Myoglobin is the heme protein primarily responsible for the colour of meat (Fox and Jay, 1966; Kim et al., 2010; Livingston and Brown, 1981). Meat colour is the most important attribute influencing purchase decisions (Mancini and Hunt, 2005), with deviations from the bright cherry red colour of meat responsible for product rejection (Suman and Joseph, 2013). Therefore myoglobin concentration is an important consumer trait. Myoglobin is the primary oxygen-carrying pigment of muscle tissues
53 supplying oxygen to the mitochondria within muscle fibres (Wittenberg and Wittenberg, 2007). Other heme proteins, including haemoglobin (Rickansrud and Henrickson, 1967) and cytochrome-c (Girard et al., 1990) are present but unlike myoglobin have little effect on the colour of meat (Ledward, 1992).
There are four major chemical forms of myoglobin which are responsible for meat colour, oxymyoglobin, deoxymyoglobin, metmyoglobin and carboxymyoglobin (Giddings and Solberg, 1977; Mancini and Hunt, 2005) with the relative proportions of these forms of myoglobin determining the colour of fresh meat.
Oxymyoglobin has the characteristic cherry red colour that is seen when myoglobin is exposed to oxygen in the atmosphere. Oxygen is required to maintain this state. Deoxymyoglobin is the deoxygenated state of myoglobin with a purplish red colour typically associated with muscle immediately after cutting. Low oxygen tension is required to maintain this state. Metmyoglobin is brown in colour and is formed when the ferrous iron in either oxymyoglobin or deoxymyoglobin oxidises to ferric iron (Livingston and Brown, 1981; Wallace et al., 1982). Carboxymyoglobin is formed when deoxymyoglobin binds with carbon monoxide forming a bright red colour. This commonly occurs during packaging with low levels of carbon monoxide (Hunt et al., 2004; Luno et al., 2000; Sørheim et al., 2001).
The chemical reactions of oxygenation, oxidation, reduction and
carboxymyoglobination cause myoglobin to move from one chemical form to another (Figure 2-7). During oxygenation, myoglobin is exposed to oxygen causing deoxymyoglobin to convert to oxymyoglobin (Reaction 1).
Over time, as the oxygen within meat is utilised, oxymyoglobin is deoxygenated back into the less stable deoxymyoglobin, which then becomes oxidised to form
54
metmyoglobin with oxidative triggers such as low oxygen partial pressure, temperature and pH (Mancini and Hunt, 2005) (Reaction 3). This is a two-step process as oxymyoglobin does not convert to metmyoglobin directly (Reaction 2). Enzymatic and non-enzymatic processes can reduce ferric metmyoglobin back into ferrous deoxymyoglobin (Reaction 4) with oxygen consumption, metmyoglobin reducing activity and nicotinamide adenine dinucleotide (NADH) availability significantly influencing the reaction.
Carboxymyoglobination occurs when carbon monoxide binds to deoxymyoglobin (Reaction 5). The chemistry of this reaction is not completely understood however deoxymyoglobin is more readily converted to carboxymyoglobin than oxymyoglobin or metmyoglobin are. When carboxymyoglobin is exposed to atmospheres free of carbon monoxide the carbon monoxide will slowly dissociate from myoglobin (Mancini and Hunt, 2005).
Figure 2-7 Interconversions of myoglobin redox forms in fresh meat. Source: AMSA Color Measurement Guidelines (AMSA, 2012)
55 2.6.5.1 Myoglobin concentration and fibre type
Myoglobin concentration is linked to muscle fibre type with type I muscles having a greater concentration of myoglobin (Pethick et al., 2005c). Therefore, along with ICDH activity, myoglobin concentration can be used as an indicator of oxidative metabolism. Furthermore, factors which influence oxidative capacity are likely to influence meat traits linked to muscle fibre type.
The increase in myoglobin concentration with age (Gardner et al., 2007; Pethick et al., 2005c) is in line with a change to an increasing proportion of type I muscle fibres and a more oxidative muscle metabolism (Brandstetter et al., 1998a; Greenwood et al., 2007; Suzuki and Cassens, 1983; White et al., 1978). As expected, higher levels of glycolytic type IIX fibres are associated with a reduction in myoglobin concentration (Gardner et
al., 2007).
2.6.5.2 Myoglobin concentration and growth
Myoglobin concentration is reduced in slower growing breeds of lamb which are less mature at a given age, such as Merinos (Gardner et al., 2007) and has been shown to be higher in fast growing lamb which are likely to be closer to their mature size (Gardner et
al., 2007).
Nutrition has been shown to impact muscle fibre type (Greenwood et al., 2006b; Moody
et al., 1980), which may subsequently alter oxidative capacity including myoglobin
concentration. Nutritional restriction in lambs, which in turn will restrict growth rates, has been shown to reduce myoglobin concentration with a magnitude of effect four times greater than as genotypic effects (Gardner et al., 2006b).
Selection for increased muscle using ASBVs for increased PEMD has been shown to increase the proportion of type IIX glycolytic fibres (Greenwood et al., 2006c; Wegner
56
et al., 2000) and reduce myoglobin concentration (Gardner et al., 2006b). However,
while selection for leanness using ASBVs for reduced PFAT also increased the proportions of type IIX glycolytic fibres, an associated reduction in myoglobin concentration was expected but not seen (Gardner et al., 2006b).
2.6.5.3 Production factors which influence myoglobin concentration
Myoglobin concentration is influenced by production factors including age, sex and genotype. Age is a strong driver of myoglobin concentration in sheep with myoglobin doubling in all muscles between 4 and 22 months of age in the Gardner et al. (2007) study. In this study the greatest increase in myoglobin occurred between 4 and 8 months of age. The increase in myoglobin with age is a consistent finding in sheep (Ledward and Shorthose, 1971; Pethick et al., 2005b) with Jacob et al. (2007) demonstrating an increase between 5 and 12 months of age. As with age, myoglobin concentration increases as lambs approach maturity (Gardner et al., 2007). As higher myoglobin concentrations are associated with increased redness in meat, at maturity animals have redder meat (Hopkins et al., 2007).
The influence of sex on myoglobin concentration is not straightforward. Although wether lambs have been shown to have reduced proportions of type IIX fibres than ewe lambs (Greenwood et al., 2007) and thus greater oxidative metabolism, Ledward and Shorthose (1971) showed that ewe lambs had 10% more myoglobin than wether lambs. In contrast to these findings, and to the hypothesis based on fibre type, Gardner et al. (2007) found no impact of age on myoglobin concentration (P > 0.05).
The impact of genotype on myoglobin concentration varies, with studies showing reduced myoglobin concentration in slow growing Merinos compared to faster growing breeds (Gardner et al., 2007; Gardner et al., 1999) while Hopkins et al. (2005) showed no difference between Merinos and other genotypes.
57 The inconsistent variation between myoglobin concentration with sexes and genotypes indicates that changes in fibre type with lamb growth do not explain the entire variation in myoglobin concentration and further studies are needed to explain the associations.