Since the process of making wine involves the introduction of yeast, should “the fruit of the vine” used for the Lord’s Supper be unfermented seeing yeast represents sin in Scripture? It would seem that this would picture the sinlessness of Jesus even as the unleavened bread does.
In the Lord’s time unfermented wine was called “new wine” and there’s no mention of new wine being required for Passover. In any case this would have been impossible since Passover was in the spring and the grape harvest was in the fall.
In the natural fermentation process no outside ingredients are introduced into the mix. Naturally occurring yeast in the grape skins converts the natural grape sugar into alcohol. All kosher wines are naturally fermented.
I’ve always preferred unfermented grape juice for communion to avoid discouraging any recovering alcoholics who might be in the congregation from partaking in the ceremony.