Meat Pigment Chemistry
The color is the
first impression consumers have of any meat product and often is their basis
for product selection or rejection (Deda et al., 2008). According to Cornforth (1998), it is
the most universal quality gauge used by consumers to judge meat
freshness. Deterioration of meat color
may indicate that the product is spoiled or has lost its nutritional
valve. Although meat color is not a good
indicator of nutritional quality, it may indicate microbial spoilage. By
maintaining meat and meat product color, we can maximize consumer quality
perception (Zdzlaslaw, 2002).
The color of meat products is
determined by a combination of different factors including moisture and fat
content, but more important is the chemical form and concentration of the
hemoproteins, especially that of myoglobin (Adamsen et al., 2006). Myoglobin (80%) and hemoglobin (20%) are
the predominant meat pigments and accounts for the red color in meat (http://labs.ansci.uiuc.edu). The color pigment of the
muscle tissue is myoglobin while haemoglobin is the color pigment of the blood
(Feiner, 2006). Myoglobin is a complex protein, similar in function to the
blood pigment hemoglobin, in that they both bind with the oxygen, which is
required for metabolic activity of an animal. Although their functions are
similar, their roles are different; hemoglobin acts as an oxygen carrier in the
bloodstream, whereas myoglobin is essentially a storage vehicle for oxygen in
muscle (Feiner, 2006). In both pigments,
the heme group is composed of the porphyrin ring system and the central iron
atom bound with the globin; in myoglobin, the protein portion has a molecular
weight of 17,000, and in hemoglobin about 67,000 (Zdzlaslaw, 2002). The color
of meat from various species, such as poultry, pork, and beef, often differs in
redness, and one cause of this difference is the amount of myoglobin in the
meat (http://meat.tamu.edu/color).
Myoglobin Structure
Myoglobin is an oxygen-binding
protein (globular protein of 153 amino acids) of the muscle (Fennema, 1996). This is the pigment chiefly
responsible for the color of meat, though hemoglobin (the oxygen-binding
protein in blood) is also present in small quantities (Feiner, 2006).
Myoglobin is a monomeric protein consists of a single-chain globin
protein and a color giving heme group in the centre (Feiner, 2006; Fennema,
1996; Deman 1999). The heme group consists of a flat prophyrin ring exhibiting
a central iron atom (Fe2+). This iron atom has six coordination
bonds, each representing an electron pair accepted by the iron from five
nitrogen atoms; four from the porphyrin ring and one from a histidyl residue of
the globin (Fennema, 1996; Cornforth, 1998). The sixth bond is available for
binding with any atom that has an electron pair to donate, for example oxygen
and nitric oxide. The oxidation state of the iron atom and the physical state
of the globin play an important role in meat color formation (Feiner 2006; Zdzlaslaw, 2002). It has eight
alpha helices and a hydrophobic core. It has a molecular weight of 16,700 Daltons
(http://en.wikipedia.org/wiki/Myoglobin)
According to Feiner (2006) and
Cornforth (1998), myoglobin exists in three main forms, each producing a
characteristic color; purple deoxymyoglobin (Mb), red oxymyoglobin (MbO2),
and brown metmyoglobin (metMb).
In living tissue, the
physiologically active oxymyoglobin (MbO2) and deoxymyoglobin (Mb)
forms are maintained through the activity of metmyoglobin (metMb) reductase
enzymes (Feiner, 2006). These processes decline postmortem, and storage conditions become more important in
determining the proportion of each myoglobin form present (Warriss 2000). In muscle immediately after slaughter, beef meat color is a deep
purplish. As oxygen in the air comes in contact with exposed meat surfaces, it
is absorbed and combines with myoglobin, turning the meat a brighter color (Cornforth, 1998). This brighter red pigment is called oxymyoglobin. Oxymyoglobin (MbO2) is the pigment responsible for the
preferential bright red color of raw meat, and is formed rapidly in the
presence of oxygen at normal atmospheric pressure (Varnam and Sutherland,
1995). MbO2 predominates at fresh meat surface (Cornforth, 1998).
Deoxymyoglobin (dMMb) exists in
absence of oxygen, such as in the bulk of meat portions and in vacuum-packaged
meats (Fennema, 1996). The ferrous iron becomes oxidized by free radicals when
meat is stored for long periods of time, producing the brown pigment MetMb,
which also forms where oxygen-dependent meat enzymes and aerobic microorganisms
successfully compete with meat pigments for oxygen (Feiner, 2006). Myoglobin and oxymyoglobin
lose electrons (oxidize), turning the pigment brown colored called
metmyoglobin.
According to Zdzlaslaw (2002),
oxymyoglobin and myoglobin exist in a state of equilibrium with oxygen; the
ratio of these pigments depending on oxygen pressure. The heme pigment in meat
is slowly oxidized to metmyoglobin and the formed metmyoglobin cannot bind
oxygen (Cornforth, 1998).
The Effect of Temperature on Meat Color.
Myoglobin (only about 0.5% of the
wet weight of red meats), its response to heat largely determines the color of
cooked meat (Nicola and Rosemary 2006). Heating causes denaturation of the
globin, which then precipitates with other meat proteins. Denaturation of
myoglobin and other proteins begins between 550C and 650C
in meat, and most denaturation occurs at 750C or 800C
(Varnam and Sutherland, 1995). The rate of myoglobin denaturation decreases
with increasing meat temperature, and this is related to the simultaneous rise
in meat pH with cooking (Nicola and Rosemary 2006).
The three forms of myoglobin differ
in their sensitivity to heat. Deoxymyoglobin is the least sensitive to heat
denaturation, followed by Oxymyoglobin, then by metmyoglobin, though the latter
two (MbO2 and metMb) have fairly similar heat sensitivity. As the
globin is denatured, metMb forms the brown globin hemichromogen
(ferrihemochrome) and the other myoglobins are denatured to the red globin
hemochromogen, (ferrihemochrome) (Nicola and Rosemary, 2006). The latter is
readily oxidized to the former, so ferrihemochrome is present in larger amounts
in cooked meats (Varnam and Sutherland, 1995). Adequate cooking of meat
produces a color change to off-white, grey, or brown hues, depending on the
type of muscle (Nicola and Rosemary, 2006). The ultimate color depends on the
extent of ferrihemochrome formation, which in turn is a product of the initial
proportionality of the myoglobins, and the final concentration of undenatured
oxymyoglobin (Gorgulho, 2009). Myoglobin, oxymyoglobin, and metmyoglobin can all be changed from one
to the other when the appropriate conditions exist. (Nicola and Rosemary, 2006;
http://meat.tamu.edu/color).
A
brown pigment, which is denatured metmyoglobin, is formed with cooking, which
normally cannot be changed to form another pigment (Nicola and Rosemary, 2006).
The
Influence of pH on Cooked Meat Color
Normal fresh meat has a pH ranging
from 5.4 to 5.6 (Varnam and Sutherland, 1995). The amount of ferrihemochrome
formation from myoglobin during cooking is affected by initial meat pH
(Gorgulho, 2009). The muscle contains glycogen but with postmortem, glycogen is
broken down to lactic acid, lowering the pH due to a reduction in oxygen supply.
This acidification process continues until either the glycogen is consumed or
the low pH inactivates glycolytic enzymes (Varnam and Sutherland, 1995).
Meat with a pH above 6.2 tends to
have a tightly packed water-retaining fiber structure that impedes oxygen
transfer and promote longer survival of oxygen-scavenging enzymes, favoring Mb
rather than MbO2 (Varnam and Sutherland, 1995). The purple-red
myoglobin combines with the closed structure of the muscle to absorb rather
than reflect light, making the meat dark, firm and dry (DFD) and for the pale,
soft, exudative (PSE) meats, postmortem
glycogen levels are reasonably high, and the acidification is accelerated so
that the pH falls rapidly, while the muscle is still warm (Feiner, 2006; Adams
and moss, 2000). The combination of high temperature and low pH causes protein
denaturation, water loss and an open muscle structure. The low pH also tends to
promote oxidation of MbO2 and Mb to brown metMb, which combines with
high light scattering from the meat surface, giving the meat its pale color
(Adams and Moss, 2000).
The extent to which pH affects the
cooked color of meat varies between species (high pH lowers myoglobin
denaturation and meat becomes more red) (Nicola and Rosemary, 2006). Meat pH
also influences other factors that affect cooked meat color. According to
Nicola and Rosemary (2006), these factors include; fat content, freezing and
rate of thawing, the initial form of the myoglobin (for example, Mb is less
heat-sensitive and more stable at higher pH than other myoglobin forms), the
condition and structure of the muscle fibers (for example, DFD vs. PSE), and
the denaturation processes of other meat proteins, including enzymes.
Note that all references used in all postings related to the topic of sausages, meat and meat product colorings will be posted in the last article about this topic.
About the author
Mr. Sempiri Geoffery, the author of this article graduated from Makerere University with a Bsc In Food Science and Technology Degree in January, 2011.
Mr. Sempiri Geoffery, the author of this article graduated from Makerere University with a Bsc In Food Science and Technology Degree in January, 2011.
