Sous-vide (//; French for "under vacuum") is a method of cooking in which food is placed in a plastic pouch or a glass jar 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. The temperature is 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.
Three characteristics of sous-vide cooking, each of which developed separately, are low-temperature cooking, containerized cooking that separates the food from its heating environment, and pressurized enclosure using full or partial vacuum.
Low-temperature cooking was first described by Benjamin Thompson, Count Rumford in 1799. He used air as the heat transfer medium in his experiments while attempting to see if he could roast meat in a machine which he had created to dry potatoes. In Thompson’s own words, the meat was: “Not merely eatable, but perfectly done, and most singularly well-tasted.”
Preparation of food under pressure, with or without heat, was developed by American and French engineers in the mid-1960s as an industrial food preservation method. As with Rumford, the researchers learned that the food had distinctive improvements in flavor and texture. As this method was pioneered, applying pressure to food through vacuum sealing was sometimes called "Cryovacking." The pressure notably concentrated the flavors of fruits, even without cooking. 
The method was adopted by Georges Pralus, a French chef, in 1974 for the Restaurant Troisgros (of Pierre and Michel Troisgros) in Roanne, France. He discovered that when foie gras was cooked in this manner, it kept its original appearance, did not lose excess amounts of fat, and had better texture. Another pioneer in sous-vide is Bruno Goussault, chief scientist of Sterling, Virginia-based food manufacturer Cuisine Solutions, who further researched the effects of temperature on various foods and became well known for training top chefs in the method. He developed the parameters of cooking times and temperatures for various foods. Goussault and Pralus independently worked on development of sous vide in the 1970s, and eventually became collaborators. It was Goussault who pioneered the marriage of vacuum sealing with low-temperature cooking. Pralus, considered the father of modern sous vide, cooked at higher temperatures. 
Sealing the food in sturdy plastic bags retains juices and aroma that, otherwise, would be lost in the process.
Placing the 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. 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 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 and 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, as 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.
The degree of accuracy and constancy of cooking temperature required varies with the food cooked. In some cases it is not critical. A 15-millimeter-thick (0.6 in) piece of fish will cook in 17 to 18 minutes at any temperature from 44 °C (111 °F) to 61 °C (142 °F). Such food can be cooked in a switched-off slow cooker filled with hot water and a thermometer. But for an egg, which has proteins that denature at different temperatures, maintaining precise, constant temperature is more critical.
Non-sous-vide cooking times are determined by when the center of the cooked item reaches a few degrees below the targeted temperature. Then heating should be stopped immediately. While the food rests, residual heat will continue to cook it for a while. If the heating continues, the food will be overcooked. Sous-vide cooking continues until the center of the food has reached its target temperature. If it continues after this, the food will not be overcooked, and it will not cook more after it stops being heated. The time taken for the center of food to reach the target temperature depends on the initial temperature, the thickness and shape of the food, and the temperature of the bath.
One limitation of sous-vide cooking is the fact that browning (Maillard reactions) happens at temperatures above the boiling point of water (100 °C (212 °F)). The flavors and ‘crust’ texture developed by browning are generally seen as very desirable in the cooking of certain types of meat, such as a steak. The flavors and texture produced by browning cannot be obtained with only the sous-vide technique. In many cases, chefs will brown meats and other foods before placing them in the water bath or after the sous-vide cooking, using techniques such as grilling or searing on an extremely hot pan. This secondary browning is done briefly, and sometimes at higher heat than normally used, so as to affect only the surface of the food and to avoid overcooking the interior. Similarly, the skin of fish can be cooked at high temperatures after the sous-vide to make the skin crisp.
Food safety is a function of both time and temperature; a temperature usually considered insufficient to render food safe may be perfectly safe if maintained for long enough. Some sous-vide fish recipes, for example, are cooked below 55 °C (131 °F). However, people with compromised immunity should never eat food that has not been properly pasteurized. Women eating food cooked sous-vide while pregnant expose themselves and/or their unborn children to risk and thus may choose to avoid unpasteurized recipes.
Clostridium botulinum bacteria can grow in food in the absence of oxygen and produce the deadly botulinum toxin, so sous-vide cooking must be performed under carefully controlled conditions to avoid botulism poisoning. Generally speaking, food that is heated and served within four hours is considered safe, but meat that is cooked for longer to tenderize must reach a temperature of at least 55 °C (131 °F) within four hours and then be kept there for sufficient time, in order to pasteurize the meat.
Pasteurization kills the botulism bacteria, but the possibility of hardy botulism spores surviving and reactivating once cool remains a concern as with many preserved foods, however processed. For that reason, Baldwin's treatise specifies precise chilling requirements for "cook-chill", so that the botulism spores do not have the opportunity to grow or propagate. Pasteurized food can then be stored for up to two weeks at around 3 °C (37 °F) sealed within the vacuum pack.
Some plastic bags may have plasticizers with endocrine disruptor properties that are released when heated. The degree of danger posed by plasticizers, when present, is controversial. However because of improving standards for food-grade plastics, plasticizers are less common than in earlier uses. According to Baldwin, the plastic pouches made for sous-vide cooking must be food-grade to limit the migration of plastic constituents into the food. He adds that they must also be heat-resistant with a softening point of 195°F (90°C) or higher, have low gas permeability to prevent freezer burn, and have adequate mechanical strength.
The sous-vide method is used in many high-end gourmet restaurants by chefs such as Heston Blumenthal, Paul Bocuse, Michael Carlson, Thomas Keller, Grant Achatz, Ferran Adrià, Joël Robuchon, Philippe Rochat, Alessandro Stratta, Charlie Trotter, Michael Mina and Joan Roca. Amtrak also uses this method for meals served on their trains, including the Acela Express.
Sous-vide has become common on cooking TV shows such as Iron Chef America and Top Chef, and even in restaurants such as Panera. It has also been used to quickly produce significant quantities of meals for hurricane evacuees. Non-professional cooks are also beginning to use sous-vide cooking.
Initially, enthusiasts used laboratory-grade thermal immersion circulators, often bought used on eBay, which required very careful cleaning and, even then, were not recommended for kitchen use.[by whom?] Beginning in 2008, Auber Instruments and Fresh Meals Solutions made available comparatively inexpensive yet highly accurate PID controllers with attached thermocouple probes that could be used to control commercial rice cookers, slow cookers, electric stock pots, and similar apparatuses. In late 2009, several machines intended for home use and less expensive than laboratory-standard equipment went on sale.
More expensive but more universally usable solutions are available in the form of combi steamers, which combine convection and steam cooking methods into a single device. Instead of water baths, they feature tightly-controlled steam generation systems (not requiring the use of a sealed bag). It is possible to duplicate some effects of sous-vide techniques through the use of a beer cooler filled with warm water, checked with an accurate thermometer, and resealable bags with the air removed to package the food for cooking. However, the heat loss involved in this technique makes it unfeasible for long-term (four-plus hours) cooking.
Sous vide, French for "under vacuum," implies that food should be sealed in a plastic bag with all the air removed. An alternative method is to place the food in an open-sided plastic bag and partially submerge the bag into the water, forcing out the air. This method involves clipping the open side of the bag to the side of the pot to keep water from leaking into the opening. The goal is to have the food completely in contact with the hot water to assure even cooking.
Lower-cost units are now available in the form of a stick which has a clip to attach it to the side of the cooking vessel. Sous Vide sticks provide a heating element, controller, and circulating pump. They are available online for $75 or more with the higher-cost units providing a higher degree of control over the water temperature. Devices are becoming more user-friendly, with apps affiliated with most of the devices, including brands such as Nomiku, Anova Culinary and Joule. Apps guide people throughout the steps of cooking, with some units having voice-control integration. They are connected to WiFi to control the device remotely.
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