Make a starter with water, sugar, flour and some aspen bark and/or Oregon Grape berries. Both have natural yeast on their surface the culture will be sour in a few days.
Hmmm..., I don't really have access to those. Maybe though I will do an experiment and toss in frosty, concord grape. Maybe that will work too.
I ask as I simply expected my second sponge to sour just as the first did..., but it was natural sour probably from bacteria in the air and for some reason it didn't sour the second time.
Somebody tried to tell me that as long as the sponge is natural and I let it sit about three days before baking each following batch, I was making "sour dough". I told him I was pretty sure a bacteria that gave a special smell and sour flavor was necessary..., not simply some sort of time thing.
I also like to use brewing yeast when making historic bread..., I like Edme Ale Yeast, as it would be the same stuff used back then. It's funny stuff though. You need about 10 hours before the first rise... so make the dough at about 10 p.m., and set it aside, covered, if you want to bake at 06:00.
LD