Collagen

Collagen is the most abundant protein in mammals, forming the structural framework of connective tissue, tendons, and the sheaths surrounding muscle fiber bundles. In steak cuts, collagen exists primarily in the thin connective tissue sheets (perimysium and epimysium) that separate and surround muscle fibers.

Collagen's dual nature: At room temperature and moderate cooking temperatures, collagen is tough — it's what makes a raw steak chewy. But at sustained temperatures above 150–160°F, collagen denatures and converts to gelatin, which is soft and lubricating. This is the entire principle behind braising tough cuts: long, slow cooking converts collagen to gelatin, making tough meat tender.

In steak cuts: Most tender steak cuts (ribeye, NY strip, tenderloin) have relatively little collagen compared to braising cuts (chuck, brisket, shank). The collagen that is present doesn't have time to convert to gelatin during the brief cooking of a steak — it barely begins to denature before the steak is done. This is one reason tougher cuts benefit from longer cooking and tender cuts should be cooked quickly to a lower temperature.

Collagen and the 150°F threshold: Around 150°F, collagen begins contracting, which squeezes the muscle fibers and expels moisture. In a steak cooked past medium, this contraction contributes to the dry, firm texture. It's another reason to keep steaks below 140°F internal for optimal juiciness.

Practical tip: If you're eating a cut with visible connective tissue (like a strip steak's fat cap margin), the collagen in that tissue won't have time to soften at medium-rare temperatures. Trim it before eating, or accept the chewy texture.