What is Sous Vide?
Sous-vide is a method of cooking in which food is vacuum-sealed in a plastic pouch and then placed in a water bath or steam environment for longer than normal cooking times (usually 1 to 7 hours, up to 48 or more in some cases) at an accurately regulated temperature much lower than normally used for cooking (typically around 55 to 60 °C (131 to 140 °F) for meat, higher for vegetables). The intent is to cook the item evenly, ensuring that the inside is properly cooked without overcooking the outside, and to retain moisture.
As may also be done in traditional poaching, sealing the food in sturdy plastic bags keeps in juices and aroma that otherwise would be lost in the process.
Placing the food in a water bath, with the temperature having been set at the desired final cooking temperature of the food, avoids overcooking, because the food cannot get hotter than the bath it is in, as in bain-marie. In conventional high-heat cooking, such as oven roasting or grilling, the food is exposed to heat levels that are much higher than the desired internal cooking temperature; the food must be removed from the high heat prior to its reaching the desired cooking temperature. If the food is removed from the heat too late, it becomes overcooked, and if it is removed too early, it is under-cooked. As a result of precise temperature control of the bath and the fact that the bath temperature is the same as the target cooking temperature, very precise control of cooking can be achieved. Additionally, temperature, and thus cooking, can be very even throughout the food in sous-vide cooking, even with irregularly shaped or very thick items, given enough time.
The use of temperatures much lower than for conventional cooking is an equally essential feature of sous-vide, resulting in much higher succulence: at these lower temperatures, cell walls in the food do not burst. In the case of meat cooking, tough collagen in connective tissue can be hydrolysed into gelatin, without heating the meat’s proteins high enough that they denature to a degree that the texture toughens and moisture is wrung out of the meat. In contrast, with the cooking of vegetables, where extreme tenderness or softness is seen as undesirably overcooked, the ability of the sous-vide technique to cook vegetables at a temperature below the boiling point of water allows vegetables to be thoroughly cooked (and pasteurized, if necessary) while maintaining a firm or somewhat crisp texture. While the cell walls will generally not burst, the de-polymerization of the pectic polysaccharides that connect the vegetable cells together and/or the gelatinisation of starch in the vegetable can be achieved without overcooking.
From a culinary viewpoint, the exclusion of air is secondary, but this has practical importance: it allows cooked food to be stored, still sealed and refrigerated, for considerable times, which is especially useful for the catering industry, and it excludes oxygen from food that requires long cooking and is susceptible to oxidation, e.g., fat on meat, which may become rancid with prolonged exposure to air.