Sealing the food in sturdy plastic bags retains juices and aroma that otherwise would be lost in the process. Placing the packaged food in a water bath, with the temperature set at the desired final cooking temperature of the food, prevents overcooking, because the food cannot get hotter than the bath it is in, as in bain-marie.

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 and very thick items, given enough time.
How do you cook sous vide?
By contrast, 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, and it must be removed from the high heat prior to 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 undercooked.

The use of temperatures much lower than those used for conventional cooking is an essential feature of sous vide. When cooking meat, tough collagen in connective tissue can be denatured into gelatin, without heating the meat's proteins high enough that they denature to a degree that the texture toughens and moisture is exuded from the meat.

In contrast, soft vegetables are often considered overcooked. Low-temperature cooking allows plant matter to remain more succulent, as the cell walls do not burst. Cooking vegetables at temperatures below the boiling point of water allows them 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 depolymerization of the pectic polysaccharides that connect the vegetable cells together and 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. It also improves the transfer of heat between the water bath and food, without the thermal insulating properties of any trapped air in the bag.

Apart from ensuring uniform cooking, sous vide cooking facilitates development of desired organoleptic flavors and limits off flavors due to oxidation