So you already know how important it is to rest your meat, and you may have even gone and cooked your steak sous-vide. Surely, the only thing left to do is cut it and eat it, right?

Not so fast.

One of these steaks is not like the other.*

*Okay, okay. For all you language and logic pedants out there, it's true that if one of these steaks is not like the other, then both of these steaks is not like the other. But you know what I meant, right?

Can you spot the difference between the two hanger steaks? They were both cooked to a perfect 130°F medium-rare in the same pan, they are both cut from the same piece of meat, and they both sport a beautiful brown, crackly crust. Yet one of them is more tender than Otis Redding on a good day, while the other has more in common with a rubber band.

What's the difference? It's all got to do with the angle at which it's sliced.

We read it in cookbooks all the time: "Slice thinly against the grain." But what does slicing against the grain really mean? Well, meat is made up of bundles of long muscle fibers that are laid out parallel to one another.

Take a close look at your meat, and you'll see that just like wood, it's got a grain. In some muscles, like the loin (where NY strip and rib-eye come from) or tenderloin (a.k.a. filet mignon), that grain is very fine: the muscle fiber bundles are thin enough that they don't form a significant grain. Cuts from weak muscles like these will be soft and tender pretty much no matter how you slice them.

On the other hand, cuts from harder working, more flavorful muscles, like skirt steak, hanger steak, or flank, have thicker muscle fiber bundles with a clearly defined grain. Take a look here:

In this picture, I've labeled the three features you're most likely to notice on a piece of grilled meat.

Natural faults can occur at the interface between larger muscle groups, where the connective tissue meets the muscle, where the meat may have been folded during packaging or transport, or where a careless butcher may have made a nick in the meat (as is the case here).

can occur at the interface between larger muscle groups, where the connective tissue meets the muscle, where the meat may have been folded during packaging or transport, or where a careless butcher may have made a nick in the meat (as is the case here). Grill marks are probably the lines most often confused with grain. Many a time, I've seen backyard chefs start slicing meat at a 90° angle to the grill marks, rather than to the natural grain of the meat (which may or may not coincide with those grill marks).

are probably the lines most often confused with grain. Many a time, I've seen backyard chefs start slicing meat at a 90° angle to the grill marks, rather than to the natural grain of the meat (which may or may not coincide with those grill marks). The grain is the most important characteristic: it is the direction which the muscle fibers are aligned, and properly identifying it can make the difference between tough and tender.

You see, the fibers themselves are tough cookies. They have to be. Their job is to move all the moving parts of an animal that is much much bigger than you. Try and tear a single muscle fiber by stretching it along its length, and you'll have a pretty hard time. On the other hand, pulling individual muscle fibers apart from one another is relatively easy.

Try it: Get yourself a flank steak, cut off a small square of it, and try yanking it apart by holding it with the grain running between your hands. Can't do it, right? Now rotate it 90 degrees so that instead of pulling along the length of the muscle fibers, you are pulling them apart. Much easier.

So before putting a piece of flank, hanger, or skirt steak in your mouth, the goal should be to shorten those muscle fibers as much as possible with the help of a sharp knife. If you cut with your knife parallel to the grain, you end up with long muscle fibers that are tough for your teeth to break through. Slicing thinly against the grain, however, delivers very short pieces of muscle fiber that are barely held together.

Ah, tenderness...

Really, that's about all you need to know, so you have full permission to stop reading right now.

But! For those of you, who like me, had the greatest geometry teacher in the world in 9th grade and have thus been instilled with a preternatural desire to draw triangles and measure stuff, well, in the words of Mr. Sturm, get your gas masks, because we are climbing Mount Elegance, and the air up there is quite thin!**

** These words were usually immediately followed by "Kenji, don't get too excited, or I shall be forced to deliver a spanking!"

So final question to answer: quantitatively, how much of an effect does this actually have my meat? I mean, how much does it really matter which way I slice it?

Let's set up some definitions:

Let w be the distance you move the knife between slices (i.e. the width of the slice).

(i.e. the width of the slice). Let m be the length of the meat fibers in each slice.

Finally, let θ be the angle between the knife blade, and the meat fibers.

Given a bit of high school trigonometry, you can quickly come up with the following formula:

m = w / sin(θ)

So what are the implications of this? Well, if our goal is to minimize the length of the meat fibers (m), then we need to maximize sin(θ). In order to demonstrate, I cut a 1/2-inch window out of a regular piece of paper and layed it across a flank steak at various angles.

In retrospect, I should have used some grease-proof paper or plastic. Ugh.

Anyhow, as you can see in this first image, when the meat is cut 90 degrees to the direction of the meat fibers, sin(θ) is equal to 1 (i.e. maximized), and the meat fibers are exactly as long as the slice is wide. Now take a look at this:

In this picture, I've rotated the paper to simulate what a cut made at a 45-degree angle would do the meat fibers. This time, while the width of the slice is still .5 inches, the length of the meat fibers has reached .707 inches long (that's .5^(1/2), for all you nerds out there who get excited over 45-45-90 triangles). That's an increase of almost 50%!

Now take it to the extreme: if you were to cut perfectly parallel to the meat fibers, then sin(θ) will be equal to 0, and according to the unbreakable laws of mathematics, your meat fibers would stretch all the way into infinity (assuming the steak came from a really really really big cow, that is.)

So one last look at the first two steaks. Now can you spot the difference?

If not, I know the names of several good doctors who specialize in Attention Deficit Disorder.

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