This is mostly an experimental question, but here are some thoughts which might help:
0. When you compare different fruits, you'll need to be careful to have the same length of wires in them, at the same distance.
1. The electrical resistance will come partly from cell walls,
which are hard for the (electrically charged) ions to cross. I bet
you'll get different numbers for conductivity if you meaure juice or
fresh fruit. Mushed-up fruit may be in-between.
2. If you measure resistance with a standard dc meter, there will
be some charging effects at the wires. That may increase the
resistance. If you can find an ac resistance meter, that would work
That's all I can think of for now.
I'll be willing to bet that a lot of variation can be in how good
the contact is between the wires and the fruit. The contact resitance
should be inversely proportional to the surface area of contact, and
this really means the amount of contact that's wet. So make sure you
stick the probes or wires the same distance into the fruit or juice.
(published on 10/22/2007)