What is it?
The name translates as ‘under vacuum’, and it is usually associated with cooking vacuum-packed food in a heated water-bath, however the vacuum packing is not its most important feature.
Its most important feature is precise temperature control of the water.
Temperature is an important aspect of cooking, but it is almost never properly controlled for.
Usually, instead of measuring temperature, temperature is estimated indirectly by measuring time. Thus the instructions for cooking a roast (of a certain weight) might be “180C for 1.5h”. The recipe assumes that 1.5h is the time required for heat from the oven air (@180C) to conduct to the centre of the roast (without exceeding 70C, a well-done temperature). Note that the temperature of the roast is never actually measured, even though the temperature of the roast (and not the oven) is what determines its doneness. Furthermore, the internal temperature of the roast (70C) has got nothing to do with the temperature of the oven (180C).
I can see how this strategy evolved from days long-passed when time could be measured but temperature was more challenging to measure. The glacial progress of cooking means that this approximation still persists today. Sous-vide removes this guesswork.
With sous-vide, the meat is vacuum-packed to remove air (a very poor heat conductor), and the package is placed in a water bath (water is an excellent heat conductor). The water-bath is pre-heated to the temperature desired for the meat. For a medium-rare steak this is 55C. Heat-diffusion through the meat will bring the entire piece to 55C, uniformly.
One of the best illustrations of sous-vide is salmon. I tend to like it at 48C, however that is probably a bit well-done for many tastes: 45C might be preferable. That’s 3C. See how much control you have? These three degrees are more than enough to modify the texture and doneness of the final product.
What’s more, because the water is at the target temperature (e.g. 48C) the ‘thin’ bit of a salmon fillet cannot exceed this temperature while you wait for heat to diffuse into the middle of the ‘thick’ bit. The result is complete uniformity in texture across the fillet.
Sous-vide is ideal for a home cook. Food purchased can be vacuum-packed and stored in the fridge or freezer. Stored in this way it doesn’t oxidise, dry out, get freezer burn, or absorb other flavours, and it will last longer. When needed, it is popped into the water-bath and you go and watch TV, or make a sauce to go with it etc. Cooking is unattended time. If you cook too long it makes little difference, the food cannot get hotter than the water and so it can’t be overcooked. Then you just serve it and your done. If it was a steak you can give it a quick sear to create a crust (or blast it with a MAP-propane blow-torch, the kind you find in hardware stores not kitchen shops). If you change your mind, you can cool the cooked package (ice-water bath), refrigerate, and reheat it in a bath again when it suits you (which might be the next day). It will stream-line your cooking.
Unfortunately, the term sous vide (under vacuum) looks like it’s here to stay; however it is misleading. While a partial vacuum may have been used to extract air and seal the bag, the food is not in a vacuum. It is just in a bag without air. Neither is it under pressure (except atmospheric pressure). Furthermore, the vacuum is not the point — precision cooking, or controlled temperature cooking, would be more relevant and less mysterious terms.