Marbling: Management of Cattle to Maximize the Deposition of Intramuscular Adipose Tissue

by Bridget Wasser, Executive Director, Meat Science Technology, National Cattlemen’s Beef Association and Dani Shubert, Associate Director, Meat Science, National Cattlemen’s Beef Association, a contractor to the Beef Checkoff

Summary

Beef palatability is a substantial driver of consumer demand for beef, and marbling is one of the largest contributing factors to beef palatability as it affects beef tenderness, juiciness, and flavor. For many years, marbling research has focused on understanding both how marbling contributes to eating quality, as well as how to increase marbling levels in cattle through genetics and feeding. A checkoff-funded white paper titled “Marbling: Management of cattle to maximize the deposition of intramuscular adipose tissue” seeks to summarize the beef industry’s progress toward understanding marbling in beef and how the beef industry can increase marbling to create a more tender, juicy, and flavorful product. 

Background

Consumers in the United States and abroad have valued highly marbled beef for nearly a century. In spite of growing interest in pasture-fed (or grass-finished) beef in the United States, most consumers still prefer beef that is reasonably marbled and juicy. Scientists have taken a two-pronged approach to understand the biology of marbling development. Biochemists, molecular biologists, and geneticists have worked to understand the intracellular and extracellular factors that regulate the development of marbling, whereas beef cattle nutritionists have created feeding regimens to provide high-quality beef carcasses without increasing carcass subcutaneous fat, or fat trimmed from steaks before they reach the consumer. 

Discussion 

The contribution marbling to overall palatability has been established for decades. Savell and Cross (1988) established a “Window of Acceptability” for beef (Figure 1), indicating that overall palatability of beef is optimal between 3 and 7.5 percent intramuscular fat (marbling). The relationship between percent fat and overall palatability highlights the importance of marbling in beef quality. What was not addressed in this research is that as percent fat increases, there is a dramatic change in the fatty acid composition of marbling. As intramuscular fat percentage increases, the proportion of saturated fatty acids (SFA) and trans-fatty acids decrease, and the proportion of oleic acid and other monounsaturated fatty acids (MUFA) increase. These changes in fatty acid profile have a direct effect on the palatability of beef. 

Figure 1. Window of acceptability for fat content of meat (palatability versus grams of fat, two servings, raw [8 ounces]). The window is based on fat content range of 3.0 to 7.5 percent. This is equivalent to beef from the Longissimus muscle (12 – 13th rib) that grade USDA Select (3.0 – 4.27 percent fat content) to those that grade USDA Choice (4.28 – 8.0 percent fat content), Savell and Cross (1988).

The fatty acid profile of meat influences beef palatability in a variety of ways. Early studies demonstrated that increased oleic acid (a MUFA) in beef led to an increase in overall palatability of the beef. One theory used to explain this shift in palatability is that oleic acid is “softer” than SFA, providing a more fluid mouthfeel, which most consumers perceive as more desirable. The melting point of fat directly affects the perception of juiciness in beef, and the ratio of MUFA to SFA dictates the melting point of fat. Saturated fatty acids have higher melting points and are “harder” at refrigerated or room temperatures, whereas MUFA have melting points below room temperature and thus are perceived as “softer” fats.

More recent studies have discovered that the fatty acid profile of beef contributes to the formation of flavor through the cooking process. Fatty acids react with heat, sugars, and proteins to create different beef flavors. Oleic acid has been identified as the fatty acid that contributes most to positive, buttery beef flavor.

Marbling represents a unique fat depot, and can be distinguished from other fat depots by its location within the muscle (Figure 2a & 2b). Marbling consists of clusters of individual fat cells (adipocytes), and these fat cells increase in number and in size as the beef animal progresses from being practically devoid of marbling, to having higher marbling scores.


Figure 2a. Marbling adipocytes lying alongside muscle fibers in bovine Longissimus muscle. Moody and Cassens (1968)

Management of cattle through growth and during feeding can also influence the extent and composition of marbling development in beef. In the early stages of marbling development, glucose contributes 70 percent of the energy and carbon needed to create marbling. As cattle become fatter, the contribution of glucose decreases while the use of acetate for fat synthesis increases, especially in the creation of marbling. Thus, providing sources of dietary glucose at early ages may promote marbling development more than if glucose is fed at later stages of development. Early weaning of beef steers may result in higher marbling scores at slaughter than normal weaning of steers. Researchers hypothesize this may be caused by increased glucose availability (from grain-based rations) at the early stages of marbling development. 

Figure 2b. Marbling adipocytes located in seams of connective tissue, the oval central structure is an arteriole, which is flanked by pockets of marbling adipocytes. Brooks et al. (2011)

Calf-fed steers are typically fed high-concentrate finishing diets at weaning, whereas yearling-fed steers are typically fed native pasture until approximately 12 months of age and are then transitioned to a grain-based diet. One study reviewed in this white paper explored these management strategies in greater detail. Calf-fed steers reached harvest weight at 16 months of age, whereas yearling-fed steers reached similar weights at 17.5 months of age. Although slower to reach harvest weight, yearling-fed steers at the time of harvest had the same average external fat thickness and marbling score when compared to calf-fed steers. Beef from yearling-fed steers showed a greater proportion of SFA and lower proportions of MUFA, including oleic acid, which may have influenced palatability of the beef.

Conclusions

For beef cattle, the development of marbling is more complex than the development of subcutaneous fat. The results of the studies outlined in this white paper indicate that grain-based diets are necessary to promote the development of marbling. Additionally, grain-based diets increase the juiciness and flavor of beef by promoting the production of oleic acid in marbling and other fat depots.

Additional Resources 

  • For more information, please visit BeefResearch.org and read the full White Paper, titled “Marbling: Management of cattle to maximize the deposition of intramuscular adipose tissue.”
  • Brooks, M. A., C. W. Choi, D. K. Lunt, R. K. Miller, C. B. Choi, and S. B. Smith. 2011b. Carcass and meat characteristics and M. longissimus thoracis histology of beef from calf-fed and yearling-fed Angus steers. Prof. Anim. Scientist 27:385-393. 
  • Moody, W. B., and R. G. Cassens. 1968. A quantitative and morphological study of bovine longissimus fat cells. J. Food Sci. 33:47-55.
  • Savell, J. W., and H. R. Cross. 1988. The role of fat in the palatability of beef, pork, and lamb. In: Designing Foods: Animal Product Options in the Marketplace. pp. 345-355. National Research Council, Washington, DC.
     

Tags: Beef Issues Quarterly, Research Findings, Spring 2016

March 25, 2016