Role of Animal Genetics and WHC
The meat industry has known for years that some animals are more susceptible to a variety of production stressors than others. Pigs that are homozygous or heterozygous for the halothane (Ryanodine) gene are very susceptible to stress, which causes changes in the meat that predispose it to abnormal meat qualities ranging from pale, soft, and exudative (PSE) to dark, firm, and dry (DFD) pork. Carcasses with PSE meat have a faster than normal decline in pH post-mortem while the carcass temperatures are relatively high. These conditions lead to excessive protein denaturation, early cellular membrane disruption (that is, increased conductivity in PSE meat), and a lower than normal WHC. Rapid chilling will help minimize the severity this condition. At the other extreme, carcasses that are DFD have meat that is abnormally high in pH 24 hr post-mortem because the live animal metabolizes most of stored muscle glycogen before slaughter. This results in lower amounts of lactic acid accumulation during the conversion of muscle to meat. Chilling rate does not affect this condition because DFD meat is caused by ante-mortem stress.
Animals with the stress genotypes are often fast growing and heavy muscled, but because of the lower quality, breeding companies and pork industry organizations recommend eliminating this gene from breeding herds. WHC varies more between genetic lines of pigs than within genetic lines, so selecting breeding stock for higher WHC is possible.
Pigs with the RN gene (also known as the Napole gene; Lundström and Malmfors, 1985) have normal color and firmness, but have lower WHC because these animals characteristically have a higher than normal amount of glycogen in the muscle at slaughter. Hence, these carcasses develop a lower than normal pH (often near 5.2 to 5.4 (see Fig. II.4). Because their pH is closer to the isoelectric pHI (5.1), the lower WHC is mostly due to protein insufficiently charged to bind water and to higher than normal protein denaturation. However, pigs with the Napole gene may be slightly more heavily muscled and have more tender meat than pigs without this gene. Thus, while WHC and processing yields decline, some measures of quality improve.