So if the flavor of a steak doesn't come from fat, where does it come from?
That, it turns out, is an easy question to ask but a difficult one to answer. Unfortunately, it doesn't boil down to anything so simple as a single macronutrient like fat.
At best, beef flavor is poorly understood. Despite thousands of meat scientists working in universities all over the world, we are still a long way away from a complete scientific understanding of why a given steak tastes the way it does. (And this has a lot to do with the fact that meat science, just like the meat industry, cares more about things like the rate at which cows gain weight or how to improve their fertility than it does about flavor.)
But there are two things we do know about beef flavor. And the first is that fat does have something to do with it. Just not the fat you—or the USDA—is thinking of.
In 1982, a British food scientist named Don Mottram undertook an interesting experiment. He set out to find exactly what fat had to do with flavor. So he performed an experiment where he removed different kinds of fat from beef, and then cooked and assessed the result. He tested the cooked de-fatted beef with a gas chromatography machine, which measures the "volatile aromatic compounds" that create the perception of flavor, and also subjected it to a panel of "13 assessors experienced in evaluating meat flavour."
When Mottram removed the visible fat from steak—the fat in the trim and marbling, the fat a USDA grader is on the lookout for—he didn't find much of a difference. The beef was still beefy. It had the characteristic nutty and roasty notes we all love.
Then Mottram removed a different kind of fat called phospholipids. This fat is invisible. It resides in cell walls and, compared to marbling fat, it tends to be less saturated. When Mottram cooked this phospholipidless beef, he found it didn't taste like beef at all. The profile of volatile aromatic compounds had changed completely. There was, he noted, "a highly significant difference in cooked aromas."
But phospholipids aren't the whole story. Remember the lean wild venison steak I mentioned earlier? Its intense taste compared to a USDA Prime steak isn't just about phospholipids. Deer, after all, don't possess a great deal more structural cellular fat than cattle, their ruminant cousins. The other important aspect to flavor is the organic substances that steep into an animal's flesh as it gets older. These are complex chemicals like terpenes (which are found in herbs), flavinoids, and carotenoids. When exposed to the heat of the pan or grill, they react with amino acids, proteins, sugars, and so forth to form yet more volatile aromatic compounds. Scientists call these chemicals "secondary compounds." In the case of cows, they originate in the forage an animal eats, and they are what make beef a food that varies according to its terroir.