To read Mark Schatzker's description of the birth of his quest for the world's best steak, click here for the first item in this series.

"Fat is flavor." I presume you're familiar with this phrase—nay, mantra. You may have even uttered it. "Fat is flavor" is what you might call a core grilling value, the one rule about red meat that everyone can agree on. Its universality is such that you almost expect to find "fat is flavor" enshrined in the Constitution.

And the truth is, it practically is. The government believes in that fat is flavor the way it believes in taxes. When a grader from the United States Department of Agriculture assesses a carcass of beef, the feature he prizes above all others is fat. Specifically, he's looking for marbling: the little dots and curls of fat within meat. A more marbled steak is a tastier steak. Everyone knows that.

And everyone is wrong.

Want proof? Put a teaspoon of butter in your mouth. It's rich, certainly. It tastes thick and fills your mouth the way down fills a pillow. But is it flavorful? Is butter "intense"? Now try it with vegetable oil or lard. Same thing.

Or better yet, go buy yourself a USDA Prime steak and eat it along with a wild venison steak. You'll observe two things. First, the venison is way leaner than the beef. Second, it is exponentially more flavorful.

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.

When Mottram cooked this phospholipidless beef, he found it didn't taste like beef at all.

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.

(But note: when you round up cattle off pasture and stick them in a feedlot to feed them grain shipped in from the Midwest, any sense of terroir goes right out the window.)

So why is the whole world hung up on fat? Because decades ago, when the USDA began grading carcasses, fat was a decent indicator of eating quality. A fat carcass indicated a steer that had been raised well and had plenty of good food to eat. Truly lean beef—something most people today have never tasted—can be disgusting.

The cattle world of today, however, is a vastly different place. Now we fatten cattle in feedlots on steamed, flaked corn and bulk them up with hormones, antibiotics, and drugs. The USDA asked for fat, and the industry has become exceedingly good at giving it to them. Beef today looks good, but it doesn't have much flavor.

Before anyone runs out and declare war on marbling, however, keep in mind that fat isn't the enemy of a good steak, either. It may not add much in the way of beefy flavor, but it does make a steak richer, smoother, and juicier. So as far as that lean piece of venison goes, I suggest frying it in butter.

Also by Mark Schatzker:
In Search of Steak Glory