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Strategies for Increasing Oxidative Stability of (Fresh) Meat Color

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Strategies for Increasing Oxidative Stability of (Fresh) Meat Color O X I D A T I V E P R O C E S S E S I N M E A T Strategies for Increasing Oxidative Stability of (Fresh) Meat Color CAMERON FAUSTMAN*, W.K.M. CHAN, M.P. LYNCH and S.T. JOO Introduction moglobin and myoglobin) and for electron transport during respiration (cytochromes) in the living animal. Skeletal Fresh meat color is determined by the oxidation status muscles which obtain energy primarily by aerobic means of myoglobin. Several reviews of myoglobin chemistry and contain relatively high concentrations of heme proteins when meat color stability, including cured and/or cooked meat compared to those which depend primarily on anaerobic color, have been published (Livingston and Brown, 1981; glycolysis. Giddings, 1977; Faustman and Cassens, 1990a; Renerre, Of the heme proteins found in post-mortem skeletal 1990; Cornforth, 1994). The purpose of this presentation is muscle, myoglobin is the most important for considerations to emphasize recent research findings which impact the oxi- of meat color. The majority of hemoglobin found in the liv- dative stability of myoglobin in fresh meat. Specific atten- ing animal is lost during slaughter as a result of exsan- tion is given to metmyoglobin reduction and antioxidant ap- guination. Some blood is retained in meat (Fleming et al., proaches for minimizing oxymyoglobin oxidation. 1960) and Warriss and Rhodes (1977) estimated the aver- Maintenance of oxymyoglobin and thus a desirable age residual blood content of butcher’s meat to be 0.3%. appearance in fresh meat has significant economic impact. Han et al. (1994) recently published a procedure for deter- Liu et al. (1995) noted that reduction of discoloration dis- mining the concentrations of hemoglobin and myoglobin in counts at the retail level could result in added value to the meat. While visible blood spots are considered undesirable U.S. beef industry in excess of $700 million per year. In in meat (Lyon et al., 1986), hemoglobin can provide a source addition to this sensory aspect, the redox chemistry of myo- of pigmentation in processed meat products. Both blood and globin is critical for considerations of lipid oxidation and spleen which contain high concentrations of hemoglobin appears to play an important role in the extent to which have been utilized for this purpose (Bittel et al., 1981; Mielnik internal color predicts cooking doneness in ground beef and Slinde, 1983; Oellingrath and Slinde, 1985). The use of (Hunt et al., 1995). blood at high concentration may result in excessive pigmen- Heme-Containing Proteins in tation and an unacceptable dark product (Caldironi and Muscle-Based Food Products Ockerman, 1982). Cytochromes are present in low concentrations in skel- Muscle-based food products are derived from the skel- etal muscle and not considered to be of primary importance etal muscle of livestock and fish species following slaugh- to meat color (Ledward, 1984). Pikul et al. (1986) investi- ter/harvest. Historically, there has been a differentiation be- gated the cytochrome c content of several different species tween red (dark) meat and white meat. The basis for this of poultry. As a percent of total heme pigments, cytochrome difference is the concentration of heme proteins found in c ranged from 2.33% to 2.58% in breast meat and 2.14% to particular muscles. Of specific interest are the heme pro- 4.23% in thigh meat. In cardiac muscle, cytochromes con- teins hemoglobin, myoglobin and cytochromes. These com- tribute a substantial portion of the pigment present (Drabkin, pounds are responsible for oxygen-binding and storage (he- 1950); heart muscle has been utilized to improve the color of bologna (Lozano and Cassens, 1984). Cytochrome c con- centrations may also affect darkness of breast muscle color in turkeys subjected to pre-slaughter stress (Ngoka and *C. Faustman, W.K.M. Chan, M.P. Lynch and S.T. Joo, Froning, 1982). The relative contributions of these heme Animal Science Department, University of Connecticut, proteins in a given muscle food may become more discern- Storrs, CT 06269-4040. ible in the near future as Boyle et al. (1994) recently pub- Reciprocal Meat Conference Proceedings, Volume 49, lished a procedure for quantifying the reduced and oxidized 1996. forms of hemoglobin and cytochromes in biological samples. 49th Annual Reciprocal Meat Conference 73 Structure ferences also exist and in part, reflect the need for oxygen storage and delivery. Red myofibers contain more myoglo- Heme proteins contain a globin moiety or amino acid bin than white myofibers (Cassens, 1977) and within a given sequence and heme. Myoglobin (MW ca. 17,000) contains carcass, the concentration of myoglobin will differ among a single polypeptide chain and heme group. Hemoglobin muscles based on their myofiber composition. The breast (MW ca. 64,000) can be essentially considered a tetramer muscle of domestic poultry is typically very white and ex- of myoglobin. tremely low in myoglobin, while thigh muscles are more The heme structure within myoglobin contains an iron red and contain higher concentrations of myoglobin. The atom bound within a protoporphyrin ring. Protoporphyrin is longissimus muscle of the pig contains a substantially lower a cyclic compound formed from the joining of four pyrolle concentration of myoglobin than that of the lamb or cow/ rings (porphobilinogen) by methene bridges (Kalyanasun- steer (Lawrie, 1985). The blue whale, which makes long dives daram, 1992). The binding of iron within the planar proto- and thus requires large stores of oxygen, contains greater porphyrin yields the metalloporphyrin, protoporphyrin IX, concentrations of myoglobin than terrestrial animal species. known commonly as ‘heme.’ In general, metalloporphyrins The control of myoglobin concentration in meat has are highly colored as a result of their cyclic conjugated been critical to the veal industry. The husbandry of ‘special- tetrapyrolle structure (Kalyanasundaram, 1992). The solu- fed’ veal calves is targeted in part to obtain a muscle food bility of myoglobin and hemoglobin in aqueous solution is product of relatively low myoglobin concentration without quite high while that of free heme is very low. Methods used anemia; dietary iron is controlled carefully (Bremner et al., to quantitate heme within meat products will typically em- 1976; McFarlane et al., 1988). Veal calves raised on iron- ploy protein denaturing conditions with subsequent extrac- supplemented milk diets display higher myoglobin concen- tion of heme into organic solvents (Hornsey, 1956). trations than their non-supplemented pen mates (MacDougall Heme iron may exist in a ferrous (+2) or ferric (+3) state. et al., 1973). Grain-fed calves generally receive more di- The use of the prefixes ferro- and ferri- to describe heme etary iron and yield meat which is more red in color than structures indicates the redox state of the central iron. Six that from special-fed calves (Buege, 1989). The addition of coordination sites exist on iron. Four of these anchor it to an iron chelator appears to assist in development of low the four pyrolle rings of heme. The fifth site, which exists at pigmented muscle in grain-fed veal calves (Pommier et al., right angles to the other four, serves to attach the heme to 1992). the amino acid sequence of the protein. In myoglobin (and hemoglobin), the sixth coordination site which is directly Myoglobin Chemistry and opposite to the fifth typically binds oxygen in the normal Perceived Color in Fresh Meat physiological functioning of the molecule. However, a vari- ety of chemical compounds may bind to the heme iron of Heme iron in myoglobin generally exists in a ferrous myoglobin and yield substantially different spectra and per- (+2) or ferric state (+3). A ferryl (+4) form may result from ceived color. hydrogen peroxide activation of the ferrous/ferric states, but The heme moiety of cytochrome c is covalently bound this is short-lived and is more important for considerations to the globin portion of the protein via thioether linkages of heme protein catalysis of lipid oxidation than meat color between cysteine residues and the heme molecule. The 5th (Kanner, 1994). and 6th ligands of cytochrome c are linked to the apoprotein The ferrous forms of myoglobin include deoxymyoglobin - thus the 6th position is not available to bind ligands as in and oxymyoglobin. Deoxymyoglobin heme iron lacks a hemoglobin and myoglobin (Lemberg and Barrett, 1973). ligand at the sixth coordination site (Dickerson and Geis, 1983). The color of deoxymyoglobin is purplish-red and is Myoglobin Concentration typically viewed when the deep interior of a muscle is ex- posed during meat cutting, or when freshly-cut meat is Myoglobins have been characterized in a wide variety vacuum-packaged. When meat is exposed to the atmosphere, of animal species (Drabkin, 1950; Rickansrud and oxygen binds to heme iron and forms oxymyoglobin, the Henrickson, 1967; Hapner et al., 1968; Satterlee and cherry-red pigment in beef. The process of myoglobin oxy- Zachariah, 1972) and used as the basis for identifying spe- genation is referred to as blooming and appears to differ in cies origin in meat products (Janssen et al., 1990). In gen- rate and extent among different species (Millar et al., 1994). eral, a high myoglobin concentration results in more red Ferric myoglobin results from heme iron oxidation in color within skeletal muscle. Myoglobin concentration in a either of the ferrous myoglobins. Partial or complete loss of given muscle is influenced by both genetics (Enfield, 1968; tertiary structure results in an increased rate of oxymyoglo- Boccard et al., 1979) and environment (Thomas and Judge, bin oxidation (Sugawara et al., 1995). 1970) Occasionally as a result of bacterial contamination, The effects of animal species, age and muscle type on green discolorations may form. These have been attributed myoglobin concentration have been summarized by Lawrie to the presence of sulfmyoglobin. Berzofsky et al.
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