Color Changes During Storage and Packaging

Red meats vacuum-wrapped in oxygen-impermeable films maintain a dark purple color. Consumer education has not reached the stage whereby this supposedly unattractive color is associated with the inherent advantages of either vacuum or anaerobic packaging for chilled meat, and gasimpermeable packaging for frozen meat. When vacuum or anaerobically packaged meats are removed from their package and exposed to air, the myoglobin oxygenates and the meat color changes from purple to bright red. When this color change occurs, the meat is said to bloom.

When fresh meat is exposed to high concentrations of oxygen—for example, as occurs in air, in a high oxygen modified atmosphere pack, or in a pack with an oxygen permeable wrapping—the myoglobin is converted to the bright red oxymyoglobin, which is resistant to further oxidation. However, with packaging in materials of low, but measurable, oxygen permeability, the low partial pressure of oxygen facilitates the formation of another more stable oxidized compound, metmyoglobin. Metmyoglobin is brown, giving the meat an unattractive color. In effect, the myoglobin of meat exposed to either high oxygen or no oxygen remains in a satisfactory state.

Even with exposure to high oxygen levels, the meat will finally turn brown. Metmyoglobin is slowly formed first in the deeper tissues where the critical low levels of oxygen occur. Eventually enough browning is produced that it can no longer be masked by the bright red oxymyoglobin lying above. Presumably, consumer experience with long storage of chilled meats, with eventual browning, has led to the association of dark meat color with "old meat" and spoilage.

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