WHEN I was growing up in Illinois a few decades ago, my father always cooked a roast of some kind for our Sunday afternoon dinner. My brother and sisters and I especially loved what we called “red meat,” which was beef cooked rare. The color was a big part of it, a vivid promise of juicy, mouth-filling flavor, nothing like the dull look of pork or chicken or pot roast. It may not have been up there with the loaves and fishes, but it seemed miraculous to me that red meat could come from the stolid creatures we saw — and smelled — on our after-dinner drives through the countryside.

In recent months I’ve been marveling at meat’s several shades of red, and at their creation stories. They all start with a meat protein called myoglobin. Like the hemoglobin in blood, myoglobin is a molecule that stores oxygen, and it has an iron atom at its center to which the oxygen can bind.

Oxygenated myoglobin is red, but when its structure is changed by heat or by other molecules, it changes color. That’s why redness in cooked meat signifies juiciness: As meat cooks, the heat causes the other meat proteins to coagulate and squeeze out their moisture. Myoglobin stays unchanged and red as the meat juices flow, then turns from red to gray-brown as the release of moisture ends and the meat becomes dry.

Because myoglobin exposed to air eventually turns brown, red surfaces on raw meat also signify freshness. Until recently, that is. Two and a half years ago, the word got out that fish processors were treating some tuna sold in the United States with small amounts of carbon monoxide gas to keep it red well past its usual shelf life. Last year we learned that the same thing was being done to some precut beef in the supermarket.