calling food scientists--question about meat
Hmm, I'll try. It's about what's inside the meat in terms of connective tissue and fat. In a stereotypically tender cut of meat like beef tenderloin there is very little fat and essentially zero connective tissue. When exposed to heat this solid, relatively fat-less meat begins to tense up. All meat will do this whether it's fatty or not. The key is how long it's cooked. Put it on the grill and only partially cook it (say medium rare) you'll still have tender meat. Over do it and you have a expensive piece of "brick." When cooked too long the meat has tensed so much it has squeezed all water and fat out of itself and there is nothing left to provide moisture. The difference with a tough cut of meat, like a pork shoulder, is that it comparitively has lots of fat and connective tissue. Now this kind of meat also shrinks and tenses when exposed to heat, but when this cooks for a long period of time the fat and especially the connective tissue melts and dissolves. #1, This kind of bastes the meat from the inside out. #2, The dissolved tissue is no longer there in it's original form to toughen the meat and it basically falls apart - you've no doubt seen this in a pot roast or BBQ pork.
Thank you both for very informative responses. I don't eat meat, but prepare it once in a blue moon for my long-suffering husband. This explains the terrible result of the belgian beef stew I made last night with steak tips. I knew it was the wrong cut, but I never envisioned how utterly bad it would turn out. Oh well--I guess I owe him a proper stew now...
That's almost right, but fat is not connective tissue, and vice versa. Prime, or loin and rib meat, has way more marbled fat than the "cheaper" cuts or lesser grades.
What makes steak tender is, broadly speaking, interspersed almost microsopic streaks of fat - what's called "marbling", within limits, the more the better. The fat, easy to chew, breaks up the less-easy-to-chew raw-ish meat fibers. And it doesn't hurt that humans are genetically predisposed to love the mouthfeel of fat. ;) Even and heavy marbling are what you pay big bucks for in prime meat. It's existence and prevalence are a matter of genes and husbandry. Needless to say, it's a small percentage of all beef.
What makes pot roast "tender" but not dry, on the other hand, is not fat marbling, but mostly the gelatin-rendered-from-connective tissue intespersed throughout the meat and added water (in some form) improving what would otherwise be dry, stringy meat tissue. This is why simply adding fat to a lean cut doesn't work, and larding is only somewhat effective.
When you braise a good steak cut, the lack of connective tissue - ideal for grilling - makes for a dry, stringy meat. If you grill a braising cut, the tough connective tissue makes it unpleasant if not functionally impossible to eat.
"Marbling" and connective tissue, the opposite ends of the meat spectrum in a sense, can look disconcertingly similar until you start looking a little more closely, but the end result is quite different, even if it takes the same "effort to chew."
Good information from both HaagenDazs and MikeG, to which I would only add that the chemical reaction both alluded to that converts tough connective tissue (collagen) to soft gelatin occurs slowly at a temperature at or above approximately the boiling point of water (at sea level) and in the presence of moisture. That's the reason that long stewing or braising makes tough cuts like chuck or short ribs soft and falling-apart tender. At the same time, the gelatin keeps the meat from becoming dry as the liquid is forced out of the muscle fibers by the heat.