I know from experience that too much acidity will cause chicken to
toughen up rather than tenderize. Some time back I did three different
marinades on a "family-size" package of boneless/skinless chicken
breasts. The ones that were soaked in a marinade based on apple cider
vinegar were noticeably stringier than the other ones. One of the other
two marinades only had a bit of lemon and lime juice for acid. The
third marinade was yogurt-based, so probably not very acidic at all.

Some time back I saw a good article and series of pictures that show
what "meat tenderizers" do to meat over time. Suffice it to say that
the results of the longest contact time were not pleasant to look at or
contemplate. Basically meat mush. I can't find the link right now, but
I'm sure the article is still out there somewhere on the web.

Bob
===============================
In article >,
says...
> In the case of the apple juice, I've found that an hour or so for
> chicken or overnight for ribs gives an extremely moist meat with just a
> little bit of a sweet taste that goes well with smoke. Maybe, since
> apple juice is slightly acidic, a longer time would result in chemical
> changes to the meat, but the shorter time just makes it juicy. I've
> done no research into the changes in meat texture as a function of
> acidity or papaya, so if anyone has more observations, I'd sure enjoy
> reading about them.
>