with his Bachelor of Science in International Culinary Arts and thesis in Marco Pierre White, also takes a studious approach, although he originally left school with no qualifications.

Like most academics, Heston's so enthused about his subject that he fails to see some restaurant customers might balk at the thought of eating chlorophyll. He happily lists it on the menu along with the risotto of crab and rocket, red pepper cassonade, crab ice cream and passionfruit seeds it has recently accompanied. "Unfortunately it sounds like chloroform," he says when asked about diners' reactions.

Anyone remembering school biology has every reason to feel confused because what appears at the table in this case is not the green plant molecule C55H72O5N4Mg, but a red treacly substance.

"Some people think the rocket in the risotto is the chlorophyll because it's green," Heston admits. "From a chef's point of view, however, chlorophyll is pigmentation and there are several colours we use." He extracts red chlorophyll from red peppers, green from parsley and uses them as colouring agents, garnishes and, to a limited extent, flavourings. "The red chlorophyll balances the sweetness of the other ingredients in the risotto," he explains.

Culinary chlorophyll can also be obtained from carrots and olives.

John Campbell has experimented with several different methods of extracting chlorophyll, having first come into contact with its use as an ingredient through Phil Howard of The Square in London. John's method involves blanching herb leaves for 40 seconds at a precise temperature, then liquidising them to give a green pasty substance. This is diluted in hot water and stirred constantly until the green specks and water separate completely. Then it's poured over ice and strained through muslin, leaving "something like oil paint". Makes the production of chicken stock seem like child's play, and it's also volatile; professional kitchens can only store it for two days.

"I wouldn't put the term chlorophyll on the menu," says John, who uses it for mash and salad dressings. "But I think that while it might seem strange now, in four or five years it could be the norm as intelligent people become more attracted to the cooking profession."

While chlorophyll may seem like a sci-food gimmick, it represents a welcome return to what restaurants used to be all about - the careful and time-consuming preparation of dishes too complex to make at home. Blumenthal and Campbell have the perfect antidote to those chefs charging high prices for the mozzarella, tomato and pasta combinations we can whip up ourselves. any night after work.

For that - and everything else, according to The Good Food Guide - they get top marks.