Fruit cell emf varying with temperature

[D_o_S]

Homework Statement

Hi all,
In school, I tried to investigate how the emf (voltage) of a fruit cell varies with temperature. I took ~200ml of freshly squeezed orange juice, 2 copper electrodes, a voltmeter, heater and thermometer.

I set up everything, and got an initial voltage of somewhere around ~0.2mV or so IIRC.
I started heating the orange juice, and the voltage went down. It seemed to be going down and down, until some bubbles around the copper electrode started to rise to the surface, when fluctuations in the voltage were noticed, sometimes up to 30mV.

Homework Equations

What is the relationship between the temperature and the voltage of a cell? I was thinking that the voltage (or perhaps current?) should rise, since the molecules are moving faster. On the other hand, the higher the temperature, the higher the resistance, and so in turn there should be a lower current?

All in all, should the voltage be affected by the change in temperature at all?

The Attempt at a Solution

I am lost. As proposed above, I have found 2 possible theories, but I cannot make sense of why the voltage would jump about.

Thanks in advance.

 

[Astronuc]

Increasing the temperature would increase the resistance in the copper electrodes.

A sudden onset of an increase in voltage could indicate the initiation of some electrochemical reaction, the sudden increase in conductivity of the solution, or the breakdown of any protective oxide on the electrode.