By now everyone has either heard of or consumed omega 3 eggs. Chicken farmers can…
A while ago, I saw a picture of off-colored ground beef in a supermarket posted on Facebook. There were plenty of comments complaining about how awful it was, but no one seemed to know much about meat color, why it changes or what it means, so I figured I’d post a summary.
Meat color is not actually a good indicator of spoilage. It’s more accurately assessed by smell. Meat color is dependent on the oxidation status of the pigment in meat called myoglobin. Oxygen is transported in the blood by hemoglobin and transferred to myoglobin in the muscle. When myoglobin is bound to oxygen it forms oxymyoglobin, which is right cherry red in color. Myoglobin without oxygen (i.e. deoxymyoglobin) has more of a purple color, and when the iron in deoxymyoglobin is oxidize it forms metmyoglobin, which is brown.
When meat is freshly cut, it has very little oxygen penetration and has the colour of deoxymyoglobin (purple). If the meat has undergone normal muscle to meat conversion, the pH is reduced and the enzymes that use oxygen don’t function well. As a consequence, oxygen from the atmosphere builds up and binds to deoxymyoglobin to from cherry red oxymyoglobin. When meat is graded, it is cut and allowed to “bloom” from 20-30 minutes before grading to allow the color to develop. In beef, if animals are stressed before slaughter and muscle sugar (glycogen) is used up, there is no sugar present to be converted to lactic acid, and the pH doesn’t drop. As a result, enzymes using oxygen continue to use it, no oxymyoglobin is formed, and the meat doesn’t turn bright red.
When meat colour changes to brown, it means the iron in the myoglobin has been oxidized and the process is dependent on the oxidation/reduction status of the meat. It is well known that feeding cattle vitamin E (a powerful antioxidant) can extend the shelf life of beef, particularly hamburger. Something else that will turn the myoglobin a bright cherry red is carbon monoxide, and the color formed is very stable. For this reason, it’s not allowed to be used to develop/maintain beef color, as the beef might be bright red in color but spoiled rotten.
Smell is a better indicator of spoilage. Things that you can smell indicating spoilage are aldehydes and ketones from fatty acid peroxidation, and products of bacterial amino acid metabolism (i.e. ammonia and some really gross biogenic amines as indicated by their names…putrescine and cadaverine).
Typically, in a store if hamburger is brown, it gets thrown out as consumers are not keen to buy it. At home, if you’ve had some burger in the fridge that’s been kept cold and it’s started to turn brown, but doesn’t have an off odour, it’s likely still OK to eat but should be used as soon as possible. And with the price of beef these days, it’s nice to know where to draw the line.