Why meat turns gray or brown

HostI was looking at some ground beef in my fridge this morning and noticed the outside was bright red but the middle of the brick was a dull gray. It made me wonder if I should throw the whole thing out or if it was just playing tricks on my eyes. What's actually going on with the way meat changes colors like that?

GuestIt's one of those things that seems like a warning sign, but usually it's just a bit of chemistry at work. The main thing to understand is a protein called myoglobin. This is the stuff that gives meat its color. A lot of people see that red juice in the bottom of the tray and think it's blood, but it's not. All the blood is gone long before the meat hits the store. That liquid is just water mixed with this protein. Its whole job is to store oxygen in the muscles of the animal. The fun part is that this protein acts like a mood ring. It changes its color depending on how much air it's touching. When it's tucked away where there's no air at all, it actually looks kind of purple or even a dark blue.

HostPurple meat? I don't think I have ever seen a purple steak at the grocery store. It sounds like something that has gone very wrong.

GuestYou actually have seen it, but probably only if you buy those packs that have all the air sucked out of them. When you tear that plastic open and let the meat sit on the counter for a few minutes, you can watch the color change right before your eyes. It goes from that deep purple to a bright, cherry red. We call this the bloom. The protein is grabbing oxygen from the air and changing its shape, which changes how it looks to us. That bright red is the color we all look for because we have been taught that red means fresh. But the irony is that the red color only happens once the meat is exposed to the air.

HostOkay, so purple is fresh and red is also fresh. But what about the gray I saw in my burger meat? That's the part that usually makes me want to order pizza instead.

GuestThat's where it gets tricky. In a big pack of ground beef, the meat on the outside is touching the air that gets through the plastic wrap, so it stays red. But the meat in the middle is suffocating. It's not getting enough air to stay red, but it's also not in a total vacuum, so it can't stay purple. It gets stuck in a middle ground where the protein starts to break down and turn a muddy gray or brown. It's still perfectly fine to eat. It just hasn't had its chance to bloom yet. If you were to break that meat apart and let it sit out, some of that gray might even turn back to red as the air hits it.

HostI don't know if I buy that. If I see a gray steak sitting on the shelf at the store, I'm not going to think it just needs to breathe. I'm going to think it has been sitting there for a week and it's starting to rot.

GuestAnd you would be right to be cautious. There's another way meat turns brown or gray, and that's from being out too long. After a few days, the protein loses its ability to hold onto that oxygen. It basically rusts. Just like a piece of iron turns brown when it sits in the rain, the iron in the meat rusts and turns the whole thing a dull, flat brown. This is what we see with older meat. So, you have two different paths to the same color. One is just a lack of air in the middle of a fresh pack, and the other is a sign that the meat is past its prime.

HostThat feels like a bit of a gamble for the person buying dinner. If gray can mean fresh and gray can mean old, how are we supposed to tell the difference?

GuestWell, the stores know we love that bright red color, so they use some tricks to keep it that way. Some places use a special mix of gases inside the packaging. They might pump in a little bit of carbon monoxide. It sounds scary, but it's safe. It locks that red color into the protein so it stays bright for weeks, even if the meat isn't actually that fresh anymore. This is why you can sometimes find a steak that looks beautiful and red but smells like it should've been thrown out days ago.

HostThat feels a bit like they're lying to us. If they're forcing the meat to stay red even when it's old, they're taking away the only tool we have to see if our food is safe.

GuestIt's a way to make the shelves look better, but you're right that it makes the color a bad way to judge. You have to use your other senses. Fresh meat should be firm to the touch and it shouldn't have much of a smell at all. If it feels slimy or if it has a sour, tangy scent, the color doesn't matter. It's time to toss it. Also, when you cook meat, it turns brown or gray anyway. That happens because the heat is finally breaking those proteins down for good. The color change we see in the pan is just the final version of what happens slowly in the fridge.

HostSo the red we see in the store is really just a snapshot of one moment in a long cycle of changes.

GuestThe most important thing is that a brown spot in the middle of your roast is just a sign that the meat is packed tight and the air can't reach it.

HostMy burger meat might look a little dull in the center, but as long as it smells like nothing, it's ready for the grill.