Soaked some decomposing oak leaves for 36 hours in preboiled water with some yeast flakes. Stirred, put 1-2 ml into test tubes with 5 ml of malt beer ("Karamalz", I just picked a random brand), filled up with equal amounts of 5 % vinegar and whisky (didn’t have any vodka at hand). Put a thin layer of neutral vegetable oil on top.
Should keep out the vast majority of bad microbes and provide a mostly oxygen free environment for yeast.
IIRC, I’ve also read on suigeneris brewing that oak leaf litter has a 50 - 80 % chance of catching bretanomyce, which I’d be thrilled to find.
The overall liquid amount is like 9 - 10 ml, of which ~1.5 ml is whisky. That’s still not little, but it’s more like 6 % abv, so far from the 15 you calculated. But I agree that even that is probably a tad much. On the other hand, if I want to use it in a co-fermentation or even staggered pitch, it has to be able to take a punch.
ahh, ok, you said equal amounts of vinegar and whiskey so I was calculating it as being ~1/3rd beer and ~1/3rd whiskey. I'd still recommend just putting your yeast source into hopped wort and letting it ferment.