I wanted to look into this:
Here is a good summary of what I found (this is copied from a website I found):
According to the U.S. Department of Agriculture (USDA) Agricultural Marketing Service, Angus beef must meet the following requirements:
2.1 Genotype. Cattle eligible for Angus influence beef programs based on genotype must have positive identification (ear tags, tattoos, brands, etc.) and be traceable back to provable (e.g.; registration papers) Angus parentage. Qualifying cattle must be traceable to one registered parent or two registered grandparents. Programs which claim a specified percentage of Angus heritage must use this method.
2.2 Phenotype. Cattle eligible for certification in Angus influence beef programs based on phenotype (appearance) must be predominately (51 percent) solid black. Blue roan, gray, etc., are not considered to be black or a percentage of black. Such variations can qualify only when it occupies 49 percent, or less, of the body area with the remaining 51 percent, or greater, being solid black. (1) Angus influence cattle may be either horned or polled. Carcasses of certified live animals which display certain non-Angus characteristics (e.g.; dairy conformation, Brahman humps) shall be excluded as specified in the carcass specifications for approved programs.
(1) At times, a black hair coat can become sun bleached and appear to be a shade of brown, particularly on the back. If the base of the hair close to the skin is black then that entire brown tipped area should be considered solid black. However, if the hair color is brown to the roots, it should be considered as brown in color and the area will not contribute to the 51 percent black requirement.
"Certified Angus Beef" (CAB) is a special industry designation developed in 1978 that involves standards for marbling, tenderness, age, and color. According to the National Cattleman's Beef Association, only about 8% of U.S. beef is entitled to the label "Certified Angus." Just because something is labeled "Angus" or "Black Angus" doesn't mean it's the same quality as "Certified Angus Beef." Angus beef is further differentiated by USDA grades such as "prime," "choice," and "select," giving us such labels as "Certified Angus Prime," indicating the best Certified Angus Beef.
That tells us what qualifies as "Angus" but not why we would want Angus beef. A lot of it comes down to genetics--specifically, the genes that control a protein called myostatin. Myostatin inhibits the growth of muscles in cattle. According to David Elstein and Erin Peabody (see reference below), "If the gene responsible for producing myostatin is altered so that it makes an inactive form of the protein, or the gene is intentionally suppressed, the result is more muscle and less fat." Angus and Hereford cattle have more myostatin, so their meat is fattier and more marbled. But fat content and marbling alone don't tell you if the meat is likely to be tender--you have to look at other things, such as how fine the marbling is and how well distributed through the meat, and the toughness of the fat and connective tissue. Here again there is an advantage for Angus beef, as it tends to have finely textured marbling and thus can be more tender than meat from other breeds.
Marbling of meat figures prominently in USDA beef grading standards, as this excerpt shows:
Quality Grades:
•Prime grade - is produced from young, well-fed beef cattle. It has abundant marbling and is generally sold in restaurants and hotels. Prime roasts and steaks are excellent for dry-heat cooking (i.e., roasting, broiling, and grilling).
•Choice grade - is high quality, but has less marbling than Prime. Choice roasts and steaks from the loin and rib will be very tender, juicy, and flavorful and are, like Prime, suited to dry-heat cooking. . . .
•Select grade - is very uniform in quality and normally leaner than the higher grades. It is fairly tender, but, because it has less marbling, it may lack some of the juiciness and flavor of the higher grades. . . .
•Standard and Commercial grades frequently are sold as ungraded or as "store brand" meat.
•Utility, Cutter, and Canner grades - are seldom, if ever, sold at retail but are used instead to make ground beef and processed products.
USDA meat grades are illustrated in the following chart. Note that when they refer to "maturity," they are referring to pre-slaughter age, not post-mortem aging. Category A, for example, is typically from cattle that were less than 30 months old when slaughtered.
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