“I think my butcher trying to put one over on me.”
“What do you mean?” I queried the caller.
“I think he is packing fresh hamburger around old meat.”
I began to sniff where this discussion was heading. “You mean when you dig into your packaged hamburger you find that while the surface is red, the inside has turned brown?”
“Exactly!” came the reply. “Should I throw it out?”
It was time for a little chat about the chemistry of meat colour. In a live animal, just like in humans, blood picks up oxygen as it passes through the lungs. Specifically, it is an iron ion embedded in a complex protein called hemoglobin that binds oxygen and delivers it to cells around the body. In muscle cells, haemoglobin transfers the oxygen to myoglobin, another complex protein that stores oxygen until it is needed. Every cell needs oxygen for respiration, the process by which glucose is “burned” to release energy.
Myoglobin is dark purple but converts to red oxymyoglobin when exposed to oxygen. The ratio of myoglobin to oxymyoglobin at any given time depends on the amount of oxygen available. When an animal is slaughtered, its blood is drained and no more oxygen is delivered to tissues. That is why the colour of freshly butchered meat is actually dark purple except on the surface where it is contact with oxygen from the air. However, if the meat is now wrapped in a material that does not allow oxygen to pass through, the surface turns brown.
There is some interesting chemistry taking place here. The small amount of oxygen that still remains in the air surrounding the packaged meat, instead of binding to the iron in myoglobin, ends up stealing an electron from it. This reaction, which can be enhanced by the presence of bacteria, results in the formation of “metmyoglobin” which is brown. This does not mean the meat is spoiled, although since it takes time for this reaction to occur, the appearance of brown colour means the meat is not totally fresh although this has no consequence when it comes to taste or safety.
What is the solution to keeping the surface from turning brown? Use packaging that allows oxygen to pass through, keeping the surface of the meat red. Polyethylene plastic wrap serves this purpose. But oxygen is a double-edged sword. It will keep the meat red, but it will also react with fat and cause rancidity and off-flavour. That’s why polyethylene wrap is suitable for a few days but not longer. Although the surface stays red thanks to the formation of oxymyoglobin, very little oxygen diffuses into the meat. That little oxygen results in the formation of metmyoglobin, which is the reason that the inside turns brown. This is what was noted by my caller.
Neither should meat be frozen in its original polyethylene wrapper because this plastic allows moisture as well as oxygen to pass through. Loss of moisture results in “freezer burn.” For freezing, meat should be wrapped in freezer paper that is coated with a special plastic layer such as polyethylvinylalcohol (EVOH) that is impervious to moisture.
As far as marketing goes, retailers would like to keep meat red for more than a couple of days in the refrigerator case. This is where modified atmosphere packaging (MAP) comes into play. In this case, plastics that are impervious to the passage of gases are used so that the headspace around the meat can be filled with a higher level of oxygen than is normally present in air. Such plastics usually are composed of different layers of polyethylene, EVOH and sometimes PVC that prevents the oxygen from escaping, keeping myoglobin in its oxygenated form longer.
The extra oxygen also suppresses the growth of dangerous Clostridium botulinum bacteria, but it can also foster the growth of bacteria that prefer a high-oxygen environment such as pseudomonas. The growth of these bacteria, however, can be hindered by including carbon dioxide in the headspace. There is also a trick that can be used to keep meat looking red longer. When carbon monoxide reacts with myoglobin it forms red carboxymyoglobin, so introducing this gas into the package will maintain the red colour. This is not allowed in Canada.
Vacuum packaging is a way to keep meat fresh longer. It prevents microbial contamination from the outside, and excludes oxygen to prevent fat oxidation. This also means that there will be no oxymyoglobin and the meat will be a dark colour. However, when removed from the packaging and cut, it will react with oxygen in the air and “bloom” to produce the red colour.
After hearing about the colourful science behind meat packaging my caller absolved her butcher of any questionable activity but she was still “going to play it safe” and discard the meat. It seems you can lead people to science but you can’t make them digest it.
By Joe Schwarcz
Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.
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