When meat is cooked for a long time, it generally tends to soften rather than toughen. The process of slow cooking or braising meat at low temperatures for an extended period breaks down the tough connective tissues and collagen present in the meat. As these tissues break down, they convert into gelatin, which adds moisture and tenderness to the meat.
The key to achieving this softening effect is to use low and slow cooking methods, such as braising, stewing, or slow-roasting, which allow the meat's collagen to dissolve gradually. This is particularly important for tougher cuts of meat that have a lot of connective tissues, such as beef chuck, pork shoulder, or lamb shanks. These cuts can be less tender when cooked quickly but become wonderfully tender and flavorful when cooked for a long time.
On the other hand, if meat is cooked too quickly at high temperatures, it can result in toughness because the muscle fibers tighten and contract, squeezing out moisture and making the meat chewier. For tender cuts of meat, like tenderloin or sirloin, quick cooking methods like grilling, pan-searing, or broiling are more suitable to retain their tenderness.
In summary, long, slow cooking methods help to soften tougher cuts of meat, while quick, high-temperature cooking is best suited for more tender cuts.