Why does tough meat turn tender after hours of cooking?

Tough meat turns tender in a long braise because its connective tissue, mostly collagen, slowly breaks down into gelatin. Muscle protein includes connective tissue made of collagen and elastin, and after death muscle proteins denature to varying degrees, except the collagen and elastin. If collagen is denatured enough, for example by heating, its three strands separate, and this describes the formation of gelatin.

That is why stewing suits the least tender cuts, which become tender and juicy with slow, moist heat. Cuts with marbling and gelatinous connective tissue make moist, juicy stews, while lean meat may easily become dry.

Braising works this way: the food is typically browned at high temperature, then simmered in a covered pot in cooking liquid, at a very low simmer, until the meat can be cut with the gentlest pressure from a fork. A common misconception says braising adds moisture to meat. The opposite is true, and the appearance of moisture comes from gentle cooking breaking down connective tissue and collagen, which lubricates and tenderizes fibers.

The word collagen comes from the Greek kolla, glue, and means glue producer, because boiling the skin and sinews of animals gave early glue.