Food Chemistry

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 aut-
oxidation:

(12.4)

The oxygen molecule dissociates from the heme,
taking along an electron from the iron, after pro-
tonation 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 pro-
ton 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

−→

←−

k− 1

Myoglobin (12.5)

Soon after slaughter, the meat has a pH at

or near 7, at which the equilibrium con-
stant of the above reaction is K=k 1 /k− 1 =
1012 − 1015 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◦Cand5daysat0◦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 contri-
bution to the preservation of the color of fresh
meat is unclear. One proposal suggests the par-
ticipation of a NADH-cytochrome b 5 -reductase.
This enzyme which was detected in meat reduces
MMb⊕and MHb⊕. In addition, a series of non-
specific 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 pos-

sible 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 ni-

trite (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 sta-

ble complexes with Mb and MMb+, MbNO and

MMb+NO:

(12.7)

Reducing agents, such as ascorbate, thiols or

NADH, accelerate thereddening by reducing

nitrite to NO and MMb+to Mb. Nitrosylmyo-

globin, 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 com-

plex, heme occurs with NO ligands present in