In the time of Jesus there was no refrigeration. There was no pasteurization to stop yeast that grows on the skin of the grape from making alcohol. Louis Pasteur discovered this. He lived in the 1800's and Jesus much earlier. Perhaps communion should be held on Wednesday night in an upper room as per scripture and use wine. Grape juice is not mentioned in the Bible and not authorized.
This link talks about yeast on the grape skin. See section under Spontaneous Generation.
https://en.wikipedia.org/wiki/Louis_Pas ... blications
Wine
Re: Wine
Little late here but this is interesting but be 'warned' it is Presbyterian
http://www.gpcweb.org/why-do-we-use-wine-in-communion-
http://www.gpcweb.org/why-do-we-use-wine-in-communion-