What influences the amount of voltage that comes out a fruit?

The voltage of any electrochemical cell is entirely dependent on the two different electrode materials, not the type of electrolyte.

Take the same electrodes and put them in anything containing an electrolyte: e.g. any fruit, a potato, salt water, baking soda solution, vinegar, and you will get the same voltage from the cell.

The relevant property of the electrode materials is called electronegativity.