Not about grapes but natural yeast.Really? A grape has carbon dioxide in it? So, like, if we had a food fight with grapes, those things would be exploding when they hit. Cool!
This has been said before here, but I've never seen it proven. Do you have some kind of scientific evidence for this? And it seems to me that if it is true, that doesn't mean that there would be much alcohol at all for quite awhile, probably not enough to taste or even matter.
According to the Wikipedia article, "It should be noted that few yeast, and lactic and acetic acid bacterial colonies naturally live on the surface of grapes." (https://en.wikipedia.org/wiki/Fermentation_in_winemaking). Now, Wikipedia is not reliable as a source, but this article does source the information. In other words, to have grape juice ferment because of yeast on the grapes, you must purposefully put the yeast there. Furthermore, it seems like a simple process before or right after harvesting the grapes could wash the grapes and produce pure grape juice, if one wished to do so.
In Maine there are many cider mills because there are many apple trees.
Every year we would collect several bushels of apples (our mill required a minimum of 6) and have them rendered for cider with space enough for freezing. The service and plastic containers cost 25 cents per gallon (n the 80's,90's).
I remember usually getting 20-30 gallons per year.
They went in a second freezer until the temperature was less than 32 F. then they went out to a shed.
On our heated porch I would put 2 gallons (no yeast or anything else added) which would foam away and then turn clear resulting in hard cider - maybe 5% alcohol, depends on the sugar content for any given year.
HankD