Heme proteins and oxidation in fresh and processed meats - Oxidation in Foods and Beverages and Antioxidant Applications - page 81

of the chains (Jolly and Taketa, 1984). Acetylation at the N-terminus can
make it difficult to determine the amino acid sequence of the protein subunit.
4.5 Discoloration in meat products
Color is a critical factor that consumers use to decide which raw and processed
meat products to purchase. This section deals primarily with mechanisms by
which heme protein oxidation (e.g., formation of brown pigments) occurs during
refrigerated and frozen storage. Autooxidation rate (k ox ) is a term used to
describe Hb and Mb oxidation rates (e.g., metHb and metMb formation).
Sodium chloride (e.g., table salt) accelerated Mb oxidation at post mortem pH
values (Andersen et al., 1988). Pink color in uncured, fully-cooked poultry and
beef is a separate defect that has been described (Cornforth et al., 1986; Seyfert
et al., 2004).
4.5.1 Effect of pH
Soon after death, the pH of muscle drops from around pH 7.4 (physiological pH)
to values ranging from 5.5 to 6.8 (post-mortem pH). This is especially
noteworthy because Hb and Mb oxidation rates increase as pH is decreased in
this pH range (Shikama, 1998; Yin and Faustman, 1993). Rapid Hb and Mb
oxidation at low pH is related to the ability of protons (H + ) to enter the heme
crevice and protonate liganded O 2 . The neutral superoxide radical ( · OOH) then
dissociates from the heme crevice resulting in met formation (Fig. 4.2, top). An
Fig. 4.2 Pathways by which hemoglobin and myoglobin autooxidize. Protons facilitate
release of neutral superoxide radical ( · OOH) from OxyHb. O 2 and a weakly coordinated
water molecule facilitate metHb formation from deoxyHb; the superoxide anion radical
(O 2 ·ÿ ) is also produced in this reaction. The unprotonated distal histidine (E7) provides
hydrogen bonds with liganded O 2 in OxyHb and water in metHb. Adapted from Brantley
et al. (1993).