Pliny\'s grape juice bread experiment

[Epictetus]

After much trial and error, I've recently become somewhat-proficient baking sourdough bread, so I decided I wanted to try a Roman recipe.

Picenum still maintains its ancient reputation for making the bread which it was the first to invent, alica being the grain employed. The flour is kept in soak for nine days, and is kneaded on the tenth with raisin juice, in the shape of long rolls; after which it is baked in an oven in earthen pots, till they break. This bread, however, is never eaten till it has been well soaked, which is mostly done in milk mixed with honey.

Pliny, Natural History, 18.27

Martial mentions this bread, too:

XLVII. PICENTINE LOAVES.
Picentine flour teems with white nectar, just as the light sponge swells with the water it imbibes.

Martial. 13.47

Since Pliny says the flour is soaked for nine days, and we know the bread was light, I think it virtually certain that it rose with natural yeast, like today's sourdough. The grape juice might help with the raising process, too. There should be sugars and yeasts in the juice, so I suspect it will do something. I don't know what, but I'll find out because I started the nine-day process today so I can bake it the weekend after next.