The Dark Side of Dark Cutters

By Duane M. Wulf, PhD, University of Arizona

"Fear is the path to the dark side.” – Yoda

Dark cutting beef, also known as DFD (dark, firm, and dry), is characterized by a higher-than-normal meat pH which results in dark-colored lean with a firm and dry surface. In addition to an unappealing appearance to consumers, dark cutting beef has a shorter shelf life because it is more susceptible to bacterial spoilage. When cooked, dark cutting beef will appear “rare” even when cooked to “medium”, is tougher to chew, and has more off-flavors, including more metallic, musty, and bitter flavors.

USDA meat graders discount the carcass’s Quality Grade based on the severity of dark cutting and packers have specifications to keep dark cutters out of boxed beef channels; however, borderline dark cutters pass through the U.S. beef chain every day. I have recently observed on multiple occasions dark cutting beef in retail meat cases. Unfortunately, these “borderline” dark cutters that make it to the consumer have more meat toughness problems than those more severe dark cutters that are sorted out by the packers. The relationship between meat pH and tenderness is not linear: normal beef (pH 5.5) is tender, borderline dark cutter beef (pH 5.8 - 6.1) is tough, and more severe dark cutter beef (pH 6.3-6.7) is tender.

Estimates of dark cutting incidence within the U.S. fed steer and heifer carcass population range from 0.5 to 3.0%. The most recent National Beef Quality Audit reported a nationwide overall incidence of 1.9% dark cutters, which equates to about a half million dark cutting beef carcasses annually. Given the current USDA-reported average discount of $323 per head, the U.S. beef industry loses approximately $160 million per year, or $6.14 per head (averaged across all fed cattle) due to dark cutters. While this might not seem especially significant in a multi-billion-dollar industry, dark cutters often occur in clusters and can result in devastating economic losses in certain pens or producer lots.

To understand what causes dark cutters, we must first understand the “normal” conversion of muscle to meat that occurs during the first 24 hours after death. Living muscle is a neutral 7.0 pH and drops to a pH of 5.5 in normal meat over a period of several hours after slaughter. After the blood supply is removed, metabolism within the muscle switches from aerobic to anaerobic, utilizing glucose to produce ATP and lactic acid. Lactic acid accumulates and causes this normal pH decline. As muscle pH approaches 5.5, the low pH deactivates the metabolic enzymes, stopping the entire process, and causing the onset of rigor mortis.

Lactic acid is produced from glucose, which is stored in muscle in the form of glycogen (glycogen is a “branching tree” of many glucose molecules). If there is not enough glycogen in the muscle of the living animal, there will not be enough lactic acid produced in the meat, and the meat pH will remain too high, resulting in a dark cutter. The severity of glycogen depletion that occurred in the living animal determines the severity of dark cutting; that is, less glycogen means less lactic acid produced which leads to a higher pH, resulting in a dark cutter.

Cattle can vary widely in muscle glycogen amount. According to our research, if their muscle glycogen drops below a threshold of 50 µg glucose per g of muscle, there will be insufficient lactic acid production to achieve a normal meat pH, resulting in a dark cutter. Some cattle could be very close to crossing the threshold while others have abundant glycogen, and we have no way of knowing by visual observation.

When cattle are stressed, the fight-or-flight response kicks in, cortisol and epinephrine increase, and glycogen is mobilized within the muscle, decreasing glycogen levels. Depending on animal diet and how depleted these glycogen stores are, it can take a few days or up to two weeks to replenish glycogen levels to normal. Therefore, dark cutters can result from stress events occurring several days prior to slaughter. Stress that depletes glycogen can either be psychological (isolation, mixing cattle, excited handling), physiological (weather, estrus, lack of water), or physical (exercise, long transportation, mounting/fighting). Physiological stress can occur even if we are doing our best to provide for comfortable, happy cattle and even though the cattle do not appear stressed.

Some factors that contribute to a higher incidence of dark cutters:

• Sex (bulls have much higher incidence; heifers have higher incidence than steers)

• Longer transportation time

• Mixing unfamiliar cattle

• Isolation (one animal by itself)

• Season (September/October has 70% more dark cutters than other months)

• Lower-energy diets (grass/forage-fed)

• Greater proportion of Type I muscle fibers (dairy breeds)

• Aggressive implant strategies or short time between final implant and slaughter

When dark cutters occur, determining the cause can be difficult or impossible. While we know that a stressor caused the depletion of muscle glycogen, we may not be able to identify the stressor or determine why some cattle were dark cutters and others were not. What we can do is minimize risk factors. Avoid mixing, sorting, or re-penning cattle within the final weeks before slaughter. If possible, avoid long transportation distances to packing plants. Ensure cattle are eating well and the diet is high in carbohydrates during the final weeks.

Research efforts to create interventions that increase muscle glycogen or prevent glycogen depletion have not been very fruitful. We understand factors that have negative effects (reduce glycogen) but have not identified methods to increase glycogen. Newly available technologies that reduce an animal’s response to stress, such as appeasing pheromones, have been shown to decrease the cortisol response, which could be a very promising tool for reducing dark cutters. Reducing the fight-or-flight response is the path to reducing dark cutters. Yoda was right – “Fear is the path to the dark side.” The exciting thing about finding methods that prevent muscle glycogen depletion is that not only will dark cutters be reduced, but steak tenderness and consumer satisfaction should also improve within the non-dark-cutting population. As we continue to understand this problem more thoroughly and work on it together, we can defeat the dark side.

Dr. Duane Wulf has been a faculty member at The Ohio State University, South Dakota State University, and currently the University of Arizona. His research focused on ante- and post-mortem factors affecting meat quality has attracted numerous graduate students and resulted in 46 refereed journal articles. For ten years, Dr. Wulf served as a missionary businessman in Sonora, Mexico, starting a meat processing plant, a cattle ranch, and a restaurant, with the purpose of providing training opportunities to the fatherless and less fortunate. Dr. Wulf has worked across all production and processing phases of the meat industry and has been hired as a consultant domestically and internationally by both small and large companies.