The Food Lab: Better Home Cooking Through Science

Here’s the reason: it has to do with the conversion of one of
the muscle pigments, myoglobin, and its exposure to
oxygen. Immediately after being cut, meat is a dark purplish
color—the color of myoglobin. Soon oxygen begins to
interact with the iron in myoglobin, converting it to
oxymyoglobin, which has a bright cherry-red color. Have
you ever noticed how when you cut into a rare steak in an
oxygen-rich environment (like your house) it starts out dark,
then “blooms” into redness? Now try the same thing in the
vacuum of outer space. See the difference? So, even though
the bright red color is the one most associated with
freshness, it’s really got nothing to do with it—purple meat
can be just as fresh. You are particularly likely to notice this
dark color in vacuum-sealed meat.
Eventually enzymes present in the meat will cause both
myoglobin and oxymyoglobin to lose an electron, forming a
pigment called metmyoglobin. It’s got a dirty
brown/gray/green color. While it doesn’t necessarily
indicate spoilage, it does mean that the beef has been sitting