Vegetarians beware, this post is all about meat.  More specifically, we look at the what makes meat the colour it is.  It all comes down to a handy molecule called myoglobin.  But first, a bit about twitching.

The difference between white and dark meat or white and red meat is a consequence of the different muscle cell types.  Muscle cells are commonly called muscle fibres.  White muscle fibres are also known as "fast-twitch" muscle fibres, and are geared towards (as their name implies) quick, sudden movements like a short burst of flight.  Most of the energy for white muscle fibre movement comes from the metabolism of glycogen, a networked polymer of glucose.  Glycogen metabolism doesn't require oxygen, but results in a buildup of lactic acid, which limits the length of time the cell can work before it needs rest to get rid of the accumulation of lactic acid and restore glycogen stores.

Red or, "slow-twitch" muscle fibres, by contrast, dominate in muscles that require prolonged constant effort, such as the legs of most animals.  Their primary source of energy is fat stores by way of cellular respiration.

Most animal muscles are made of a combination of white and red muscle fibres.  At one extreme, frog legs are almost exclusively white muscle fibres, since they make nothing but sudden fast movements.  Animals that are constantly chewing their cud, such as cows, have cheeks that are made up of only red muscle fibres.  Birds such as chickens or turkeys fly rarely, and only for short periods, so their breast muscles are mostly white fibres, while their legs are a combination of white and red.  Ducks are migratory birds, so their muscles contain a high proportion of red fibres to support extended periods of flight.

Most of raw meat's colour comes from a pigment called myoglobin, which is related to hemoglobin and binds oxygen to transport it around the cell.  Myoglobin, like hemoglobin, contains a heme group (pictured above) which contains a central iron atom, usually in the +2 oxidation state.  The colour of myoglobin is determined by whatever the iron atom is bonded to:  if it's bonded to an O₂ molecule, the myoglobin is bright red, whereas in the absense of oxygen it bonds to water and is a purple colour.  If the iron atom becomes oxidized, or loses an electron, the myoglobin turns brown.  This can happen after a prolonged time without access to oxygen, or in an acidic environment.

When meat is cooked, some of the proteins in it denature and become opaque, turning red meat pink.  At 60 degrees C, the myoglobin itself denatures and becomes tan-coloured, giving well done meat a brownish-grey colour.  Freezing for long periods of time can also denature the myoglobin.

Finally, curing meat can cause other molecules to bond to myoglobin.  Nitrite, used in cured meats like ham and bacon, reacts to form nitric oxide.  Myoglobin bonded to nitric oxide is pink in colour.  Smoking or barbequeing meat can also turn it pink‚ nitric oxide (named Molecule of the Year in 1992) is the culprit again.  This is the characteristic smoke ring‚ of smoked and barbequed meats that is prized by barbeque aficionados.

This post was generously contributed by long-time Jacks of Science fan Kate Cook studying at University of Waterloo. Thanks for the meat knowledge Kate!