Before slaughter, animals should be free from undue stress. Long-term stress on animals prior to slaughter will produce meats that are dark firm and dry (DFD). The long term-stress may be due to transportation exhaustion, extreme climactic conditions e.g. too cold or too hot, hunger due to prolonged withholding of feed, mixing of unfamiliar animals in the holding pen, and extreme frightening and agitation of the animals. This meat will exhibit a dark, purplish-red to almost black color, and have a dry surface. The darkness is as a result of high water retention which reflects less light, and the high water retention is due to higher pH in the meat. Stress causes the glycogen (carbohydrate store) in the meat to be used up. When this happens, there is not enough glycogen for conversion to lactic acid which normally happens during anaerobic respiration after death. Production of lactic acid generally reduces the pH of the meat from 7.0 to 5.7. With less glycogen to convert to lactic acid, the pH will remain high (5.9-6.5) resulting in DFD meat characteristics.
Another type of meat defect is called pale, soft and exudative (PSE). In this case the meat has a pale color and is soft. While DFD meat is caused by a lack of lactic acid, PSE is caused from high levels of lactic acid while the meat is still warm. This occurs due to short-term excitement and agitation of the animals prior to slaughter. The high lactic acid content, denature the proteins causing them to be less soluble. In protein chemistry we would explain the reduction in solubility by the pH shifting towards the meat protein’s isoelectric point. The protein therefore loses capacity to hold and retain water. Therefore there is significant drip loss.
