Subject: What type of acid(s) is in fruits to be able to generate electricity?

I did my science fair project on fruit batteries, & I thought ascorbic 
acid was the acid that reacted w/zinc and copper to make electricity, but 
the pears made more electricity than the lemons!(~0.85V compared to ~0.73V)
I put together 2 pineapples and 3 lemons and generated 3.5 V, but that 
still didn't power a bulb which only required 1.4 V.  What went wrong with 
my experiment? (The bulb works with a real battery, I tested it)I did have 
to use a galvanized nail and a tubular piece of copper in place of zinc 
and copper electrodes, but shouldn't that have worked just the same?

Re: What type of acid(s) is in fruits to be able to generate electricity?