Recipes often stress this when the need to add water arises. Why is that? And just why does it take stews and other boiled dishes so long to cook and tenderize the meat? Isn’t boiling water more than enough to make the internal temperature of the meat reach at least 70 degrees C?
You’re asking two different questions here.
1)Why shouldn’t you add cold water to a boiling pot? Because you’ll drop the temperature and it’ll take longer to get back up to boiling.
2)Why does it take so long for stews to tenderize meat? I can talk about the specifics of that, but you’re cooking off the fat and connective tissue (which is chewy), that takes time. If all you had to do was heat the meat to tenderize it, then steaks wouldn’t…well steaks wouldn’t be steaks.
It takes time for the connective tissue in meat to break down. That’s why real BBQ can have cooking times in double digits.
Because if you add cold water,
Cold water brings the temperature down, so it takes time to bring it back up.
But the rest of the answer is indeed collagen breakdown. Meat doesn’t tenderize because it gets hot, meat gets tender when the collagen breaks down and becomes gelatin. This takes time far more than it takes temperature. And in fact, if you’ve got your stew at an actual boil, you probably have it too hot - your proteins are going to tighten, squeeze out the internal moisture and toughen the meat, and it’s going to take even longer to tender it up again. Turn your stew down to a simmer - roughly 6-10 bubbles per minute should gently break the surface of the liquid (95C) - and you’ll end up with more tender meat.
Also, make sure you’re using cheap tough meat. If you’re using “good” meat for stew, there’s probably not enough collagen in it to begin with.