Even with the same metal in each electrode you will get a voltage that depends on either differences in pH or in oxygen concentration only. Different pH will have a different electrochemical half cell voltage, and this also depends on the actual acid or base in that particular area, most of them being weak organic acids or bases. Different levels of diffused oxygen ( or hydrogen evolved in chemical reactions as well) also created electrochemical half cells that will be at different potentials.
You will never be able to measure voltage of a fruit or vegetable alone, it is the difference in electrochemical half cells ( measured with reference to a standard half cell which is a hydrogen gas in solution with a black platinum electrode, defined as 0V) of each side that is measured. You cannot easily separate the metal electrochemical cell from the solvent solution, as it varies with temperature, concentration and time and the current drawn as well.
