I heard it said that some will only have grape juice for Passover because it does not contain yeast, a.k.a. a leavening agent.
I found this:
Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is normally already invisibly present on the grapes. The fermentation can be done with this endogenous (or wild) yeast ; however, this may give unpredictable results depending on the exact types of yeast species that are present. For this reason a pure yeast culture is generally added to the must, which rapidly predominates the fermentation as it proceeds. This represses the wild yeasts and ensures a reliable and predictable fermentation.[
It appears that there would be yeast already there on the grapes regardless of man's intervention of adding more yeast to turn it into wine. I wonder how this applies in the spirit?