It is noteworthy that the use of unfermented grape juice in communion is a fairly recent development historically and in America, for example, Welch's was partially responsible for the support of "temperance" in both individual consumption and also in sacramental celebration. This is not to say of course that strictly on this basis the use of grape juice is illegitimate, but the historical practice does weigh against it. I understand that "conscience" is a factor here, but it should not be the ultimate factor—we believe God speaking in Scripture is the final authority.
In Scripture "wine" is the "fruit of the vine," so technically speaking the juice one could make from the grapes immediately harvested is "wine" although unfermented. However in practice citizens of Israel (along with the rest of the Ancient Near East) allowed this to ferment as refrigeration was not a convenient possibility. Thus in the vast majority of instances when "wine" is referred to it is the "fermented fruit of the vine." The warnings against drunkenness (cf. Eph 5:18) use the same word to describe what Jesus made the water into in John 2, which means that alcoholic content is assumed (so the addition of sugar to stop the fermentation is a modern invention).
Thus, in conclusion, I believe that while unfermented grape juice is "legitimate" as it qualifies as the "fruit of the vine" I do not believe it is "original" or "ideal" from how "wine" is used given the example and instruction of Scripture.
For additional perspective, I would point you to an article published in our online magazine Ordained Servant "Wine or Grape Juice: Theological and Pastoral Reflections on the Fruit of the Vine in Communion" by John W. Mahaffy.
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