Dark cutting beef is purplish black rather than bright red in colour. Dark cutting beef actually resembles vacuum packed beef. However, vacuum packed beef will brighten up (or “bloom”) after the package has been opened, but dark cutting beef will not. Dark cutting beef looks unappealing to consumers, may spoil faster, and is not sold at retail stores. As a result, dark cutters are severely discounted at packing plants.
How common is dark cutting?
Dark cutting is most common during the late summer and early fall. Dark cutting used to be about three times more common in Eastern than in Western Canada. This changed in the fall of 2005, when an unusually high spike in the incidence of B4s occurred in both Eastern and Western Canada. Since then, the seasonal incidence of dark cutters has fallen below historical levels in the East, but has increased in the West. Dark cutters were more common in Western than in Eastern Canada in 2009 and so far in 2010.
Why does dark cutting occur?
Muscles store energy in the form of glycogen. Normally, glycogen is metabolized into lactic acid as the carcass cools after slaughter. The buildup of lactic acid causes the muscle pH to drop and the meat to turn red. In severely stressed animals, the muscle glycogen stores may have already been used up before they die. This means that the metabolic processes that normally turn the beef bright red will not happen, the pH will remain high, and the meat will remain dark. It can take up to a week for muscle energy stores to return to normal levels in severely stressed cattle.
Predicting exactly which stresses will cause dark cutting is difficult. For example, some cattle that have been in truck rollovers may not become dark cutters, and some very calm 4-H calves will grade B4. Most loads of fat cattle have no dark cutters. Some loads will have one or two, and sometimes a quarter of the load may grade B4, even though all the cattle have been managed similarly. Some factors that have been shown to influence the risk of dark cutting include:
Mixing unfamiliar cattle (especially bulls or stags) several hours before slaughter;
Heifers that are in heat and riding. This has implications if MGA is used to prevent cycling. MGA has a 24-hour withdrawal before slaughter, but heifers may come into heat 72 hours after MGA withdrawal if there are delays in transportation or slaughter scheduling.
Some breeds or bloodlines may have a higher incidence of dark cutting, but the role of genetics is generally believed to be quite minor, compared to environmental and management factors;
Aggressive implant regimes may increase the incidence of dark cutting for some animals, in some cases;
Switching cattle from a high-energy to a low-energy diet shortly before slaughter;
Keeping cattle off feed or water for too long before slaughter;
Holding cattle at packing plants over the weekend before slaughter;
Temperature stress (dark cutting is usually most common in late summer and early fall);
Electrolyte therapy at slaughter plants may help reduce the incidence of dark cutting. Researchers at Agriculture and Agri-Food Canada’s Lacombe Research Centre developed and patented an electrolyte product (Nutri-Charge) to reduce transport stress use in cattle prior to shipping and slaughter. Data from over 1,400 bulls and steers indicated that Nutri-Charge reduced the incidence of dark cutting by 50 per cent.
These factors can also interact with each other. In 1998, Colorado State University researchers studied three years of management and grading records from 2,672,223 cattle (15,439 pens from nine feedlots). They identified sex, implant management and weather as three of the main risk factors for dark cutting. The risk of dark cutting in steers increased if combination implants were used as both the initial and terminal implants, especially if the terminal implant was given less than 100 days before slaughter. In heifers, the risk of dark cutting increased if estrogenic implants were used as the terminal implant less than 100 days before slaughter. In both cases, temperatures higher than 35C in the day or two before slaughter increased the risk of dark cutting still more.
Researchers at the Lacombe Research Station and the University of Alberta suspect that carcasses that are rapidly chilled may bloom more slowly. Recall that the lactic acid produced by post-mortem muscle metabolism helps to turn the meat red. These metabolic reactions slow down at lower temperatures, so carcasses that are chilled too quickly may take longer to bloom. These researchers are funded by the Alberta Beef Producers and Canada’s Beef Science Cluster to measure muscle temperature and pH changes in carcasses with dark-coloured lean. If this research indicates that the rate of carcass chilling does affect lean colour, it is possible that adjustments to cooler management may help reduce the incidence of B4 grades. More information on this and other research supported by Canada’s national checkoff is available atwww.cattle.ca.
Reynold Bergen is the science director of Beef Cattle Research Council.