If you spend much time reading cooking sites or even watching cooking shows on TV, you've probably encountered some version of the idea that "searing meat seals in its juices."
Whether it's expressed in passing or explicitly stated as a fact, it probably doesn't raise much of an eyebrow. After all, the idea that cooking a piece of meat over high heat results in a juicy piece of meat pretty much corresponds with our real-life experience.
And yet, because it's the internet, people love to argue. Does searing meat seal in its juices? Or is that, as some enthusiasts would have you believe, merely a myth?
In some sense, the answer depends on what you mean by "searing" and what you mean by "sealing." This means, in a way, it's a little bit like arguing about whether barbecuing and grilling are the same thing or how to really pronounce the word pho.
As in, as long as your steak turns out good, what difference does it make?
Is Sealing in Juices Even Possible?
Still, facts are facts, and if you're wondering whether any cooking technique whatsoever can produce a barrier on the surface of a piece of meat that will hold in its juices, the answer is no. Cooking meat causes it to lose its juices.
The real question is, of all the ways it is possible to cook a steak, is there one method that consistently causes the smallest amount of juice loss? Or to put it another way, what cooking method produces the juiciest steak?
And the answer is, searing.
What Is Searing?
So what is searing? Strictly speaking, searing is a dry-heat cooking technique that involves cooking a piece of meat over very high heat on a flat cooking surface like a skillet or griddle, using very little or no added fat or oil. Just heat up a dry skillet, set your steak down on it and you're searing it.
And while the discussion of sealing in juices mostly comes up in the context of cooking steaks, searing is also one of the first steps in braising a piece of meat.
Braising is a moist-heat cooking method in which a tough piece of meat like beef chuck or oxtail is slowly simmered in liquid until it becomes extremely tender and falls off the bone. But because simmering is a low-temperature cooking method, it doesn't cause meat to turn brown. The browning of meat (caused by a phenomenon called the Maillard reaction) only takes place at temperatures of around 300 F or so. Simmering is around 180 F.
Which is why it's customary to brown the meat before braising it—both for the sake of appearance (browned meat is more appealing) and also for flavor, since the Maillard reaction also produces all kinds of new and complex flavor compounds. In other words, browned meat tastes better.
The point is, because the goal of searing is to produce a flavorful, brown crust over as much of the meat's surface as possible, a flat cooking surface will do this better than, say, the ridges of a grill. You want as much hot metal in direct contact with the meat as possible.
And generally speaking, the hotter the better. Since you're not adding oil to the skillet, you don't have to worry about smoke points. This means if you can heat the surface of your skillet to 500 F or hotter, you'll get a better sear on your meat.
What Are Those Juices?
Now, when we said that cooking meat causes it to lose juices, what does that mean? What juices? And lose them how?
Meat is made up of muscles and muscles are in turn made up of bundles of muscle fibers, and these fibers are made up of individual cells, shaped like long tubes. These cells contain a small amount of liquid held in by a thin membrane. This liquid is what forms the meat's juices.
Now, heating a piece of meat causes the muscle fibers to contract, as if being squeezed. This contraction forces the liquid out of the cells, causing the meat to lose its juices. Some of it leaks right out during cooking, some of it is converted to steam and is lost through evaporation.
But this doesn't all happen at once. It takes time. This means that the longer a piece of meat is cooked, the less juicy it will be. Conversely, the less time a piece of meat is exposed to high heat, the more juicy it will be.
This is why searing produces juicy meat. Because it's an extremely high-temperature cooking technique, it cooks the meat so quickly that it doesn't have time to lose much liquid. Therefore, seared meat = juicy meat.
Don't Forget to Rest
Another thing about those muscle cells: cooking squeezes out the juices, but taking the meat off the heat and allowing it to cool will cause some of those juices to be reabsorbed into the cells.
That's why resting your meat after cooking it is so important. If you slice straight into it, those juices pour out onto your plate or cutting board. But if you wait a bit, those juices are reabsorbed into the cells, which means not as much spills out onto your plate.
The key, though, is that excessive exposure to heat will damage the cells, making this reabsorption impossible. That means if you overcook your steak, like cooking it medium-well or well-done, not only will most of its juices have cooked away, but what little remains can't be reabsorbed by the cells no matter how long you rest it.
But with that said, assuming you cook your steaks to medium-rare (or at most medium), it's fair to say that sealing in juices is as much a function of resting your meat as what method you use to cook it.