A Grain of Salt by Dr. Joe Schwarcz

Author:Dr. Joe Schwarcz
Language: eng
Format: epub
Publisher: ECW Press
Published: 2019-10-08T00:00:00+00:00

Butchering Science

“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 color. 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, hemoglobin 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 color means the meat is not totally fresh. But 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-flavor. 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.

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