I don't think there is much (if any) tannin in raisins. In white wines, golden raisins are used for body building. Dandelion wine, in particular, is very light bodied, almost watery. It benefits greatly from a pound of raisins in it. For some of my white wines (apple included), I add powdered tannin at about 1/8 tsp per gallon.
Lorena