Color Changes During Processing

In cattle and sheep, the normal color of living muscle tissue is purplish. This is a consequence of both the pigment myoglobin being at a high pH and any oxygen present carried by the blood being rapidly used up by the living tissues, effectively ensuring an anoxic state. Immediately after slaughter, the situation continues, so that for prerigor meat in an unoxidized anoxic state, the myoglobin is dark purple. It changes to a paler, bright red when at rigor with oxygen present. From this point, the color is determined by the proteins myoglobin and hemoglobin, which are affected by muscle temperature and pH postslaughter (111113). Changes in muscle proteins during rigor development change the light scattering properties of the muscle tissue, so that it becomes paler as the muscle pH falls to pH 5.5, that is, as the pH approaches the isoelectric point of the proteins (114). Translucent meat, which appears dark, occurs when the pH is greater than pH 6.0 and light scattering diminishes (111,114). Thus, antemortem depletion of muscle glycogen results in meat of high ultimate pH and hence darkening in color—so-called dark, firm, and dry (DFD) meat. With high-pH meat as well, there is a closer packing of the myofibrils, with a resultant greater translucency, so the meat is darker in color (14). The as

1 day aging at 10°C

6 days aging at 10°C

1 day aging at 10°C

6 days aging at 10°C

6.0 6.5 Ultimate pH

Figure 8. These results, at two aging times, were obtained from an experiment where the aging of some samples was inhibited by injection of zinc before rigor mortis. The shear forces of these muscles were high, were of similar values, and did not change with ultimate pH (O). There is a clear influence of ultimate pH on meat tenderness, where both high and low ultimate pH values result in tender meat, but meat with a moderate ultimate pH is tougher (•). After six days aging at 10°C, the meat with aging inhibited did not change, and all meat reached approximately the same levels of tenderness. The tenderness changes resulting from ultimate pH appear to relate to differences in aging rate, although there are still outliers arising from other factors. In the range of ultimate pH from 5.5 to 5.9, a small increase in pH results in large increases in toughness. Source: Ref. 108, used with permission.

6.0 6.5 Ultimate pH

6.0 6.5 Ultimate pH

Figure 8. These results, at two aging times, were obtained from an experiment where the aging of some samples was inhibited by injection of zinc before rigor mortis. The shear forces of these muscles were high, were of similar values, and did not change with ultimate pH (O). There is a clear influence of ultimate pH on meat tenderness, where both high and low ultimate pH values result in tender meat, but meat with a moderate ultimate pH is tougher (•). After six days aging at 10°C, the meat with aging inhibited did not change, and all meat reached approximately the same levels of tenderness. The tenderness changes resulting from ultimate pH appear to relate to differences in aging rate, although there are still outliers arising from other factors. In the range of ultimate pH from 5.5 to 5.9, a small increase in pH results in large increases in toughness. Source: Ref. 108, used with permission.

sociated physical conditions of firmness and dryness are also associated with closer spacing of the myofibrillar lattice.

With normal ultimate pH values of around pH 5.5, the rate of cooling will also affect the muscle proteins. Portions cooled quickly scatter less light than those cooled slowly, such as regions in the muscle center (eg, beef hind quarter). In addition, aging of the meat also lightens meat color (111). The appearance of the meat is modified by electrical stimulation, as the rapid pH fall results in a more youthful appearance of the muscle, which improves the quality grade in the United States (115).

The most dramatic effect on meat color occurs in pork from stress-susceptible pigs (116). When these animals are stressed before slaughter, there is an extremely rapid fall in pH to pH 5.4 while the muscle is hot. The end result is extreme light scattering via denaturation of myofibrillar and sarcoplasmic proteins, to produce PSE meat.

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