Raw honey is full of "things" including bacteria and wild yeast so I suppose it's not impossible that honey contain tannic acid, although it's certainly not enough to get your yeasts off. I'll bet buckwheat honey probably has some tannins, in fact there's a contributor over at Got Mead (http://www.gotmead.com) who always includes a couple ounces of buckwheat honey in his "quick" mead recipes, although I always figured it was for color because he always supplements with processed nutrients and "energizers". If Vikings can do it. . .