Raisins are a common yeast source in older beer & wine recipes. The correct yeast for wine is naturally found on the skins.
Whether modern packing methods would kill that is something I'd know nothing about, however.
btw, Prauger (sp?) winery in Napa claims to be the only commercial operation in the U.S. to use only the natural yeast on the wine (the rest may use that yeast, but culture it to speed the start). The consequence is hitting the market a year later, but the wine is spectacular.
hawk