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Instructions on how to be a man: Start large fire. Cook large steaks over large fire. Rip steaks from fire with bare hands, bite down, and allow succulent juices to dribble down chin.

Instructions on how to be a smart man: Start large fire. Cook large steaks over large fire. Rip steaks from fire with bare hands, allow steaks to rest in a warm place undisturbed for 10 minutes. Bite down, and allow succulent juices to dribble down throat.

This week at The Food Lab, we're going to explore the importance of resting meat. Asides from over/under-cooking/seasoning, not resting meat properly is probably the cooking blunder that we are all most guilty of.

You mean I have to wait before I can tuck into that perfectly charred ribeye? Unfortunately, yes.

Here's why:

This is a picture of a steak that was cooked in a skillet to medium rare (an internal temperature of 125°F or 51.7°C). The steak was then immediately placed on a cutting board and sliced in half, whereupon a deluge of juices started flooding out and onto the board.

The result? Steak that is less than optimally juicy and flavorful. This tragedy can be easily avoided by allowing your steak to rest before slicing.

I've always been told that this happens because as one surface of the meat hits the hot pan (or grill), the juices in that surface are forced away towards the center, increasing the concentration of moisture in the middle of the steak. Once the steak gets flipped over, the same thing happens on the other side. The center of the steak becomes supersaturated with liquid—there's more liquid in there than it can hold on to—so when you slice it open, all that extra liquid pours out. By resting the steaks, you allow all that liquid that was forced out of the edges and into the center time to migrate back out to the edges.

Sort of makes sense, right? Imagine a steak as a big bundle of straws, each straw filled with liquid, and representing the muscle fibers. As the meat cooks, these straws start to change shape, becoming narrower, and putting pressure on the liquid inside. Since the meat cooks from the outside in, the straws are pinched most tightly at their edges, and slightly less tightly in their center. So far so good. Logically, if the edges are pinched more tightly than the center, liquid should get forced towards the middle, right?

Well here's the problem: water is not compressible. In other words, if you have a two-liter bottle filled to the brim with water, it is (nearly) physically impossible to force more water into that bottle without changing the size of that bottle. Same thing with a steak.

Unless we are somehow stretching the centers of the muscle fibers to make them physically wider, there is no way to force more liquid into them. You can easily prove that the muscle fibers are not getting wider by measuring the circumference around the center of a raw steak vs. a cooked one. If liquid were being forced into the center, the circumference should grow. It doesn't—it may appear to bulge, but that is only because the edges shrink, giving the illusion of a wider center.

In fact, the exact opposite is the case. Since the center of a medium-rare steak comes up to 125°F, it too is shrinking, and forcing liquid out. Where does all that liquid go?

The only place it can: out of the end of the straws—or, the surface of the steak. That sizzling noise you hear as a steak cooks? That's the sound of moisture escaping and evaporating*.

* There are ways to minimize the shrinkage of muscle fibers, thus minimizing moisture loss, but that topic will be covered in a future Food Lab installment.

Give That Theory a Rest

So why does an un-rested steak expel more juices than a rested one? Turns out that it all has to do with temperature.

We already know that the width of the muscle fibers is directly related to the temperature to which it is cooked, and to a degree, this change in shape is irreversible. A piece of meat that is cooked to 180°F (82.2°C) will never be able to hold on to as much liquid as it could in its raw state. But once that meat has cooled slightly, its structure relaxes—the muscle fibers widen up slightly again, and it's this small change in shape that makes all the difference.

This image shows six steaks of identical thickness that were all cooked to an internal temperature of 125°F. I sliced one steak open every 2.5 minutes and placed it on a plate to collect any juices that leaked out.

Here's what's going on:

After No Resting: The meat around the exterior of the steak (the parts that were closest to the pan) are well over 200°F (93.3°C). At this temperature range, they are pinched tightly shut, preventing them from holding on to any moisture. The center of the steak is at 125°F. While it can hold on to some of its juices at this temperature, cutting the meat fibers open is like slitting the side of a soda bottle: some juice might stay in there (mostly through surface tension), but liquid is going to spill.

The meat around the exterior of the steak (the parts that were closest to the pan) are well over 200°F (93.3°C). At this temperature range, they are pinched tightly shut, preventing them from holding on to any moisture. The center of the steak is at 125°F. While it can hold on to some of its juices at this temperature, cutting the meat fibers open is like slitting the side of a soda bottle: some juice might stay in there (mostly through surface tension), but liquid is going to spill. After 5 Minutes of Resting: The outermost layers of meat are down to around 145°F (62.8°C) and the center of the steak is still at 125°F. At this stage, the muscle fibers have relaxed a bit, stretching open a little wider. This stretching motion creates a pressure differential between the center of the muscle fiber and the ends, pulling some of the liquid out from the middle towards the edges. As a result, there is less liquid in the center of the steak. Cut it open now, and some of the liquid will still spill out, but far less than before.

The outermost layers of meat are down to around 145°F (62.8°C) and the center of the steak is still at 125°F. At this stage, the muscle fibers have relaxed a bit, stretching open a little wider. This stretching motion creates a pressure differential between the center of the muscle fiber and the ends, pulling some of the liquid out from the middle towards the edges. As a result, there is less liquid in the center of the steak. Cut it open now, and some of the liquid will still spill out, but far less than before. After 10 Minutes of Resting: The edges of the steak have cooled all the way down to around 125°F, allowing them to suck up even more liquid from the center of the steak. What's more, the center of the steak has by this time cooled down to around 120°F, causing it to widen slightly. Cut the meat open at this stage, and the liquid will be so evenly and thinly distributed throughout the steak that surface tension is enough to keep it from spilling out on the plate.

The difference is dramatic. Just take a look at these two steaks:

In the steak on the left, all those delicious succulent juices are all over the plate. In the steak on the right, everything stays inside, right where it belongs.

But wait a minute—how do we know that those juices really are staying inside the rested steaks? Is it not possible that in the ten minutes that I've allowed it to rest that the liquid hasn't simply evaporated, leaving me with a steak that is equally un-moist?

To prove this is not the case, all you need to do is weigh the steaks before and after cooking. Aside from a minimal amount of weight loss due to rendered fat, the vast majority of weight loss comes from juices that are forced out of the meat.

The steak loses around 13 percent of its weight just during cooking. Cut it open immediately, and you lose an additional nine percent. But allow it to rest, and you can minimize this weight loss down to around an additional two percent.

Larger Meats

So that's all well and good for steaks, but what about for larger cuts of meat, say a whole roasted pork loin, or a prime rib? Well, the same principles apply here too. the main difference is they need to rest for longer. How long? Well there are various rules of thumb: five minutes per inch of thickness, ten minutes per pound, half of the total cooking time, etc.

By far the easiest and most foolproof way to test if your meat has rested long enough is the same way you can tell if your meat is cooked properly: with a thermometer.

Ideally, no matter how well-done you've cooked your meat, you want to allow it to cool down until the very center has reached 120°F (49°C). At this stage, the muscle fibers have relaxed enough that you should have no problem with losing juices. As shown in the graph, In a 1.5-inch-thick steak, this translates to around 10 minutes. For a prime rib, this may take as long as 45 minutes.

Congratulations: Your meat is now seven percent more delicious!

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