Julie, I don't think store-bought grapes would be good. The point is the wild yeast the grape skins collect while they're growing, and I suspect packaged grapes go through a cleansing/washing/sanitizing process before they hit the store shelf, which would rid them of anything that was on their surface — from not good (pesticides) to good (wild yeast). It wouldn't hurt to try, but if you did a side-by-side with and without grapes, I think you'd fine the results identical. Good question — thanks for connecting here. PJH

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