The dark, firm and dry (DFD) or dark-cutting condition causes large losses to both the cattle feedlot and meat industry due to its unattractive, dark red colour and its limited shelf life. Dark-cutting, is a direct consequence of low muscle glycogen at slaughter and results in a lack of normal acidification of meat during rigor development causing high pH (>5.8). Glycogen breakdown in muscle may be rapidly triggered by increased circulating adrenaline or by strenuous muscular activity. Several other factors have also been found to lead to the DFD condition in cattle. The aims of this investigation were to determine the incidence of DFD in cattle slaughtered at a South African abattoir and to identify potential causative factors of this syndrome. It was furthermore also important to evaluate the sensory attributes, microbiological quality and hence, shelf life of DFD meat. The incidence of DFD carcasses, based on pH measurements (threshold ~ 5.8) within I h post mortem on 22 178 cattle in the M. longissimus dorsi, was 11.8%. The estimated overall incidence measured over a 5 -day period, testing only cattle supplied by the four main feedlots, was 7.83% (N= 5 659), measuring pH 24 (~5.8), confirmed by colour grades (values of 7 or 8 were classified as DFD). The incidence of dark-cutting beef, also seemed to be feedlot related with the highest being 12.37% and the lowest 1.75%. It is suspected that the incidence of DFD carcasses is higher in extremely hot or cold weather or large fluctuations, and should decrease during the months with milder temperatures. Transport distances did not affect the DFD incidence. In experiment 2, cattle from Feedlot C, transported over a short distance (45 km) had a higher incidence of DFD (9%) than those of Feedlot A (1.7%), transported over the longer distance of 290 km. Long holding periods in the holding pens at the abattoir (>24 h and even as high as 54 h), appeared not to have a significant influence on the occurrence of DFD. There was no correlation between bruised carcasses and DFD carcasses (the respective means of bruising were 6.7% and 3%, and respective mean incidences of DFD were 1.7% and 9%). Using Scheffe's test to compare the effect of gender on the incidence of DFD, significantly more bulls and steers were affected by dark-cutting than heifers (P< 0.0001). Mounting behaviour among young bulls, and social regrouping of steers during the pre-slaughter period, have been shown to increase the incidence of dark-cutting. Avoidance of mixing unfamiliar bulls and steers during transport and holding periods has been recommended to minimise stress. The low incidence of dark-cutters (P > 0.05) in heifers could possibly indicate a non-¬oestrus situation in the majority of the heifers, since a significant association between the presence of oestrus and DFD was shown previously by other researchers. The effect of gender on the incidence of dark-cutting, related to feedlot, was also significant. Higher incidences of DFD were found with steers of Feedlots B, C and 0 (14.5%, 11.6% and 13.7% respectively) than in Feedlot A (1.9%). The lowest overall incidence of dark-cutters was found in Feedlot A (1.7%). It appeared as if the breed type of the cattle had an effect on the dark-cutting beef. The Bos indicus (Brahman-type) animals had a significant effect (P< 0.001), on the colour of the beef when compared with European breeds. Cattle of two feedlots, fasted for periods of 40 hand 60 h, showed that a shorter fasting period had a significant effect (P < 0.001) on the pH. It appeared as if pre-slaughter stress, induced dark-cutting more readily in fasted than in fed cattle. Fasting also inhibited muscle glycogen re-synthesis during recovery of stress. The effect of pre-slaughter handling on the blood composition and muscle glycogen content of cattle of three different feedlots was studied. After transport (phase 1), the glycogen depletion was significant for all three feedlots (P < 0.05). Further glycogen depletion was found after slaughter (phase 2) (P < 0.01). The hematocrit increased after transport, which indicated that animals of Feedlot B (P < 0.0001) experienced stress. During the second phase, a significant decrease in hematocrit for Feedlot B was observed (P < 0.05). For Feedlot A, a non-significant increase was found in phase 1, with a significant (P < 0.05) increase in phase 2. The hematocrit of cattle of Feedlot C, however, showed a significant decrease during both phases. The blood glucose concentration for all three feedlots increased during phase 1, but was not significant. However, the blood glucose concentrations for all three feedlots increased significantly during phase 2 which probably indicated that the animals experienced stress during this period. The increase in creatine kinase was significant (P < 0.01) during both phases 1 and 2 for Feedlot A. Fluctuations were found in the creatine kinase concentrations of Feedlot C. Feedlot B, however, showed a significant decrease in the creatine kinase concentrations. Free fatty acid concentrations for all feedlots increased linearly over the different stages. The high free fatty acid concentrations in blood samples of phase 2, of Feedlots Band C confirmed that these animals experienced stress, especially from the period being held in the pens to after slaughter (P < 0.01 for both). The free fatty acid concentration, appear to be the best blood parameter to relate to meat quality. The DFD incidence during this experiment, for Feedlots Band C, is 35%. For Feedlot A, with the lowest DFD incidence (16%), the increase of free fatty acid concentration was not significant. Although the effect of growth promoters was not tested, the use of beta-agonists as growth promoters may increase the incidence of dark-cutting meat unless a sufficient withdrawal period before slaughter is observed. Feedlots A and C implanted Revelor in 9 month old cattle, while Feedlot B implanted both Synovex S (at 6 weeks) and Revelor (at 4 months). Consumer sensory evaluations indicated that the general appearance (P < 0.05), colour (P < 0.001) and acceptability (P < 0.01) of raw normal steak were preferred significantly to those of raw DFD steak. Twice as many panellists preferred the raw normal steak than raw DFD because of the more attractive red colour, compared to the almost black colour of raw DFD steak. The hedonic ratings of the sensory attributes of fried normal steak versus fried DFD steak were non-significant for all attributes, although it was expected that consumers would find the DFD steak more tender and less flavoursome than normal steak. DFD meat samples (N= 84) were tested over a 7-week period for total aerobic count, Pseudomonas, coliforms and Eschericia coli. Normal beef samples were expected to be acceptable, both on microbiological grounds and sensory evaluations until at least the seventh week. Microbiological spoilage of vacuum-packaged DFD meat took place during the second week and deterioration was more evident during the fourth week, due to the presence of cheesy odours. During the fifth week, green discolouration of the meat was observed.
Dissertation (MSc Agric (Food Science))--University of Pretoria, 2007.