Chemistry Reference
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Effect' is used. This creates a large pool of deoxyHb that is in equilibrium with a
small pool of oxyHb and ample O
2
reactant. These conditions facilitate rapid Hb
oxidation at pH values near 6 because the relatively high concentration of
deoxyHb reacts with copious amounts of O
2
to produce metHb and superoxide
anion radical (
·ÿ
O
2
) (Fig. 4.2, bottom).
4.5.2 Effect of oxygen partial pressure
If the Root Effect Hbs are disregarded for a moment, low O
2
partial pressures
(PO
2
) are required for substantial amounts of deoxyMb and deoxyHb to be
present in post-mortem muscle. Ground beef turned brown rapidly in O
2
-
depleted atmospheres (PO
2
of 7mm Hg) compared to normal atmospheric
pressure (PO
2
of 160mmHg) (Ledward, 1970). Apparently the O
2
-depleted
atmosphere creates an environment in which a substantial fraction of the Hb and
Mb present exist as deoxygenated heme proteins and there still is enough O
2
available to promote met formation as described in the lower half of Fig. 4.2.
O
2
-depleted atmospheres (e.g., low PO
2
values) can occur just below the
surface of intact muscle since O
2
from the atmosphere only penetrates 1±4mm
into the tissue (Lawrie, 1974). Consequently it is sometimes observed that the
interior of a beef steak contains high levels of met heme protein (brown
pigments) while the surface and deep interior contains only reduced heme
proteins. The surface will be red in color due to oxy-heme proteins while the
deep interior will have purple hues due to the presence of only deoxygenated
heme proteins. The deep interior of post mortem muscle is anaerobic.
O
2
-depleted atmospheres can also occur at the interface of sliced products
that are `shingled' and when multiple pieces of muscle are pressed against each
other. Stacking of slices or pressing of muscle pieces causes there to be
substantial amounts of deoxyHb and deoxyMb that react with the small amount
of O
2
present at the interfaces which facilitates met formation.
Sodium ascorbate (vitamin C) and sodium erythorbate (a synthetic isomer of
vitamin C) have O
2
scavenging properties. Thus the potential of ascorbate to
lower the PO
2
to a point that accelerates Hb and Mb oxidation should be kept in
mind. If all the O
2
were to be scavenged by ascorbate, this should inhibit
browning since met formation-mediated by deoxygenated heme proteins
requires O
2
as a reactant (Fig. 4.2).
Respiring mitochondria at the surface of muscle can also compete for
atmospheric O
2
which can increase deoxyHb and deoxyMb concentrations at the
surface of the muscle. Consumption of O
2
by mitochondria occurs more readily
at elevated pH. This is the reason that beef at elevated pH can appear dark (with
purple hues) due to deoxygenation of the heme pigments at the surface.
4.5.3 Autooxidation of different Hbs
Perch Hb and trout IV Hb autooxidized 26-fold and 19-fold faster compared to
bovine Hb, respectively at pH 6.3 (Aranda et al., 2009). This was mostly
attributed to variation in amino acids surrounding the porphyrin group when
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