Acidic ingredients don't tenderize meat, they actually will toughen it if left in too long or the marinade is strong.
Originally Posted by Haggis
In my experience Balsamic Vinegar has no effect on the meat apart from helping to tenderise the meat by breaking down the protein strands. Of course it does impart a delicious flavour and colour.
I have marinated meat in balsamic and cook on a very hot grill, have never noticed any bitter taste.
Mmmmm stew, another thing not to eat in summer!
Here are exceprts from Shirley's article (which can be found here:http://www.taunton.com/finecooking/pages/c00157.asp
"There is a commonly held belief that soaking a tough cut of meat in a marinade will make it tender. Sadly, this just isn't true much of the time. While some marinades are very successful at adding flavor to meat, chicken, and fish, they are, with one exception, a disaster at tenderizing.
... highly acidic marinades can actually toughen food, while enzymatic marinades can turn the surface of the food to mush.
Acidic marinades "denature" proteins. Imagine the protein in raw meat, chicken, or fish as individual units of coiled ribbon, with bonds holding each coil in a tight bundle. When these proteins are exposed to an acidic marinade, the bonds break and the proteins unwind. Almost immediately, one unwound protein runs into another unwound protein and they bond together into a loose mesh. (This is the same thing that happens when proteins are exposed to heat.)
At first, water molecules are attached to and trapped within this protein mesh, so the tissue remains juicy and tender. But after a short time, if the protein is in a very acidic marinade, the protein bonds tighten, water is squeezed out, and the tissue becomes tough.
In limited cases, mildly acidic marinades can add wonderful flavor to fish and meat, especially if you enhance the mixture with fresh herbs, spices, or perhaps another liquid like Worcestershire sauce. The key is to use the correct strength acid for the food you're marinating.
A fairly tight-textured cut of meat like flank steak can survive a more acidic marinade. Since the marinade only penetrates a fraction of an inch, it won't toughen the meat.
Shirley O. Corriher is the author of CookWise (Morrow).