2009_Book_FoodChemistry

576 12 Meat
NO, N − 3
), low-spin complexes are formed, the
spectra of which are similar to those of MbO 2 .
The change of Fe 2+ → Fe 3+ is designated as autoxidation:
(12.4)
The oxygen molecule dissociates from the heme,
taking along an electron from the iron, after protonation
of its outer, more negative oxygen atom
to form a hydroperoxy radical, the conjugate acid
of the superoxide anion (cf. 3.7.2.1.4). The proton
may originate from the distal histidine residue
or other globin residues or from the surrounding
medium. Autoxidation is accelerated by a drop in
pH. The reason is the increased dissociation of the
protein-pigment complex:
Globin + Heme k 1
−→
←− Myoglobin (12.5)
k−1
Soon after slaughter, the meat has a pH at
or near 7, at which the equilibrium constant
of the above reaction is K = k 1 /k −1 =
10 12 −10 15 mole −1 . Since, during post-rigor,
glycolysis decreases the pH of the meat to 5–6,
myoglobin becomes increasingly susceptible to
autoxidation.
The stability of MbO 2 is also highly dependent
on temperature. Its half-life, τ, atpH5is2.8h
at 25 ◦ C and 5 days at 0 ◦ C. Fresh meat has
a system which can reduce MMb + back to Mb.
Although various enzymatic and non-enzymatic
reactions take part in this system, their contribution
to the preservation of the color of fresh
meat is unclear. One proposal suggests the participation
of a NADH-cytochrome b 5 -reductase.
This enzyme which was detected in meat reduces
MMb ⊕ and MHb ⊕ . In addition, a series of nonspecific
reductases, also known as diaphorases,
are supposed to play a role in this system. The
slow formation of MMb + can be reversed at
the low partial pressure of O 2 which is found
inside the cut of meat or in packaged, sealed
meat. Therefore, for color stabilty, packaging of
meat in O 2 -permeable materials is not suitable
since, after a time, its reduction capacity is fully
exhausted. A non-O 2 -permeable material is
suitable for packaging meat. All of the pigment
is present as Mb and is transformed to the bright
red MbO 2 only when the package is opened.
Stabilization of the color of meat is also possible
under controlled atmosphere packaging.
A gaseous mixture of CO and air appears to be
advantageous.
Copper ions promote autoxidation of heme to
a great extent, while other metal ions, such as
Fe 3+ ,Zn 2+ or Al 3+ , are less active.
12.3.2.2.4 Curing, Reddening
Color stabilization by the addition of nitrate or nitrite
(meat curing) plays an important role in meat
processing. Nitrite initially oxidizes myoglobin to
metmyoglobin:
Mb + NO ⊖ 2 → MMb⊕ + NO (12.6)
The resulting NO forms bright-red, highly stable
complexes with Mb and MMb + , MbNO and
MMb + NO:
(12.7)
Reducing agents, such as ascorbate, thiols or
NADH, accelerate the reddening by reducing
nitrite to NO and MMb + to Mb. Nitrosylmyoglobin,
MbNO, is formed, which is highly stable
when O 2 is absent. However, in the presence of
O 2 , the NO released by dissociation of MbNO is
oxidized to NO 2 .
The color of cured meat is heat stable. Denatured
nitrosylmyoglobin is present in heated meat, or,
due to dissociation of the proteinpigment complex,
heme occurs with NO ligands present in