- Have you ever wondered why just as you finally finished cooking a delicious steak and are ready to serve your guests, someone inevitably reminds yo to let the meat rest.

Does meat need to rest?

Is it actually tired in the first place?

It turns out that letting meat rest can make eating it more enjoyable and that's according to science.

So let's explore why.

I'm Sheril Kirshenbaum and on this episode of Serving Up Science, we're cooking up some steak and exploring the science behind why most so-called experts want you to let your meat rest.

Think about the last time you didn't like eating a steak or a burger.

Was it too tough, overly dry, dare I say leathery?

Yeah that doesn't sound good at all.

What it probably lacked was moisture.

When it comes to cooking, roasting, broiling or grilling meats, for the best flavor letting it rest helps keep what we often call the juices inside.

Now notice I didn't say blood, it's not blood.

The red colored liquid you see when cooking or cutting meat is actually water and myoglobin which is another protein that stores oxygen and gives beef its pinkish red color.

What we call white meats like chicken and turkey don't have as much myoglobin which is why they appear lighter in color and can taste particularly dry.

Now back to why we let meat rest.

Joining me in the kitchen today is my husband and sous-chef David.

Hi honey.

- Hi.

- He'll be cooking up some meat for us, well for science.

Let's get our ingredients for a medium rare skillet steak.

It's not all that complicated, we just need olive oil, salt, and a good skillet on a medium heat.

Looking at our steak, all of that meat was once muscle on an animal and when we add heat, a lot happens.

It shrinks and gets firmer, the color changes as fat breaks down and the water and myoglobin that we love, seeps out of the steak.

There are fibers in the muscle that contract and relax which has once allowed the animal to move.

These are what give meat what we tend to call its grain.

Within muscle fibers are two protein threads or filaments called myosin and actin.

During muscle contraction, the myosin threads grab on to actin threads, pulling them closer together.

For our steak in a pan, turning up the temperature actually changes those fibers.

First, the heat breaks down myosin threads altering their shape in a process called coagulation and making the meat shrink.

Water gets squeezed out of the muscle which begins to happen at around 100 to 120 degrees Fahrenheit, notably before the meat is cooked to a safe temperature to eat which is at 145 degrees according to the USDA.

The actin breaks down at higher temperatures, from 150 to 163 degrees Fahrenheit.

When that happens, your meat is going to get very firm as the process pushes even more water out.

If we remove all of that moisture, we wind up with a dry, overcooked steak that's not going to impress anyone.

And this is the reason professional chefs aren't so pleased when a customer requests their dish well done.

It won't be juicy and might even get sent back to the kitchen.

- It's well done.

- We're cooking our steak medium rare, so we'll remove it once it hits the critical 145 degrees Fahrenheit.

At that point, you can safely remove your steak without worrying about getting sick.

And now, it's all about how pink around the center you're aiming for.

Once I take the steak off of the heat, it gives those coagulated myosin proteins a chance to relax a little bit.

As the meat rests, the moisture that had been squeezed out of the muscle fibers have the opportunity to seep back in.

- Sorry about that.

- There's a lot of smoke.

Just about done.

- 145, all right.

- 145.

Now the kitchen smells great, I'm ready to eat but we need to let the meat rest.

So we're gonna cover it with some foil but how long do we wait before preparing and eating it?

Let's start the clock.

Recommended resting times may vary and will depend on the thickness of the cut and cooking methods used.

Thin steaks or chops should rest five to 10 minutes while thicker cuts could sit 20 to 30 minutes.

Whole turkeys or large roast are best left resting for 40 minutes before carving.

So how do you spend that time is up to you.

I did it when I was a kid, you can do it.

- Well what am I supposed to do?

- Over.

- Look at this.

- Grab that.

- Okay.

- Down.

Perfect, that looks great.

- Is that your card?

- No.

- This one?

- Still no.

Hold, okay.

There, look, look at you.

Oh wait that's not really a move.

Time to wake you up.

- You wanna try?

- No, that's pretty grisly.

- This looks like more your size.

- It is moist, you've done well.

It's got its juices and I wouldn't call it leathery.

Well done, but not well done, but well done.