Liquid gold

The most important ingredient in cooking keeps a remarkably low profile. Chefs spend most of their hours at the stove trying either to keep it or get rid of it; in the oven, its presence or absence can ruin a roast; in the freezer, it lives in suspended animation; while at the sink, its purpose becomes more mundane.

The most important ingredient in cooking keeps a remarkably low profile. Chefs spend most of their hours at the stove trying either to keep it or get rid of it; in the oven, its presence or absence can ruin a roast; in the freezer, it lives in suspended animation; while at the sink, its purpose becomes more mundane.

To say that without water there would be no cooking is a little daft, since without water there would be nobody to cook for, but you take my point. Boiling water has the twin virtues of cheapness and a constant temperature, making it perfect as a cooking medium for all sorts of foods; in dried foods, such as rice, pasta and pulses, for example, it also rehydrates.

The water we drink or cook with is not as chemically simple as H ₂O might suggest. Aside from dozens of minerals and other impurities, water also contains a certain quantity of dissolved air, which helps to give it a fresh flavour, and is why a glass of water left standing for a while will develop small bubbles.

One man who has investigated water more thoroughly than most is Heston Blumenthal, the science-loving owner of The Fat Duck, in Bray. Trying to keep a fresh green colour in French beans has been an obsession for many years, and one he thinks he has cracked. "The secret is the calcium level in the water: if it's too high, the beans take longer to cook and start to oxidise." This produces the muddy, dark-green colour familiar to anybody who lived through the Dark Ages of British vegetable cookery.

Blumenthal now uses Volvic mineral water to cook vegetables: it has a notably low calcium level. My own experiments seemed to bear out his theory: I boiled four lots of green beans for exactly five minutes in tap water (of the hard, Clapham variety), in Volvic, in San Pellegrino (which has a very high calcium content) and in distilled water (obtained by a long-overdue defrosting of the freezer). I then stopped the cooking process by dunking them in iced water (tap water, not Volvic: I am not made of money).

All the beans came out more or less the same colour, but the high-calcium waters (tap and San Pellegrino) did not cook them so thoroughly: I then repeated the experiment with Volvic and tap water, cooking the beans for four minutes. The Volvic beans were perfectly tender and a good bright green; the tap water beans were distinctly al dente. Am I really going to cook with Volvic from now on? Well, no, but it was an illuminating exercise. I did not, by the way, eat the beans cooked in freezer water, although it is worth saving to top up the iron or the car radiator, assuming there are no frozen peas lurking in it.

The chef at Umu, London's smartest and priciest new Japanese restaurant, Ichiro Kubota, takes things a stage further by importing water from Japan to make his miso soup. It is, it must be said, a masterful miso - the water travels well - but it is a little ironic that the tanker bringing his water from Kyoto probably passes one going the other way, full of peaty Highland water for the Japanese whisky industry.

There seems to be a trend these days away from using meat stocks in soup (not the beefy French onion soup, of course, the curse of holidaying vegetarians) and towards using water. As Mark Broadbent, chef at the new Bluebird Club says: "Water is a great neutraliser - it allows the flavour of the main ingredient of a soup to shine through." He also repeats the well-known chef's tip for rescuing sauces. "If a sauce splits - hollandaise, for example - you can often rescue it by whisking in a few drops of cold water."

Vinaigrette also benefits from a few drops of water, especially a mustardy, emulsified dressing: water "lets down" the vinaigrette, makes it easier to handle, and unlocks flavour.

Water can also be the enemy in the kitchen. Take crackling, for example: a watery leg or shoulder of pork also has wet fat surrounding it, which will start to crackle only when the meat is overcooked to the point of being inedible. Try a joint of proper pork (Middlewhite or Gloucester Old Spots, not the ubiquitous Landrace) and you will see what I mean: actually, with a good pork roast, an occasional splash of water as the skin starts to crackle helps it to bubble.

Bacon, of course, only sizzles and spits because it is wet: if it has been cured by the injection of an unattractive cocktail of water and chemicals, this fluid will seep out during cooking. Dry-cured, or traditionally wet-cured, bacon will fry crisply and contentedly in its own fat.

Iced water is one of the main tricks of the professional kitchen: salads are bathed in a sinkful for an hour or two to clean them and perk them up, and vegetables are routinely cooked until nearly done, refreshed in iced water, and then saved until needed. Peeling strips of cucumber straight into iced water, then draining them and dressing at the last minute with a little oil and lemon juice gives an excellent crunchy tangle of cucumber to act as a nest for a salad of, say, smoked fish and horseradish. Immersing the cucumber strips in iced water actually keeps the cucumber's own water inside its cell walls: take them out for just a few minutes and the cell walls will collapse, allowing the water to escape.

Now that purified water is on tap - or at least in a jug - in many households, and is more and more being used for making drinks (hard water does not make good tea), it is only a matter of time before home cooks and restaurant chefs start using it for cooking as well. Maybe it's time to buy a few shares in Volvic.