An animal experiencing stress will have physiological changes including changes in heart rate, blood pressure, body temperature and respiration. Several stress hormones are released into the blood stream including epinephrine and norepinephrine. Epinephrine helps to break down glycogen (stored form of glucose in the liver and muscles) into glucose. The stored chemical engergy in glucose and oxygen are converted to energy for the animals muscles. The normal byproducts of this conversion are carbon dioxide and water. When there is not enough oxygen present for this conversion, glucose is still converted to energy, but the byproducts are lactic acid and water.
To understand the effects of stress on final meat quality, it is important to understand the relationship of glycogen and lactic acid to pH decline in meat after slaughter. An animal which has not been stressed will have normal levels of glycogen in its body. When the animal is slaughtered and exsanguinated, the metabolic process continues, however there is no longer circulating oxygen. Without the presents of oxygen, the breakdown of glycogen/glucose results in a buildup of latic acid which then causes a drop in pH of the meat.
The final quality of meat is greatly affected by the rate of pH decline in the meat after slaughter. If there was a great lactic acid buildup before slaughter, the pH of the meat declines too quickly after slaughter and a Pale, Soft, Exudative (PSE) condition may develop. As suggested by the name, the affected meat is pale, soft, and fluid may drip from the surface.
At the other extreme, if the animal is glycogen depleted before slaughter the pH may not drop quickly enough after slaughter because there is not enough lactic acid produced. In this case the meat will be very dry and dark in color. This condition is known as Dark Firm Dry (DFD) meat. An additional problem with this type of meat is that it is more susceptible to spoiling since it lacks the lactic acid which normally helps retard growth of microorganisms after slaughter. Note that glycogen deficiency may also be the result of too much physical activity or inadequate diet before slaughter.
