"Sous Vide" is a French phrase that means "under vacuum" and it refers to the practice of cooking food in a vacuum-sealed bag for a relatively long period of time at controlled low temperature - usually the "target temperature" of the final product.
The main cooking device used in this technique is called an "immersion circulator" - you immerse it in the water, it heats the water to a pre-set temperature, and circulates the water to make sure that all of it stays at that temperature.
What happens when we cook meat? Meat is a composite of around 75% water, 20% protein (muscle fiber and connective tissue) and 5% fat. Each of these reacts differently to heat. Some of those reactions happen quickly and others only a little at a time. The fastest reaction is the denaturing of the muscle protein, which causes it to turn gray and tighten up, expelling the moisture contained within it. The melting of fat takes slightly more time. The rendering of tough connective tissue from collagen to gelatin takes far longer. (There is also the "maillard" or "browning" reaction which is a special chemical reaction of sugar and protein that takes place at high heat and makes new flavor compounds.)
In traditional high heat cooking methods, in the amount of time needed to cook meat to "medium" barely any collagen has rendered. So the only cuts you can quick-cook are those that are already tender. And cuts that need a lot of time to tenderize have already been well-done for hours before they are ready. Sous vide lets you render collagen by using a lower temperature for a much longer length of time.
Shabbat-Related Halakhic Questions
1) Can you cook sous vide over Shabbat if you plan to eat it on Shabbat?
2) Can you cook it over Shabbat if you plan to eat it AFTER Shabbat?
מתני׳ כירה שהסיקוה בקש ובגבבא נותנים עליה תבשיל בגפת ובעצים לא יתן עד שיגרוף או עד שיתן את האפר בית שמאי אומרים חמין אבל לא תבשיל ובית הלל אומרים חמין ותבשיל בית שמאי אומרים נוטלין אבל לא מחזירין ובית הלל אומרים אף מחזירין:
MISHNA: With regard to a stove that was lit on Shabbat eve with straw or with rakings, scraps collected from the field, one may place a pot of cooked food atop it on Shabbat. The fire in this stove was certainly extinguished while it was still day, as both straw and rakings are materials that burn quickly. However, if the stove was lit with pomace, pulp that remains from sesame seeds, olives, and the like after the oil is squeezed from them, and if it was lit with wood, one may not place a pot atop it on Shabbat until he sweeps the coals from the stove while it is still day or until he places ashes on the coals, so that the fire will not ignite on Shabbat. Beit Shammai say: Even after one has swept away the coals, it is only permitted to place hot water on it, as it is sufficiently hot and does not require additional cooking, but not cooked food. Since, in general, one prefers that food will cook more, there is concern lest he come to ignite the fire by stoking the coals. And Beit Hillel say: Both hot water and cooked food may be placed. Beit Shammai say: One may remove a pot from the stove on Shabbat but may not return it. And Beit Hillel say: One may even return it.
The Gemara comments: And now that the Master said that in these cases the prohibition of placing the pot on the fire is due to a decree issued by the Sages lest he stoke the coals; with regard to this pot of raw meat, it is permitted to place it in an oven on Shabbat eve at nightfall. What is the reason for this? Since it is not fit for consumption during the night, as it will not be cooked by then, he diverts his thoughts from it and will not come to stoke the coals. And the same is true of cooked meat; it is permitted to place it on the fire on Shabbat eve at nightfall. Since it is reasonably cooked, one will not come to stoke the coals to cook it more. Meat that is cooked and not sufficiently cooked is prohibited, as there is concern lest he come to stoke the coals. And if he threw a raw bone into this pot, he may well do so, as due to the bone he will not remove the meat to eat it in the evening.
Shehiya - Leaving food on the fire from before Shabbat.
There is a Rabbinic prohibition of shehiya to leave a pot of uncooked food on a stove or oven on Friday afternoon before Shabbat because one might come to stoke the coals on Shabbat.
The underlying concern with shehiya is not that the food is being cooked on Shabbat. The first perekrevealed the opinion of Beit Hillel that holds that there is no “shvitat keilim” on Shabbat. In other words, in general, one is not concerned if a melacha is being performed in one’s utensils without human assistance. The concern with shehiya is rather that if one sees their pot on the fire and the food is a little underdone, one may be tempted to ‘stoke the coals’ or in modern terms, turn up the heat, which equates to one of the melachot.
In the Gemara (Shabbat 36b) a discussion ensues as to how these three statements relate to each other. The result of this discussion has bearing on another debate regarding shehiya. Another beraitah is quoted in which Chananya maintains that once something is cooked to the level of ma’achal ben drusai it can even be left on a stove that has not be raked or covered with ashes. The Chachamim however argue that one can only leave a pot on a stove that has not been raked or cover with ash if it is fully cooked (m'vushal kol tzarko) and leaving it on the stove would degrade the quality of the food (mitztamek v'ra lo)
So, to avoid this gezerah there are few possible options: 1) Cover the fire, 2) Make sure that the food is already cooked to the point that it is edible even if it isn’t fully cooked, OR 3) Put in the food when it is raw.
Why did the Sages say that one may not insulate hot water for Shabbat in something that adds heat, even while it is still day? It is a decree lest one come to cover it in hot ashes that contain a glowing ember. People may not differentiate between addition of heat by means of hot ashes and other additions of heat. Abaye said to him: Let him insulate it with hot ashes, what is the problem? Rava answered him: It is a decree lest one come to stoke the coals in order to make them burn on Shabbat and thereby violate a Torah prohibition.
Hatana b'davar ha'mosif hevel - Insulating food with something that preserves or increases heat.
One is not allowed to insulate a food before Shabbat with material that preserves heat, such as sand, as a gezerah that one will come to insulate the pot with coals and stoke them.
Conversely, it is permitted to insulate before Shabbat with material that doesn’t preserve heat like a cloth. However, there is a dispute if a pot that is covered with clothes can be placed on a heating element, which would cause the pot to get hot. See further.
Everyone agrees that before Shabbat it is permissible to wrap a pot that is off the fire or on top of a hot pot which is off the fire.
According to most poskim, it is forbidden even before Shabbat to wrap a pot with clothes if the pot is on a covered fire or electric hotplate.
There is no issue of insulating food inside other food if there's no separation.
Lastly, one must have intention to insulate. The covering is permitted if the intention is to protect it from bugs or other things getting in/on it.
NOTE
But can you start it before, have it go through Shabbat, so that it is ready after Shabbat? Halakhic discourse really never dealt with that question specifically, but they did deal with this question:
Can you do hatmana food over YOM KIPPUR so that it is ready and hot for eating when the fast ends.
The Gaonim said that one cannot. But others say, why would this be problem?
Shulchan Aruch says you can, Rema says the minhag is not to do. When you want it for afterwards, there is no reason to play with the coals. But maybe the problem is how it looks on Yom Kippur, or the smell of it.
But whatever the reason, they are Yom Kippur specific, so there is no problem on Shabbat.
Kashrut-Related Halakhic Questions
1) Can you use the same immersion circulator to cook meat and pareve things for dairy meals?
2) Can you use the same immersion circulator to cook meat and dairy?
איתמר: דגים שעלו בקערה, רב אמר אסור לאכלן בכותח, ושמואל אמר מותר לאכלן בכותח. רב אמר אסור נותן טעם הוא, ושמואל אמר מותר נותן טעם בר נ"ט הוא.
It was said: Fish that "went up" on a plate, Rav said it is forbidden to eat them in kutach (a dairy sauce), and Shmuel said it is permitted to eat them in kutach. Rav said it is forbidden because it gives the taste [of meat], and Shmuel said permitted because it gives secondary taste.
The conclusion of the Gemara is that the halakha is like Shmuel, that secondary taste "notein taam bar notein taam" (nat bar bat) is permitted.
Rishonim
Rishonim debate the scope of the applicability of the rule of nat bar nat. The Rivan (cited in Tosafot Chulin 111b s.v. Hilchata) cites the opinion of his great father-in-law, Rashi, who limits the applicability of the nat bar nat leniency. He relates that Rashi believed that only fish placed on a meat plate is considered nat bar nat, since only a small amount of meat taste is absorbed into the fish. However, if fish is cooked in a meat pot, then the fish is not neutral (pareve) even according to Shmuel. This is because the fish has absorbed a great deal of “meat taste” from the meat pot. Rivan relates that once someone asked Rashi if an egg that was cooked in a dairy pot can be cooked with meat, and Rashi replied in the negative.
Tosafot, however, notes that a different impression is gleaned from Rashi’s (s.v. nat bar nat) commentary to the Gemara in Hullin 111b. Rashi explains that the fish attains the status of being “meaty” only if it is cooked with actual meat. Rashi clearly implies that if the fish is only cooked in a meat pot, then the fish remains neutral. Indeed, Rashi’s grandson, Rabbeinu Tam, and his great-grandson, the Ri, both believe that the nat bar nat rule applies even in a case of cooking, so that even if the neutral item was cooked in a meat or dairy pot, the cooked item remains neutral (see Hagahot Ashri, Hullin 8:29).
The Ran (Chullin 51a) explains that although generally, ta'am issur (taste of a forbidden thing) can transfer its prohibited status to food and utensils even when weakened, secondary taste of a PERMITTED substance (nat bar nat d'heteira) is different. Since the secondary taste of meat is so severely weakened, it cannot create, when combining with milk, a NEW issur.
Shulhan Aruch
Maran Rav Yosef Karo, in his Beit Yosef (chapter 95 s.v. Dagim) cites many Rishonim (including Rashba, Ran, Ravya) who subscribe to the most lenient opinion, that neutral food cooked or even roasted in a meat or dairy pot is still considered neutral. Indeed, in the Shulhan Aruch (Yoreh Deah 95:2), Maran rules according to the most lenient opinion that the nat bar nat leniency applies even to neutral food cooked or roasted in a meat or dairy pot.
The Rama thereupon notes that the Ashkenazic practice is to initially (lechatchila) be concerned with the strict opinion. That means, for instance, Ashkenazim should not place a neutral item cooked in a meat pot with dairy foods. If, however, the neutral food happened to have been mixed with dairy food (i.e., bidieved), the Rama records the Ashkenazi practice to follow the lenient view.
NOTE
Cooking dairy with an immersion circulator is certainly relying on this leniency l'chatchila, something the Rama says not to do.
BUT...this is definitely a case of nat bar bat d'heteira - secondary taste of a permitted thing.
BUT...food taste has to go from the food (1 nat) to the bag (2 nat's) to the immersion circulator (3 nat's).
BUT...since it is hard to know if the bag leaked, it is best to use TWO bags when doing sous vide if you want to do dairy and meat with the same device.