(2) On exposure to air, an oxygen molecule is added directly to the iron
portion (oxygenation) of the myoglobin. This yields oxymyoglobin, which has a bright
red color. This step is often called "blooming." This is the reason oxygen-permeable
wrapping material is used with fresh meat. (Vacuum-packaged meat has a dark color
when opened due to the blocking of this reaction (oxygenation).
(3) Upon exposure to oxygen, further oxidation takes place, and the iron in
the oxymyoglobin is changed from a valence of +2 to a valence of +3 and this yields
metmyoglobin, which is a brown pigment. Dehydration and high temperatures
accelerate this reaction.
Further oxidation of metmyoglobin yields off-colored meat, which is often
g. Normal Cured Red Meat Color. Cured red meat color depends on the
reaction of nitric oxide (NO) with myoglobin to produce nitrosomyoglobin, which is a
pinkish red pigment. To obtain NO, sodium or potassium nitrate/nitrite is added to the
curing mixture. The nitrate is reduced to nitrite by bacterial reaction. The nitrite is
converted to nitrous acid and finally to NO, which is a gas. Low pH, ascorbic acid, and
other reducing conditions accelerate these reactions. The NO then reacts with
myoglobin to produce nitrosomyoglobin. Nitrosomyoglobin may be oxidized to the
undesirable brown pigment metmyoglobin. To block this reaction, cured meat is
wrapped in oxygen-impermeable paper. Normally, after curing, heat is applied to the
product during smoking. At this time, nitrosomyoglobin is converted to
nitrosohemochrome. The globin portion of the pigment is fragmented and the cured
meat takes on a bright purplish red color. See figure 2-3.
Figure 2-3. Cured red meat color changes.