One of the cool things about living in a university town is the occasional fascinating discussion on some of the more obscure topics in the firmament of knowledge. I had written a small piece for the bulletin a few months ago on why Catholics use wine instead of grape juice. As an aside, I mentioned mustum and the indult for use by alcoholic priests. A university professor replied:
To add to your discussion about wine, mustum, and grape juice:
In a warm climate like the Middle East, unpasteurized grape juice (mustum) would almost immediately spoil because of the various microbes that it would contain from picking the grapes through pressing them through filtering to clarify the juice. It would be necessary to subject it to an alcoholic fermentation with yeast to produce a sizable concentration of alcohol (ethanol) and to get rid of the sugar in the juice to prevent other fermentations. Different microbes would contaminate different batches of juice, but nearly all of them are more susceptible to alcohol poisoning than yeast are. The combination of lack of sugar and presence of alcohol stabilizes wine against further degradation.
Almost surely yeast fermentations two millennia ago were started by inoculating the grape juice with a little of the previous batch of wine.
Because of these factors, there is no chance that Jesus and his disciples ever drank grape juice rather than wine, unless they stood at the side of the filter to collect it.
The image of teetotaling early Christians sipping from a spigot at the edge of a vineyard