Raisins are very toxic to ferrets because they cannot digest them properly. Recent studies also suggest that giving raisins on a daily basis – even as a treat, can lead to kidney issues in the long term. It is best not to feed raisins or grapes for this reason alone.
Ferrets should ideally be fed a diet of fats and high quality protein. Since they have only 1/2 of the intestinal length of cats and lack an intestinal cecum, they are unable to digest vegetable and fruits properly as well.
The best diet is around 30% to 40% protein and 15-30% fat. Deviation from this baseline should only be done under the supervision of a veterinarian as doing so could lead to a nutritional deficiency.
Check the packaging of the food you are choosing to make sure they fit the lifestyle of your ferret. If ferret food is not available of if you are in a pinch, high quality kitten foods may also be fed.