It is possible that at least some raisins provide no nutrients to your must. Let's assume that all raising contain nitrogen in the same way that all grapes contain nitrogen.
1. Grapes could be deficient in nitrogen and a 100% grape wine may need additional nutrients. Even wines sometimes need dap for a healthy ferment
2. They could have enough nitrogen but not much left over to ferment any additional sugars
3. Sometimes they contain enough nitrogen to ferment the sugars in the grape itself and even some additional sugars
In case 1 the
raisins are a detriment to your mead as they provide more sugars than nitrogen.
In case 2 they
do not contribute anything to your mead, just some VERY LITTLE sugars, mouthfeel and tannins.
In case 3, and this is our
best case scenario, the raisins provide some net nitrogen to your must. However, I would be absolutely amazed if 1.5tbs of raisins is anywhere enough to ferment an 18% abv mead.
[BEGIN RANT]
I am calculating that for an 18% abv mead (if raisins are anything like grapes) you would need just enough water to rehydrate raisins until they are a thick wine. Basically you are doing an inverse raisin wine because you are rehydrating grapes rather than dehydrating
https://en.wikipedia.org/wiki/Straw_wine
Your fermenter needs to seem like nothing but grapes/raisins and minimal water in between the skins. Fermenting this is an art and contamination risk is so high because you cannot punch down the cap since the whole damn thing is raisin skins. Then you add honey... but you need so much raisins to get enough nitrogen that you're already at 14 or 15% abv, which means that your honey will not provide enough taste. It will essentially be a wine. Even raisin wines very often end up sweet, which quite possibly indicates that there is not enough nitrogen to go beyond 15% abv in the best of cases.
[END RANT]
Or I could simply say that I think Squatchy is right...
