Effect of pH on Myofibrillar Proteins and Water Holding Capacity

Myosin, a highly charged molecule, undergoes some changes related to the pH of the muscle. At the pH of living tissue (approximately pH 7.1), the negative charges dominate, but at a pH of 5.5, which is closer to the muscle protein's isoelectric point, there is a similar number of negative and positive charges, and the capacity to bind water is least. When the muscle is in rigor, the bond between actin and myosin also causes the myofilament lattice to shrink, expelling water (13-15). Thus, for muscle in rigor that has a higher than normal ultimate pH, the lattice will not shrink so much, and the meat will bind water better than at a lower pH. Such meat with a pH greater than 6.0 is termed DFD (dark, firm, and dry). It has been suggested that changes involving the myosin heads by the temperature and pH conditions that exist (namely, rapid fall to pH 6.0 and below, when muscle temperature is still about 35°C) triggered by animal factors, result in pale, soft, exudative (PSE) meat that is caused by an even greater lattice contraction and greater water exudation than normal

7.2 7.0 6.8 6.6 6.4 x 6.2 a 6.0 5.8 5.6 5.4 5.2 5.0

Figure 3. Fall in pH of unstimulated and stimulated beef ster-nomandibularis (neck muscle) held at 35°C. Unstimulated muscle reaches rigor and ultimate pH (approximately 5.5-5.6) in eight hours. Electrical stimulation for 120 s results in a concurrent fall in pH (zfpH) to about 6.5. After stimulation, muscle pH falls at an increased rate. The combined effect of these pH falls is a decreased time for the muscle to enter rigor (approximately three to four hours). Source: Ref. 11, used with permission.

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