• Thursday, May 4, 2017

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

    Today is STAR WARS day. May the 4th (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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