Why are steak tips cooked rare sometimes chewier than those cooked to medium?
Several factors contribute to the toughness or tenderness of meat, but primary among them is the size of the meat fibers (the thicker and longer they are the harder they are to bite through). Flap meat, the cut that steak tips come from, looks striated because the fibers are very thick and packed very tightly in bundles. This makes them tough when cooked to rare.
Cooking past medium-rare (130 degrees) tenderizes thick-fibered meat because the muscle fibers shrink in diameter by about 25 percent at 130 degrees, which makes them easier to chew through. However, if you cook thick-fibered meat (like steak tips) beyond medium to 140 degrees, the shrinking muscle fibers will squeeze out moisture, making the meat dry and tough. Thus, aim for the sweet spot with long-fibered meats like steak tips by cooking them to medium (between 130 and 140 degrees).
THE BOTTOM LINE: Flap meat’s thick muscle fibers make it chewy when rare. Cooking to medium (between 130 and 140 degrees but not beyond) shrinks the diameter of the meat fibers while retaining moisture, making them more tender.