Can you judge meat by its color?

Surprisingly, the answer is no.

This inquiry began when I purchased a pound of ground beef at the Jim Thorpe Market. According to the sell-by date on the package, it was fresh. Through the overwrap film, I could see the product had a bright red color.

But when I opened the package and broke the ground beef apart, I was surprised to see that portions of the interior were brown. My first thought was, "Did the butcher put older meat in the middle and cover it with fresh meat?"

I spoke with Phil Zuber, a meat cutter at the Jim Thorpe Market, who took a package of ground beef from the meat case, opened it and split the meat open. It was red on the outside and had portions of brown on the inside.

"If we wait a few minutes, it should come back in color," he said, pointing to the brown portions. "What's going on is a hard thing for people to figure out," he said.

Freshly cut, a section of a beef roast is purplish-brown in color. After a few minutes, it begins to turn red. When that meat is ground, it comes out of the grinder bright red in color. But after it is packaged, within a few hours, opening the package will reveal that the portions of the interior of the ground beef will have a brown color.

So what's going on?

I asked an expert, Dr. Ed Mills, Associate Professor of Dairy and Animal Science with a focus in the meat product area at Penn State University.

"The bright red color on the surface of meat is due to its exposure to oxygen," Dr. Mills said, "and oxygen can only diffuse into the meat a certain distance."

He explained that the color of meat is due to a protein called myoglobin, a substance in the muscle used to store oxygen. Myoglobin is similar but different from the pigment in blood used to make hemoglobin.

In the absence of oxygen, each is a purplish color. When exposed to oxygen, they turn red. When fresh meat is exposed to air, it turns red. Butchers call this "blooming," and meat scientists call this binding of oxygen to myoglobin as "oxymyoglobin."

"In ground beef, when we grind the product the individual particles are exposed to oxygen," Dr. Mills continued. "For some period of time, the whole product, the surface as well as the interior, will have a red oxymyoglobin color.

"The muscle continues to use up the stored oxygen. Metabolic processes that use oxygen are still functional-slower than in living muscle but they are still taking place. So, the oxygen that enters the product during grinding is eventually used up and the myoglobin is subjected to oxidation or browning.

"If you bought the product at that stage, you'd have a package of ground beef the surface of which would be bright red because there is lots of oxygen there. The interior, instead of being purple, might be brown because of oxidized myoglobin pigment that forms.

"That same package of product held for a longer time might actually turn back to purple in the interior. The other thing that can happen is when you break it open, the brown color might dissipate and turn back to purple or red if it is allowed to remain exposed to air. If that occurs, that will signify that this is a very fresh product in spite of the fact that it looked brown."

Dr. Mills noted that color is a poor indicator of freshness.

"The code date on the package is probably better than anything else for indicating freshness," he said. "Because the color itself is so sensitive to environmental changes, it is really hard to know what is going on with respect to color."

Note that the sell-by date is based on proper refrigerated storage. If meat is purchased on a summer day and left in the trunk of a car for several hours, its shelf life may be significantly reduced.