Research »   Dark-cutting Beef

Dark-cutting Beef

Dark-cutting is a condition that is severely penalized in youthful (under 30 months) carcasses within the Canadian grading system. Carcasses that ‘cut dark’ are assigned a grade of Canada B4 and their value is reduced by up to 50 cents per pound. This represents an estimated loss of up to $1.4 million per year to Canadian beef producers, assuming 2% of the total slaughter cattle population produced a dark-cutting carcass. In 2011, 1.3% of fed slaughter carcasses in Canada graded B4.


Sections

Canada B4 Grade

If a carcass grades Canada B4, the surface of the rib eye muscle at the 12th and 13th rib is a very dark red to purple colour and is much darker than the normal bright red colour of beef sold in supermarkets and butcher retail outlets.
Up to $300 in value can be lost per head of cattle that grade Canada B4.

Dark-cutting beef is penalized because it is visually unappealing to consumers, and due to its high pH, has a reduced shelf-life and increased toughness compared to typical beef.

Cause of Dark-Cutting

Dark-cutting usually occurs when cattle have been physically stressed prior to slaughter and have depleted the energy reserves of their muscles.

Energy reserves in skeletal muscle consist primarily of glycogen, a molecule made up of linked glucose, which is a basic sugar that is found linked to fructose (fruit sugar) in table sugar (sucrose).

Dark Cutting BeefDark cutting beef (left) is a dark red to purple colour, compared to the normal bright colour of typical beef (right). Photo courtesy of the Canadian Beef Grading Agency.

If the pH does not decline to less than 6.0, proteins in the mitochondria, the fuel-producing part of the muscle, steal the oxygen from the myoglobin leaving the myoglobin with no oxygen. Without oxygen, myoglobin produces meat with a dark red or purple colour. Therefore, when the muscles of cattle are depleted of glycogen, there is little glucose to be metabolized by the muscle post mortem, and so the muscle pH does not decline and the meat is dark red or purple.

Glycogen and glucose

After the beef animal has died during the slaughter process, its muscles still attempt to function in the absence of oxygen. To maintain function, the muscles use glucose sugar from glycogen to fuel cellular processes that prevent rigor mortis. The muscle produces lactic acid and hydrogen ions, which reduce the pH of the muscle from its normal 6.9 to a slightly acidic pH value of 5.5. At this pH, the iron in the oxygen-carrying muscle protein myoglobin can bind to oxygen from the atmosphere and produce a bright red colour

When the muscles of cattle are depleted of glycogen, there is little glucose to be metabolized by the muscle post mortem. Muscle pH does not decline and the meat is dark red or purple.

If the pH does not decline to less than 6.0, proteins in the mitochondria, the fuel-producing part of the muscle, steal the oxygen from the myoglobin leaving the myoglobin with no oxygen. Without oxygen, myoglobin produces meat with a dark red or purple colour. Therefore, when the muscles of cattle are depleted of glycogen, there is little glucose to be metabolized by the muscle post mortem, and so the muscle pH does not decline and the meat is dark red or purple.

Known Risk Factors

Aggressive activity

Bulls tend to produce dark-cutting carcasses, particularly if they are mixed prior to slaughter because they engage in fighting and mounting to re-establish social order.

Estrus

Heifers can also produce carcasses that cut dark if estrus behaviour is not inhibited by melangesterol acetate (MGA) or spaying. Heifers that have been off MGA for more than 48 hours before slaughter will start cycling and be at a higher risk of dark cutting.

Stress

Cattle that are transported long distances or have become wet and cold and shiver just prior to slaughter are also at substantial risk of dark cutting.

Growth Promotants

Extremely aggressive implant programs may be a risk factor for dark-cutting. Some growth implants skew bovine muscle metabolism toward using glucose immediately for muscle growth and so glycogen reserves are reduced and the decline of muscle pH prevented from reaching the normal post mortem pH value of 5.5.

Weather

Dark cutting is most common during the hottest months of the year, during late summer and early fall. This is true in both eastern and western Canada. It is uncertain if this is due to heat stress, or the variability between hot days and cool evenings.

Mitigation

When cattle engage in aggressive activity, estrus or stress, muscle glycogen can deplete quickly. Glycogen can only be restored by muscle after a period of rest. Cattle that have been severely stressed may require a rest period in order to completely recover.

In most cases, stressed animals should be rested overnight to prevent dark-cutting. If severe depletion has occurred, up to 4 days rest with feed can be required to return glycogen levels in muscle to normal.

Electrolyte supplements have also been found to be effective. An animal’s electrolytes are out of balanced when stressed. There are several commercial products available including the Canadian product NutrichargeTM, that are effective at assisting animals recover from transport stress and prevent dark cutting. Research found that NutrichargeTM was effect with sufficient intake, but may not be cost effective when used to prevent dark cutters as it would need to be fed to all cattle in order to prevent a loss on only 2% of the carcasses produced.

References
  1. Scanga, J., Belk, K., Tatum, J., Grandin, T., and Smith, G. 1998. Factors contributing to the incidence of dark-cutting beef. Journal of Animal Science 76: 2040-2047.
  2. Tarrant, P. and Sherington, J. 1980. An investigation of ultimate pH in the muscles of commercial beef carcasses. Meat Science 4: 287-297.
  3. Schaefer, A. L., Jones, S. D. and Stanley, R. W. 1997. The use of electrolyte solutions for reducing transport stress. Journal of Animal Science 75: 258-265.
  4. Schaefer, A. L., Dubeski, P. L., Aalhus, J. L. and Tong, A. K. W. 2001. Role of nutrition in reducing antemortem stress and meat quality aberrations. Journal of Animal Science 79: E91-E101.
Learn More

To learn more on this topic, see the fact sheets posted on the right side of this page. External resources are listed below.

Factors Contributing to the Incidence of Dark Cutting Beef

Department of Animal Sciences, Colorado State University
http://www.grandin.com

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Acknowledgements

Thanks to Dr. Heather L. Bruce, University of Alberta Associate Professor of Carcass and Meat Science, for contributing her time and expertise to writing this page.

This topic was last revised on February 29, 2016 at 09:02 AM.

Fact Sheets

Canadian Cattlemen's Association Verified Beef Production Canada Beef
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