pdswife said:
Why not make your own cactus? .
how do you make a cactus?
(i'm not touching the "plop my pork in it" line...he hee hee.)
often, thick bottled marinades are kept refrigerated after opening the bottle, thus making them even thicker. they also have ingredients to stabilize them, keeping the ingredients from seperating. thicker fluids stay mixed better.
also, they are supposed to be thick, to help the marinade cling to the object to be flavoured/tenderized. on top of that, you tend to use more marinade if it's thick, and they're in the business of seeling the stuff.
btw, many marinades do not tenderize; they are only to add moisture and flavor.
michael mentioned that fruits are a good ingredient in a marinade. certain ones, like those containing papaya or pineapple have enzymes, papain and bromelain respectively. enzymes are what tenderizes meat by breaking down proteins.
some good info:
http://www.taunton.com/finecooking/pages/c00157.asp
i tend to make my own marinades, but if i find a good bottled one, i am not averse to using that as well. i currently like a few of the world harbors brand marinades, especially the hawaiian teryaki and the fajita.
if you make a thin marinade, just put the meat and marinade in an ziplock baggie, mush it around to distribute, then suck out as much air as possible and close it up. this helps marinades stay in contact with the meat.