Pliny\'s grape juice bread experiment

[Epictetus]

Slate magazine this morning has an article about BeerAdvocate.com's adjudged best beer: Russian River Brewing Company's Pliny the Younger.

And to tie beer and bread together, Pliny says that the Gauls used beer foam as a leavening agent to make bread rise!

The way I read it, the raisin juice is only used in the neading process, and not the soaking?

Yes, that is what I think, too. Right now I'm just soaking the flour in water. In the sourdough recipes I have used, a bit more water is added when it is time to knead. I was simply going to substitute grape juice for water at this final stage.

By the way, is there such a thing as "raisin juice"? Aren't raisins basically juiceless grapes? How can you get any juice out of them? In another translation I saw a footnote here, saying "or grape juice," so that is what I was planning on using. I'll probably crush the grapes myself, because I assume anything you buy in the store has been pasteurised and stuffed full of sugar and other additives.