New Approaches to Understanding Beef Tenderness

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By: Andy King, Steven Shackelford, and Tommy Wheeler

Source: United States Meat Animal Research Center at Clay Center, NE

For many, a juicy, tender, flavorful steak is the epitome of the perfect meal. Beef is one of the most expensive protein sources, so fulfilling these expectations is critical to maintaining market share. Beef palatability is determined by tenderness, flavor, and juiciness. Failure to meet consumer expectations for any one of these attributes results in dissatisfied consumers.

Beef tenderness is impacted by numerous physical and biochemical factors occurring in the muscle, particularly during the conversion of muscle to meat at harvest. This area of work at USMARC in the Meat Safety and Quality research unit has relied on a small number of traits that directly contribute to tenderness to explain the methods by which treatments affect tenderness. This work has produced substantial gains in knowledge and has led to the implementation of technology to manage tenderness. However, under the best of conditions, these variables explained only 30 to 40% of the variation in tenderness.

Technology has evolved to determine the complete profile of proteins and/or metabolic products present in muscles.  Scientists can inventory the machinery present for muscle functions and how efficiently that machinery operates.

Studies have indicated that animals that produce tender steaks have an energy-producing machinery that favors processes that convert sugars (glucose) to energy while not requiring oxygen. In contrast, animals producing less tender beef tend to have machinery that favors energy production using processes that require oxygen, but that also can utilize other energy sources, including fats.

Another key finding is that muscles producing more tender steaks tend to have larger amounts of proteins that function to protect cells in times of cellular stress, such as when oxygen supplies are reduced or eliminated. In times of limited oxygen presence, these proteins function to avoid protein damage and maintain the functionality of the cell’s machinery.

When an animal is harvested, the ability to supply oxygen and remove compounds from the muscle cell is lost. As a result, the muscle pH drops from around 7.5 to around 5.6. Large differences in the rate and extent of postmortem pH decline have large effects on beef quality. For example, muscles that fail to undergo normal pH decline display the dark cutting condition severely discounted in the industry. However, evidence is increasing that suggests that variation in postmortem metabolism that falls well within normal ranges also has a profound effect on traits such as tenderness.

In a study published in Meat and Muscle Biology, USMARC scientists worked with collaborators to measure the metabolite profiles of steaks that were consistently tender or tough through 28 days of refrigerated storage (aging). Steaks that were classified as tender had much lower slice shear force values than those that were classified as tough. The sheer force values represent the force required to shear through a ½ inch thick slice of steak, which is highly correlated to tenderness. Tough steaks became more tender during refrigerated storage, but after 28 days, tough steaks were similar in tenderness to tender steaks stored for 2 days.

More than 2,500 metabolites were detected in this experiment. Of these, 651 were related to variation in tenderness, and 102 were very highly correlated to variation in steak tenderness. Many of these substances involved in metabolism appeared to be affecting tenderness by influencing the mechanisms USMARC scientists normally study. However, others were influencing tenderness by a means unrelated to these processes. . A total of 28 compounds that could be definitively identified were highlighted as related to tenderness, but more studies are necessary to reliably identify many of the other metabolites.

The results of this experiment are consistent with previous results that animals who favor anaerobic (absence of oxygen) metabolism of carbohydrates produce more tender steaks than those that favor metabolic processes that require oxygen. However, these metabolites also suggest that differences in efficiency at specific points of these pathways may affect tenderness that is currently not understood. In particular, scientists hypothesize that levels of some of these compounds could be used as a “molecular fingerprint” to explain tenderness differences. USMARC currently has several experiments underway to determine how best to use this information to better understand tenderness variation.

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