Order a medium-rare steak at a high-end restaurant these days, and you may slice through the meat to find that it is a perfect rosy pink not just in the center but from edge to edge and is encased in only the slimmest crust of browned meat. The secret to getting such stunning results consistently is the surprisingly simple yet powerful technique called cooking sous vide. Chefs first seal the food in special plastic bags, often in a vacuum chamber (sous vide is French for “under vacuum”) but sometimes with air or other gases. They then slow-cook the bagged food at relatively low temperature, typically 122 to 149 degrees Fahrenheit, for hours or even days in a water bath or steam oven.
To steakhouse chefs used to placing dishes under 1,800-degree broilers, this approach may seem unorthodox. But thanks in part to some famous chefs who have championed the technique, including Joël Robuchon, Joan Roca and Thomas Keller, cooking sous vide has started catching on quickly—even among home cooks.
The simple act of vacuum-packing food and immersing it in hot water changes the physics of cooking more than you might think. The usual goal in cooking is to bring the food to the specific temperature at which it is perfectly “done.” For many foods, such as fish and certain vegetables, the margin of error is quite narrow. But in traditional cooking, the high temperature of the pan, oven or grill pushes heat into the exterior of the food so quickly that a large temperature gradient forms between the surface and the core. A charbroiled steak, for example, soon becomes boiling hot just under the surface where the water in the meat is flashing to steam; that boiling zone can be a good 86 degrees F hotter than the medium-rare center, and conduction keeps transmitting the heat there even after the steak is pulled from the broiler.
When cooking sous vide, in contrast, chefs typically set the water bath temperature just one or two degrees higher than the core temperature they want to reach. A computer-controlled heater can hold the bath within half a degree of that temperature while the food slowly equilibrates. Because the temperature can’t go very high, overcooking is not really possible, so timing is much less critical. The vacuum packing prevents air from insulating the food, improves food safety and greatly slows oxidation reactions that can lead to unwanted color changes or off-flavors. Low temperatures won’t brown food, but a quick sweep with a blowtorch or a fast sear on a griddle can apply the final color and crust. The food can be done to a chef’s specifications, every time.