The microbiology of DFD fresh meats: A review.

Muscle which is deficient in glycogen because of exercise or stress prior to slaughter produces dark, firm, dry (DFD) meat. Such meat is characterized by a high ultimate pH (>6·0) and deficiencies in glucose and glycolytic intermediates. These factors can result in bacterial spoilage becoming evident at an early stage of growth of the meat flora. Spoilage becomes apparent when bacteria attack amino acids. This does not occur under aerobic conditions until bacteria exhaust the glucose at the meat surface. However, since glucose is absent in DFD meat, amino acids are utilised without delay and spoilage becomes evident at lower cell densities than in normal meat. The absence of glucose also allows the anaerobic flora to produce spoilage odours at an early stage. Additionally, the high pH of DFD meat allows growth of potent spoilage organisms which are inhibited at the usual ultimate pH of meat. Early aerobic spoilage can be prevented by the addition of glucose, but prevention of early anaerobic spoilage requires the addition of a citrate buffer which reduces the surface pH, as well as providing a carbohydrate substrate which is utilised in preference to amino acids. Comparisons can be made between spoilage of DFD red meat and spoilage of white meats from poultry and fish, which normally have a high ultimate pH.