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.)

It’s all in the package: Ground Beef

I'm not sure why I have this silly face.
I love to take #meatcounterselfies!

A few weeks ago, I made a quick stop in a local grocery store to pick up some stuff for office lunches. Of course, I had to swing by the meat counter for a #meatcounterselfie.

While I was there, I found four different examples of packaging ground beef in the retail case. So, I snapped a few pictures and made a quick facebook post. My post was so popular, I decided to recreate here in the blog.

Foam trays with over wrap.
One of the most popular types of packaging

Foam trays with over wrap. It's kinda like cling wrap. In the world of meat science, we call this aerobic packaging. It's aerobic because it allows oxygen to react with the protein and creates the bright red color consumers like to see.

This packaging type is pretty inexpensive and easy, but the oxygen makes the meat spoil in a couple of days. You also shouldn't freeze meat packaged this way because it's more likely to freezer burn.

Ground beef chubs

We call these packages ground beef chubs. These are 10-pound packages, but you can get chubs in 5-pound, 2-pound, and even 1-pound. They are not always in clear bags like this. Sometimes the chubs are white and only tie at one end.

This beef was packaged in the packing plant. That's good because it decreases the number of people that handled it and lowers the chances that it will spoil. They are essentially a vacuum package, which is why you see that purplish-red color. The vacuum isn't perfect. Sometimes a little air will get in on the ends.

Beef can stay safely in this package for several days, and you can stick it directly in the freezer. My friend, Dr. Casey Owens, commented that she likes to buy ground beef in these big chubs and divide it into 1-pound portions in zip-loc freezer bags. That’s a great way to save some money.

Modified atmosphere package

This is called a modified-atmosphere package. This ground beef was also packaged in the packing plant, so the number of times it’s been handled is decreased compared to foam tray packaging. It's kind of like a vacuum package because it's sealed, but it has a special blend of air in the package to help control the growth of bacteria and give the meat that pretty red color.

I wouldn't use this package to freeze the meat; I would re-package it in a zip-loc freezer bag or a home-vacuum packager.

Vacuum-sealed package

Last is a vacuum sealed package. This beef was packaged in the packing plant and is a completely sealed package. See how it's a purplish-red color?

This package will have the longest shelf-life, and meat will freeze in that package just fine. It's also nice and flat, so it will thaw easily, too.

The meat counter at this store had several different options of ground beef, and, as a meat head, it was exciting to me to see all these different ways to package it represented in one store. But, please know that all these packaging types are safe. Regardless of how the beef is packaged or processed or any claims made on the label, all ground beef should be cooked to 160 F and checked with a meat thermometer.

I have another neat post called 10 things you didn’t know about ground beef or you may enjoy any of my other posts about beef, food safety, or the labels you see on packages.

What questions do you have about things you see in the meat counter?

Don’t judge cooked meat by its color.

Just… don’t.

Everyone, myself included, does it. We look at the color of the inside of a burger or chicken to determine if it’s cooked. Is it pink? Nope. Chomp chomp!

Here’s the problem: Color is not a good indicator of safe cooking temperatures. You have to use a meat thermometer to be sure meat is cooked to safe temperatures.

Meat turns from pink to brown because the protein that makes meat red (myoglobin) is denatured as it is cooked. The protein is damaged and doesn’t reflect light in the same manner anymore, so the meat appears brown.

Lots of factors can affect how quickly or slowly the protein is denatured as it is heated. Meat color changes can vary due to the pH (acidity) of the meat, the age and gender of the animal, how long it was stored, the way it was packaged, whether it was frozen, even the feed and water of the animals can affect cooked meat color.

Sometimes meat turns brown too soon!

Meat scientists call this phenomenon Premature Browning. It can be really dangerous because meat looks done, but it hasn't been cooked to a safe temperature.

This photo is from a great fact sheet about beef color from the National Cattlemen’s Beef Association. Both of these patties were cooked to an unsafe temperature (55°C or 131°F). Patty A looks very undercooked, but patty B looks done.

I have a previous blog post about fresh meat color. Remember that we talked about meat being oxygenated (red), deoxygenated (purple), or oxidized (brown)? Researchers at Kansas State found that when patties were cooked in the oxidized (brown) state, their cooked color was brown, even at temperatures that were too low to kill deadly bacteria like E. coli and Salmonella. Other research has found that the packaging can cause meat to brown faster, too.

Regardless of why the meat turns brown at too low of a temperature, sometimes it happens. The best way to insure that your meat is cooked properly is to use a meat thermometer.

Ground beef should be cooked to 160°F and poultry should be cooked to 165°F.

Sometimes meat stays pink too long!

Meat scientists call this phenomenon Persistent Pinking. It is not as much a food safety issue as it is a perception and eating quality issue. When meat looks pink, even if it’s been thoroughly cooked, people will think it’s undercooked and will keep cooking it until its way overcooked. When it’s overcooked, it tastes terrible.

I am conducting research on persistent pinking in ground beef this summer. 

These patties were all cooked to exactly 160°F.
You can see how some of them are still pink in the middle.

Research has shown that a high pH (more basic) can protect the proteins at greater temperatures and keep them from turning brown. They may also stay pink because of a higher concentration of the myoglobin protein. Right now our research is creating more questions than answers, but it sure is interesting.

Persistent pinking can also be caused by outside substances interacting with the meat and creating the pink color.

Nitrites are a good example of one of these outside substances. Sometimes we want this pink color to appear, like in the case of ham, sausages, or bacon. However, very small amounts of nitrites can get into the meat (especially poultry) and create a pink color that kind of looks like ham. If you don’t expect the meat to be pink and it is, you may think it’s undercooked.

These pictures were sent to me by Dr. Jim Claus at the University of Wisconsin. He is one of the leading researchers in persistent pinking in processed meats. These are a pair of turkey slices and some tuna chunks with persistent pinking problems. All of these were cooked to a safe temperature, but some clearly have some color issues.

This is a pork chop that was cooked wrapped in bacon. Looks like the nitrites in the bacon seeped into the pork chop and created a pink color.

Sometimes the ovens that cook the meat can introduce gasses that react with the muscle and cause it to turn pink. We like it when this happens in smoked meats, but when you don’t expect it, you may be concerned it hasn’t been cooked enough.

This is a smoked sausage from an awesome BBQ restaurant in south Texas. See how the smoke has created the ring of pink around the edge of the sausage?

When you see pink color in meat, look to see where the off-color is within the pieces. Undercooked meat will be pink in the middle, but persistent pinking issues are more likely to occur on the surface.

Even what the animals (especially birds) eat or drink can introduce substances that can change the cooked color of their meat. Nitrates and nitrites that occur naturally in the feed and water can remain in the birds and create some color problems, especially around the bone.

This is another picture of some cooked chicken pieces from Dr. Claus. You can see some really severe pinking problems in them.

If you are served something that you think looks underdone, it’s OK to ask what temperature it was cooked to. Sometimes meat is just stubbornly pink, but sometimes it may actually be underdone. When it comes to my kids, if I think a piece of meat they've been served may be underdone, you bet I'll ask the waiter about it. 

The best way to know is to use a meat thermometer.

Cook it to a temperature not a color.

The USDA has a great fact sheet about meat color that gives more detail about persistent pinking and premature browning.

What’s in a food label? Uncured, naturally cured or no nitrate or nitrite added.

This year I’ve been working on a series of posts about food labels and what they mean. In earlier posts, I talked about what the Natural label means on a meat package, but I get some questions about Uncured, Naturally Cured or processed meat products that are made without nitrate or nitrite.

I’ve covered this topic before in a post called ‘What is Nitrite?’, but I wanted to cover it again in the labeling series.

Some processors want to create friendlier, less chemically labels and choose to remove nitrates. Also, when meat processors want to use the Natural or Organic labels, they are not allowed to add nitrites and nitrates as they are classified by the USDA as chemical preservatives.

 

What if you just removed these ingredients from natural products?

Just take it out. Problem solved.

Some processors do that, but without nitrite, deli ham would not be pink, it would basically be just a pork roast. Tasty meats like bacon and hotdogs wouldn’t have the same flavors we enjoy. And, most importantly, all of these products would be more susceptible to spoilage and the growth of dangerous bacteria. The nitrite helps them last longer on store shelves and in your refrigerator. Nitrite also makes them safer for you and your family.

So, removing it doesn’t work.

What is nitrite anyway and what is its purpose in meat?

Nitrite is added to processed meats like ham, bacon, and sausages (hotdogs, bologna, etc) for 4 reasons:

1.       It prevents the growth of Clostridium botulinum (the bacteria that causes botulism). Botulism can shut down your nervous system and that’s not healthy. It also helps control other dangerous pathogens and bacteria that cause spoilage, so it helps keep meat safe.

2.       It is a very powerful antioxidant and keeps the meat from going rancid. The fat in processed meat can get funky flavors if allowed to oxidize, and nitrite helps to keep that from happening. Ever notice why a package of ham can last for weeks in your fridge while leftovers go bad in a few days?

3.       It gives cured meats their distinct pink color. The nitrite reacts with the muscle protein and changes it to pink, and it stays pink for a much longer time than fresh meat stays red.

4.       It gives cured meats their distinct flavor. That unique “hammy” and smoky flavor of a ham or that unique bacon flavor in bacon comes from the nitrite.



German researchers discovered that nitrite and
not nitrate (curing cousins) was the form of
curing salt responsible for meat curing, and
started to exclusively use nitrite for curing.

Also, without nitrite, several products would completely lose their identity. The USDA has standards of identity that regulate what is a hot dog, bologna, or even bacon and nitrite is an important ingredient for making them what they are. Without it, they are no longer “cured.” This means bacon without nitrite would no longer be bacon, but would instead be cooked pork belly.

How do “Natural” and “Cured” coexist?

Even though, nitrate and nitrite are not allowed to be directly added to natural and organic labeled meat products, other ‘natural’ ingredients with high levels of naturally-occurring nitrate can be used to replace the synthetic forms.

Many vegetables contain high levels of naturally accumulating nitrate. In fact, the main human dietary source of nitrate isn’t processed meats, but actually green leafy vegetables like spinach and celery. When the nitrate is converted to nitrite, presto… meat curing can naturally happen.

Meat processors can use vegetable powder in processed meats as a source of nitrite to create the pink color and cured flavor. On the label, it may be listed as celery powder, flavoring, or natural flavoring. The nitrite derived from vegetables and found in vegetable powder and in natural meats is exactly the same compound as that found in conventionally cured meats.

However, this substitution doesn’t replace all the nitrite needed to provide important quality and safety attributes. The final nitrite levels are lower and the vegetable powder may have to be limited because it can give the meat product its own flavors, too. These lower nitrate levels mean that the naturally cured meats are not as well protected from spoilage and pathogenic bacteria like Clostridium botulinum and Listeria monocytogenes. So, other steps must be taken to help keep the product safe. Meat processors add natural antimicrobial ingredients or use extra processes like high pressure processing to protect against spoilage and dangerous bacteria.

So what’s the difference, really?

Generally, natural meats are going to be more expensive because the ingredients that go into them are more expensive. However, when your dinner hits the table, natural and conventionally-cured meats should taste the same and both are safe and nutritious for your family.

  

For this post, I want to thank Dr. Jeff Sindelar from the University of Wisconsin for helping me explain all the nitrate/nitrite chemistry. Jeff and I have been buddies since graduate school, and he is a great meat scientist who has devoted his research to naturally-cured meats. You can see him talking about it in his Meat Myth Crusher video.

 

 

 

 

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.