It turned to the DARK SIDE: Why did my meat turn brown?

Today is STAR WARS day. May the 4^th (be with you). I’ve had several questions lately about meat that has turned brown. Has it really turned to the DARK SIDE?

Everyone that has bought meat has seen this happen. Maybe you take you steaks out of the package, and a little bit of brown is under the sticker on the package. Maybe you open a package of ground beef, and a little bit of brown is on the bottom of the package. Maybe you are marinating some pork chops in the fridge and they were brown in the afternoon when you got them out to cook.

It happens, and you have questions. Is it still safe to eat? Did the butcher hide this little spot under the sticker? Why did it turn brown? As a meat scientist, this is one of the most popular questions I answer.

Is it safe? If it has been kept at a cold temperature and is not way past its best-by date, most likely, yes, it is safe to eat. Smell it. You can’t smell the bacteria that will make you sick, but smelly bacteria will let you know if the meat has been at stored unsafe temperatures. If it’s not smelly, cook it using a meat thermometer.

Now, about that color…

People ask me or tell me all the time about how the butcher was trying to fool them by putting the brown spot under the sticker or how they put the fresh meat on top of the old brown meat. I realize it looks suspect, but that’s not what’s happening. It’s actually the sticker or the package that makes the meat turn brown.

I’ve talked about the changes in meat color before. It was one of my first posts and is actually one of my favorite topics and the subject of many of my research projects.

Meat turns from red to brown due to OXIDATION. If you think way back to chemistry class, you might remember that oxidation is the loss of electrons from a molecule. Meat color is controlled by a protein called myoglobin, and in the middle of myoglobin, there is an Iron. When that iron loses an electron, the protein changes shape and looks brown. We call the brown protein metmyoglobin.

So, what causes the oxidation? The change in meat color from red to brown can be caused by several events.

You pull off the packaging, and there
is the brown spot! Why?

1.  Very low levels of oxygen. That is what is happening underneath the sticker and in some packages of meat. We know that the oxygen in the air reacts with the myoglobin to make it turn red. However, when that oxygen is blocked by a sticker or part of the package, the level of oxygen is drastically lowered, causing oxidation and the brown color.

2. Time in storage. (Warning: nerdy meat scientist answer) This one is a little more complicated. When the meat turns red, the oxygen really only penetrates a little layer of the surface of the cut. So, you have a little red layer on top of a purple layer. Between those two layers of red and purple, there is a low oxygen environment and oxidation happens, so a little bit of brown, metmyoglobin forms. 

Luckily, the muscle has the ability to give the electrons back to the myoglobin (that’s called reduction, the opposite of oxidation), turning it back to purple. But, eventually, the ability of the meat to donate electrons runs out, and the brown color remains, creating a brown layer between the red and the purple. At first, you can’t see it, but with time, that brown layer will work its way to the surface that people can see. 

These steps are sped up in meat that has been on the shelf in the cooler longer (like aged meats) or by higher temperatures.

3. Salts and marinades. People love to add flavor to their meats in the form of marinades and rubs. I had a question about this just last week. Someone had bought some pork chops and placed them in the fridge to marinate all day. When they got them out to cook them, the chops had turned brown. Salt is actually a pro-oxidant. It causes oxidation. Some spices can cause oxidation, too. So it was probably the marinade that caused the meat to turn brown.

4. Freezing. Sometimes freezing meat can cause the color to change.

5. Bacteria. Yep, bacteria may produce sulfides, peroxides or other metabolites in the meat that cause it to turn brown. They also cause the meat to have a spoiled smell. We call these spoilage bacteria. They usually grow when meat has been stored at temperatures above 40°F, or stored for too long a time. When these bacteria grow, the meat turns brown, smelly, and maybe even slimy. Brown color with a putrid smell and slime are good indicators that pathogenic bacteria have also had a chance to grow and the meat may not be safe to eat. 

So, if your meat turns to the dark side, just give it a sniff. If it’s not smelly, you can probably still cook it. 

(Also, full disclosure, I'm really more of a Star Trek gal. Live long and prosper.)

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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.

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