Why Does Raw Beef Turn Brown
A few evenings ago, I was consulted by a friend of mine about the color of meat and how to tell if it's bad. She sent me this photo….
Huh!? Brown meat!? That's right. Brown meat is OKAY to eat.. So what makes meat red in the first place? The most common answer people give me is blood. Well I hate to break it to you but there is actually no blood in muscles. All the blood is removed from the animal when it is slaughtered. That red liquid you see is actually water mixing with a protein that gives meat its red color, myoglobin. Myoglobin is a protein that stores oxygen for aerobic metabolism in the muscle. All mammals contain this protein in their meat tissues and is very similar to hemoglobin which stores oxygen in our red blood cells. This protein is normally a dark grayish-purple but when it comes in contact with oxygen, it becomesoxymyoglobin and reacts by turning a deep red color. That is why most of the meat we see has a bright red color.
But this color can vary, as we have seen before, from light red to an intense red to an almost purple color. Color in meat can change depending on the age of the animal, the species, sex, diet, and even the exercise it gets. The meat from older animals will be darker in color because the myoglobin level increases with as animals age. Exercised muscles are always darker in color. Because muscles differ greatly in activity, their oxygen demand varies which in turn means the same animal can have variations of color in its muscles. Also myoglobin levels vary by species which is why beef has more of a red color than pork or lamb. So why does meat turn brown?
Both myoglobin and oxymyoglobin have the ability to lose their oxidation which results in a brown color called metmyoglobin. This essentially means that meat can turn from a bright red color (which many associate with fresh) to a brown color from a lack of oxygen. Meat can also turn brown if any sort of contamination that would cause a chemical reaction comes in contact with it. For example, cure (sodium nitrite) turns raw meat a brownish-grey color (think of a cured, uncooked salami) if it comes in direct contact with a meat surface, but if that same meat is then heated, the sodium nitrite turns the meat a pinkish color (much like ham). In order for meat to maintain that bright red color we are familiar with, oxygen must be available at a sufficient concentration. That is why grocery stores utilize a small film over their products versus a vacuum package. Browning of meat can also occur with meat that has been chilled for a long period of time (about 5 days), ie: taken home from the grocery store and placed in your fridge for some time. This happens because as meat is chilled/frozen for long periods of time, enzyme activity decreases so the myoglobin and oxygen quit mixing together to keep meat that bright red color.
Browning of meat can also occur when oxygen partial pressure is low or basically when meat is stacked on top of one another. This is more than likely the case from the photo above. This is also the reason why your ground beef from the store may be red on the outside but brown on the inside. Oxygen can't readily make its way through or penetrate the ground beef so it begins to lose its red color on the inside after time. The changing from red to brown and even the purplish color to red occurs quite easily in meat, the reverse is much difficult. Once meat has browned, it is hard to get it enough oxygen to reverse the process. Also, this same process is the reason meat does indeed turn brown when you cook it. But once meat is cooked, it denatures the proteins so there is really no going back! All of the protein is not affected at the same time which is why you get different variations of a reddish color at different temperature points. Basically this is what gives us rare, medium rare, medium, well done, etc. Those colors associated with meat temperatures are basically denatured metmyoglobin!
So we've established once meat turns brown, it's hard for it to turn back to that red color. One myth I see commonly brought up is that old meat is dyed red. This is not anything any of us in the meat industry have heard of nor have we found information to supply this so-called practice. Since we are dealing with an enzymatic reaction here, I don't think any dye could possibly work as effectively as the reaction itself. When you see red meat in the grocery store, it's because it is a. actually fresh and b. allowed oxygen to keep it that red color.
So if color isn't an indicator of spoiled meat, what is…? The number one indicator of spoiled meat is in fact smell. An off odor will be prevalent to your senses and the most effective way to diagnose spoiled meat. Another indicator of spoiled meat is tacky or sticky to the touch. Slimy meat (not juicy) is also a great indicator of spoilage. This especially occurs if meat has been temperature abused. Raw meat that has been heated up (not cooked) and then re-cooled will often times become sticky or tacky along with possibly a color change. Use these three factors in diagnosing spoiled meat: does it smell, is it sticky, AND does it have color change? If all three or at least two of three are present (even with NO color change) than it's probably alright to toss it rather than risk getting sick.
So if you happen to open up a package of meat looking like the photo above, please don't throw it away simply because of its color. Use the three indicators given to diagnose if it's spoiled or not. If it is not spoiled, feel free to indulge without worry!
For more information on this topic, visit these sources:
The Color of Meat & Poultry from USDA
Meat Color from University of Saskatchewan
Source: https://chicolockersausage.com/2013/03/07/does-brown-meat-mean-its-bad/