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TO CREATE perfectly cooked food, you need precise control over its temperature. It is this thinking that led to the invention of the sous vide method, in which food is cooked in a water bath held at a steady temperature. If you like splashing out on gadgets, you can buy the equipment to do this at home, but DIY methods also exist.

Why bother? Suppose you are cooking a thick steak and you want it to be medium rare. How well cooked a steak is largely depends on the maximum temperature the meat reaches, rather than how long it is cooked for. If you put it in a frying pan, heat is conducted from the surface of the steak towards the centre. By the time the middle of the steak is perfectly cooked, the outer parts are overcooked and dry. You end up with a thin layer of pink in the centre of the steak, and a fair bit of grey closer to the edges.

In a water bath, you can set the water at 54掳C 鈥� the temperature at which a steak becomes medium rare. You seal the meat in a vacuum bag and leave it in the bath for an hour or more, allowing ample time for heat to reach the centre of the meat, but no part of the steak gets any hotter than this. Then all it needs is a quick flash in a frying pan to brown the outsides, but the rest stays mostly pink.

Chicken breasts, which are made dry by traditional cooking methods, are much softer and juicier when cooked at lower temperatures. Although standard food safety guidance says chicken breasts should be cooked to a core temperature of 75掳C at 60掳C for 45 minutes.

One way to do this without specialist equipment is to heat up a large pot of water, check the temperature with a thermometer and turn the stove on and off to hold it within a narrow range. This method is fiddly and requires constant attention.

In his book The Food Lab, J. Kenji L贸pez-Alt recommends using a beer cooler box instead. These are designed to stay cold, but their insulation means they are equally good at staying hot. All you need to do is fill it with water a couple of degrees hotter than your target temperature. A hot tap might be good enough 鈥� if not, add boiled water from a kettle.

Sous vide means 鈥渦nder vacuum鈥� in French, but you don鈥檛 need a vacuum-sealer. You can use a zip-sealed freezer bag; you just need to squeeze out as much air as you can, since air bubbles act as an insulator. This is best done by partially lowering the bag of food into the water, squeezing out the air and sealing the top (while keeping your hands dry to avoid scalding yourself), then placing the bag fully into the water.

Vegetables cook well sous vide, but most need to reach 84掳C to break down their tough pectin molecules, which is trickier to achieve in a beer cooler.

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