Fresh Meat Discoloration in Retail Meat Showcases
With continued exposure to light, oxymyoglobin and myoglobin oxidize (the iron atom loses an electron) to form metmyoglobin, a brown or gray pigment. Metmyoglobin is also produced when oxygen is no longer available at high concentrations because it has been absorbed by the meat during the blooming process and/or because it has been consumed by aerobic bacteria as they grow.
This oxidation and discoloration occurs in equilibrium with the oxygenation reaction that causes fresh meat to bloom and for a time is reversible, with all three pigments found in fresh meat at any given time. But as the meat ages and the reducing enzymes become exhausted, the formation of the brown metmyoglobin pigment can no longer be reversed, and the fresh meat’s appealing red or pink bloom is replaced by an unappetizing and permanent brown or grey color.
Meat pigment oxidation is initiated when the light source emits high levels of certain wavelengths of the spectrum that match Soret bands, wavelengths that are characteristically absorbed by myoglobin. Ultraviolet and yellow light are strongly absorbed by myoglobin, so any light source that emits high levels of these wavelengths will tend to accelerate the rate of meat decomposition.
Various studies have confirmed that ultraviolet light leads to the discoloration of meat by accelerating the production of metmyoglobin. Light is so crucial to this decomposition process that when packaged meat from the same animal are stacked in a refrigerated meat display case, the packages that are kept in relative darkness at the bottom of the pile will remain red or pink, while the packages at the top that are exposed to the meat display lighting will soon turn brown.
The intensity of the damaging wavelengths of light, especially 254 nm UV wavelengths and 560 to 630 nm yellow wavelengths, and the extent to which the meat package is light permeable determine the rate of meat discoloration caused by photooxidation.
Other factors that influence the rate of metmyoglobin production are the temperature of the meat, the amount of oxygen available, and the amount of bacteria present. However, even frozen beef displayed at -25° C will discolor as the myoglobin continues to oxidize under display case lighting.