What's Eating Gilbert Grape?

I wanted to make a simple battery to illustrate a voltaic cell. Put simply, two different metals, connected with a conductive wire, are inserted into a salt bridge (sea of electrolytes) and current (electrons) flow from one to the other. Because one metal is more electronegative that the other, and the salt bridge contains both positive and negative ions, we get a voltage between the two metals and flow a current in a loop between them and through the salt bridge.

In this case, I took a whopping five minutes to get what was most readily accessible to me and measure the voltage of our cell. I found a copper wire, a galvanized nail (meaning coated with zinc), and a grape. 

The copper has an electronegativity of 1.90  and the zinc coating the nail 1.65. Therefore zinc will be oxidized (lose electrons) and the copper will be reduced (gain electrons). Current will flow from the anode (nail) to the cathode (copper wire) through my voltmeter which will measure the electrical potential difference. inside the grape, there are several electrolytes. I believe the most common for a grape is Potassium bitartrate (or KC4H5O6). The K cation has a +1 charge and migrates to the cathode (copper). The bitartrate anion has a -1 charge and migrates to the anode (zinc). Other acids such as citric acid and other ions such as sodium exist in the grape.  

Electrical reactions are as follows:

Cathode: Cu2+ + 2e- -> Cu (standard electrode potential of .34V)

Anode: Zn -> Zn2+ + 2e- (standard electron potential of -.76V)

Together the potential difference is 1.1V if these reactions are under normal conditions (sufficient concentration and ion mobility at 25C).

My voltage with a cold grape measured .871V but was still slowly climbing. When I breathed on it it would always bump up a little, perhaps because the extra energy of heat put into the system allowed for more reaction (the grape was colder than 25C). Normal conditions for a grape may be more like .9V but I can't find a clear reference for this. I picked the one fruit nobody seems to have measured and published! 

Thanks for reading,

Paul

p.s. I redid the test with another firm grape, this time at room temperature (~25C). It measured .901V. It is possible that if the grape had ripened more, the voltage could go up or down from there. It is also possible that my Thompson seedless (sultana) grape varies a bit from other grape varieties. Winemaking is basically the chemistry of fermenting grapes, which like soap and baking is a whole other profession with thousands of years or art and science involved. For now, I am claiming .9V as the normal conditions for a grape with zinc and copper electrodes.