• High heat cooking methods (like the broiler) make heat move quickly
• Gentle cooking methods (like sous vide) mean heat moves more slowly
• Sous vide cooking ensures even heating—and cooking—all the way through your food
• Thick ingredients take longer to cook than thin ingredients
• Shape matters; a chicken cutlet will cook faster than a meatball
• The density of the ingredient being cooked also affects how quickly heat will penetrate to the center

When you broil a steak to perfection and then slice it open, you’ll notice that the dark brown crust covers a paler brown layer right under the surface. This gives way to a grayish-brown layer, then a grayish-pink layer, and then, depending on your taste, a rosy pink in the center. It’s not just dinner: It’s a delicious way to visualize the way heat moves. This color gradient illustrates how the meat cooks. The heat of the broiler strikes most intensely on the surface of the steak, and travels steadily from there to the center.

The trick, of course, is to time that traveling heat right. You want to broil (or sear or roast) your steak for just enough time—so that at the moment the center of the meat is cooked just to your liking, the surface is not yet dried out or charred. This isn’t simple work. A broiler can run over 500°F/260°C, and if you, like us, are into medium-rare steaks, the goal is to stop cooking the meat as soon as the center reaches 130°F/54°C. Talk about chaotic precision.

Cut open a sous-vide steak, however, and it’s the same color all the way through. That’s by design: if the water bath is set to 130°F/54°C, then the whole steak, inside and out, will become exactly 130°F/54°C—and not a degree more.

The great advantage of cooking sous vide, as demonstrated time and again, is that there is no fear of cooking unevenly; you can dial in the exact grade of doneness you prefer; and you can walk away from the kitchen for hours without overcooking your meal.

The disadvantage is that sous vide cookery can take significantly longer than the broiler method. The nature of heat is that it always moves through a material from a hotter region to a colder region—in the case of your steak, from the outside, where you apply heat, to the inside. And the bigger the difference in temperatures between the hot region and the cold region, the faster it moves. So heat will penetrate to the center of a steak that’s under a hot flame quite a bit faster than if the same steak is in a bath of warm water.

That means that in order to ensure our sous-vide meat is done—to ensure that the gentle heat has made its way all the way into the center and cooked it—we leave it in the bath for at least 1½ hours.

Since sous vide–cooked meat is the same temperature all the way through, it requires an additional step to brown the exterior. By searing it with high heat, we make sure that stage happens as quickly as possible, before the heat has a chance to travel into the interior and start to cook more than just the surface.

Because of the way heat travels, the thickness of the food being cooked in a sous vide water bath makes a big difference. A steak that’s 1‑inch thick might take 1½ hours to cook all the way through; a steak that’s twice as thick will take significantly more than twice as long.

The shape of the food is an important consideration too: Since the heat has to move from the surface to the center, a 2‑ounce cut of thin steak, which doesn’t have a lot of distance between its surface and its center, heats through a lot faster than a 2‑ounce meatball.

Finally, the type of food matters too, because heat travels at different rates through different kinds of materials. A piece of lean meat (like pork tenderloin) carries heat efficiently; fattier cuts (like pork butt) take a little longer, because fat acts as an insulator, slowing down the passage of heat. Fruits and vegetables (like rhubarb) contain a lot of air, both inside their flesh—an apple can be made of 25 percent air, believe it or not—as well as among and around their irregular geometry, such as the spaces between florets of broccoli. This is why we generally add liquid to the cooking bag. The liquid fills up those spaces, allowing for a more even cook, as well as adding flavor.