Some of the preservatives that are used in raisins could cause fermentations problems. I recently used Sun Maid brand baking raisins formy mead and they were preservative free.
Note: I used their Golden Raisins in some hard cider and didn't have any problems with fermentation.
Info from Sun Maid Website:
Q: Are there additives or preservatives in Raisins?
A: There are no additives or preservatives in our Sun-Maid Sun-Dried Raisins
or our Zante Currants. When the raisins are ready to be processed, we clean
the raisins using large aspirators (vacuums), graders, and shakers to remove
stems, dirt, and other foreign objects. We then do a very thorough washing
in fresh water which rehydrates the raisins.
Sun-Maid Golden Raisins are cleaned in much the same way, although these
raisins are treated with the preservative sulfur dioxide. This form of sulfite has
been used for centuries to prevent darkening of the fruit during the drying process.
Sulfites also help preserve the vitamin A and vitamin C contents of foods.
Edited by: masta