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The PolyScience thermal immersion heater clamped to a pan of warm water in Chef Duncan Ly's kitchen at Calgary's Raw Bar looks innocuous enough.

In the stainless steel world behind this award-winning restaurant, it's just another piece of professional equipment. But ever since this little $1,500 machine - designed for precise temperature control in a lab setting - morphed into the latest cutting edge kitchen tool, it has caused a revolution in top restaurants and consternation for the food safety experts who police them.

The equipment is used for cooking sous vide or "under vacuum" - food vacuum-packed in plastic and slowly cooked in hot water, for up to 36 hours. It's a professional trick of the trade that top chefs are increasingly using to get that extra layer of flavour and texture in food, one that results in fork-tender proteins and intensely flavoured vegetables with all of their colour and vitamins intact.

But the technique can be risky. Without careful temperature control throughout the process there is a risk of deadly food poisoning. Since last year, New York chefs have been ordered by city health inspectors to cease using vacuum sealers and immersion heaters like Chef Ly's until they have an approved food-safety plan in place.

"There is a significant health risk - namely botulism - that can be associated with this process if it is not done correctly," says Sara Markt, press secretary for the New York City Department of Health and Mental Hygiene, who confirms that four New York restaurants have been fined for continuing to use sous vide equipment after being ordered to stop.

Even Chef Thomas Keller, the father of sous vide in North America, had to refrain from making his tender butter-poached lobster, squab and foie gras in the portable temperature-controlled tanks scattered throughout Per Se's kitchen until the city's food police approved his safety plan, now the standard for the food-processing industry. Since New York cracked down on sous vide, only a handful of restaurants, including Per Se, have qualified to resume sous vide cooking, while a few others are awaiting approval.

But in many other cities across North America, it's a growing, and largely unregulated, practice.

In Vancouver, chefs like Rob Feenie of Lumiere and Brian Fowkes of Rare consistently turn out their critically acclaimed cuisine with the help of sous vide. Chef Justin Labossiere says it was likely his lobster, artichoke and poached escabeche of shellfish - all prepared sous vide - that won him the top $10,000 prize at last year's Prince Edward Island International Shellfish Chefs Challenge. He now uses the technique to turn out up to 3,500 plates during playoff games at the Calgary Flames' massive new sports bar and restaurant complex, Flames Central and Wildfire Grill.

Cuisine Technology, the company that sells the PolyScience immersion heaters has a long list of customers on its website - and it reads like a who's who of top American chefs, from Jean-Georges Vongerichten to Charlie Trotter.

Sous vide cooking was first developed in the 1970s in France to preserve the delicate texture of foie gras, but chefs soon applied it to other foods. Portions of meat, fish, poultry or vegetables are vacuum-packed in a plastic pouch with flavourings such as herbs, spices and olive oil, then gently poached in hot water (usually below 62°C - about the temperature of hot tap water) for extended periods. The computerized thermal circulator keeps the water in the pan gently moving and warmed to a precise pre-set temperature, regulated to within one-tenth of a degree, often for many hours.

The science of sous vide cooking is straightforward: The ideal internal temperature is reached without ever overheating the food, so a medium rare steak retains its pink colour and juicy texture throughout.

But if the science is simple, mastering the technique can be complex. Once food is vacuum packed, the oxygen-free environment is perfect for growing deadly bacterium, like the clostridium botulinum that causes botulism. Chilled vacuum-packaged food must be kept very cold - below 4°C - and cooking should be at temperatures above 60°C, to keep food out of the "danger zone." Once cooked, foods need to be served immediately or chilled in an ice water bath.

"With ordinary cooking there is some degree of slack, but once you begin to vacuum pack things in an unnatural environment, you have to be pretty strict about temperature," says Harold McGee, a California food scientist and author who is currently working with Thomas Keller on a sous vide cookbook. "It doesn't matter how fresh your ingredients - everything has bacteria and bacteria multiplies at body temperature."

Professional sous vide chefs use a thin hypodermic needle thermometer, inserted through a piece of spongy self-sealing tape, to test the internal temperature of the vacuum-packed foods without breaking the package's vacuum seal. Meats are usually seared before serving, which not only crisps the food, but kills any surface bacteria.

It's a time-consuming process. But the results are delicious, and the precooked, chilled meats can be kept for several days before reheating for service.

It's no wonder that ambitious chefs like Chef Ly are experimenting with sous vide cooking.

"It has its place in the kitchen - just another technique you use as a chef," says Chef Ly, who has one thermal circulator set up to pre-cook chicken breasts and steaks, and another on the line to gently reheat the meats before they are seared and finished à la minute for service.

Adventurous home cooks are already starting to experiment with sous vide cooking. Clement Lo, a 26-year-old marketing professional, engineer and hobby cook, extols the virtues of sous vide cooking in his blog at http://www.alacuisine.org. He has found a way to approximate the sous vide technique using a pot of boiling water and a basic vacuum packaging machine, designed for household use.

"I like to do salmon sous vide - just vacuum seal it with a little olive oil and seasoning - then use a big stock pot of warm water, around 60°C," he says. "I have an instant read thermometer, strapped to the pot handle on a string, and if the temperature goes too high, I add a bit of cold water."

Mr. Lo says he's cooked duck confit sous vide for nine hours, and tried shrimp, butter-poached lobster and even pineapple using his home-style set up. But without the proper equipment, he's limited to relatively short forays.

"I can't do anything crazy - like cook a carrot for 36 hours - but if they ever came out with a $50 water bath machine, sous vide could become a lot more accessible," he says.

That dream may not be far off. According to Mr. McGee, Thomas Keller's sous vide cookbook will be released as soon as PolyScience perfects an immersion heater for the home cook. It may not be fifty bucks, but it may be the hottest new kitchen gadget for the cutting edge culinary set.