Handbook of Meat Processing

Technological Quality of Meat for Processing 37

(Fe 3+ , Table 2.3 ). Oxygen can bind to heme

iron only if it is in the ferrous state (Fe 2+ ).

However, many other ligands (CN, NO, CO,

N 3 ) can bind to either the ferrous (Fe 2+ ) or

ferric (Fe 3+ ) form. Water (H 2 O) can bind to

myoglobin (Mb) only if the iron is in the

ferrous form. Under low oxygen tension con-

ditions, Mb exists in the purple - colored,

reduced form (Fe 2+ ). Exposed to oxygen for

a short period of time, the central iron (Fe 2+ )

reversibly binds oxygen, producing oxymyo-

globin (MbO 2 ), which is bright pink or red.

However, when exposed to O 2 for an extended

period, the central iron atom can lose an elec-

tron (oxidized to Fe 3+ ), producing metmyo-

globin (MetMb), which is grey - brown.

Immediately post slaughter, the oxidized

form can be reduced by endogenous reducing

systems in the meat, as long as reducing

equivalents (NADH) are available and the

globin fraction is in its native state (undena-

tured). Over time, these reducing equivalents

are depleted and the pigment is irreversibly

oxidized. Oxidation also occurs rapidly if the

globin moiety is denatured by rapidly declin-

ing pH while the carcass is “ hot ” or by exces-

sively low ultimate pH.

In pigs, color variations may have been

inadvertently selected for as pigs were bred

for high gain/feed ratios and leanness. Brewer

et al. (2002) reported that genetic line had

signifi cant effects on a * value (redness),

which ranged from 9.2 to 11 (on a 15 - point

scale) among pigs from genetic lines known

yls derived from lipid oxidation, which may
contribute to off - fl avors, decrease fl avor
identity, and increase metallic fl avor (Yancey
et al. 2005 ). It can also increase fatty fl avor
and negative attributes such as painty,
cardboard, bitter, and sour (Spanier et al.
1992 ; Gorraiz et al. 2002 ; Bruce et al.
2005 ). Positive fl avor compounds, such
as 3 - hydroxy - 2 - butanone, 2 - pentyl furan,
2,3 - octanedione, and 1 - octene3 - ol, decrease
with aging; and negative compounds, such as
pentanal, nonanal, and butanoic acid, increase
with aging (Stetzer et al. 2008 ). Aging beef
can result in changes in umami taste.
Glutamic acid content more than doubles
during the fi rst 7 days of aging (Bauer 1983 ).
The potential benefi ts of aging for selected
muscles for fl avor development and tender-
ization must be weighed against the potential
development of off - fl avors.

Color

Color and appearance of fresh meat are major
factors in consumer purchase decisions
because they are presumed to be indicators
of meat freshness and quality (Brewer et al.
2002 ). Meat color is due to the concentration
of heme pigments (myoglobin, hemoglobin),
their chemical states, and the light - scattering
properties of the meat (Lawrie 2002 ). At high
pH, the heme iron is predominantly in the
ferrous state (Fe 2+ ); low pH accelerates
ferrous iron conversion to the ferric state

Table 2.3. Characteristics of various states of myoglobin

Pigment Ligand Conditions Iron State Color

Deoxymyoglobin H 2 O Very low oxygen tension

( < 5 mm Hg). H 2 O is ionically

(reversibly) bound to Fe ++

Fe ++ Purple - red/purple - pink

Oxymyoglobin :O 2 High O 2 tension (70 – 80 mm Hg).

O 2 is covalently bound to Fe ++

Fe ++ Bright red/bright pink

Metmyoglobin — Low O 2 tension ( ∼ 10 mm Hg). An

electron is lost from Fe ++

Fe +++ Brown/grey

Carboxymyoglobin CO: CO is preferentially bound to Fe ++

(compared to O 2 ). Stable during

storage and heating

Fe ++ Bright red