Meat color is a-changin’!

Let’s say you bought some steaks or a roast at the grocery store, brought it home and stuck it in the fridge. (Hopefully, stored on a plate on the bottom shelf.) You don’t get around to fixing it (that’s Texan for preparing it) for supper for a day or two. You take it out of the fridge and it has a brownish color. Maybe a few brown spots or maybe the whole thing is just a little browner than it was when you bought it. Has it gone bad? Should you throw it out? It wasn’t cheap, so you hate to throw it out. What to do?

Check the use-by date. Smell it. Does it smell bad? If it doesn’t smell bad and if you haven’t passed the use-by date, it’s probably ok.

Why is it brown in my fridge when it was red in the store?
 

Short answer. It oxidized. Muscle has an ability to prevent (really slow down) oxidation, but that ability runs out with time. Oxidized muscle is brown.

Long answer. Well, to understand why meat is brown, we need to understand why it was red, first.

Meat contains lots of proteins. Some are structural (they hold the meat together). Some are for contraction (remember that the meat was originally for moving an animal around). Some proteins hold onto oxygen to help provide the muscle with energy.

These oxygen-holding proteins are largely responsible for meat color. The main one is called myoglobin. Myoglobin is closely related to the hemoglobin that holds oxygen as it is transported through our blood. Just like we learned in sixth grade science class that blood comes in two colors (red when it is exposed to oxygen in the arteries and blue when the oxygen is gone in the veins), meat pigment comes in those two colors, too. When it is not exposed to oxygen, meat has a purple color. You can see this color right after it is cut. Meat scientists call this ‘deoxymyoglobin’ because the myoglobin doesn’t have any oxygen. If you buy meat that has been vacuum packaged, it will be purplish in color.

The meat will slowly take up oxygen from the air and turn red. In the meat business, we refer to the time it takes go from purple to red as ‘bloom time’. Most of the meat you buy in the store has been packaged so that the oxygen in the air is available to combine with the myoglobin in the meat. That’s why it is a pretty red color in the store.

We recently did a research project studying bloom time. This is a picture of some steaks from that study. The ones in the front had just been removed from a vacuum package and were the purplish deoxymyoglobin. The ones in the back had been setting out for an hour, and you can see the bright red color.

Purple to red… what about brown?

Ok. So, even though the animal is no longer living, the enzymes in the muscle are still somewhat functional. The enzymes in charge of using oxygen to produce energy will take the oxygen and metabolize it. To do this, the pigment (actually, the iron in the pigment) is oxidized (it loses electrons). When the iron is oxidized, the meat will turn brown.

 In fresh meat, other enzymes can fix the problem by reducing the iron (give it electrons back) and it will turn back to purple. Then, it can grab some more oxygen and produce more energy and the cycle begins again. This really only happens on the surface of the meat where it is exposed to oxygen, so at first, you have a red layer with the purple underneath (everywhere on the meat that is exposed to air will be red. You won’t see the purple unless you cut it). At first, the brown coloring is not around long enough to see. As the process slows with time, a brown layer of pigment will form between the red and purple.

Eventually, the enzymes will run out completely and the muscle will not be able to fix the oxidized, brown pigment and it will turn completely brown on the surface. That is the brown coloring you see. If you leave it long enough, the oxidation could spread to the fat and cause some off-flavors, but the paying attention to the use-by date should prevent that.

Here is a picture from a study we conducted. Some steaks were left in retail cases (in our lab) for a week and allowed to turn brown. Then we cut them into little cubes. You can see the completely oxidized brown coloring on the surface and the purple coloring in the middle.

Just remember to pay attention to the use-by dates on the package and if you don’t cook it by then, freeze it by then.  Use your nose and feel if the meat is slimy. If it’s smelly or slimy, cut your losses and throw it out.

I also have a video blog about why meat is red.