Copying sourdough cultures from baked breads?
Well, I was curious if it is possible. Does the heat/sitting on the shelf de-activate the bacteria? I know that it's possible to buy cultures over the net. I've read that some bakeries/pizza places will sell some sourdough dough too. :O).
Bacteria are very localized, and are always in a state of recolonizing. If you buy a San Francisco sourdough starter it is good for about two to three weeks and then your starter will be colonized by your local bacteria. Heat will kill of almost all of the bacteria if not all.
To make your own sourdough: mix 1c flour to 2c water and let stand uncovered on your counter for 24 hours, stirring occasionally. Cover and let stand another 24 hours, stirring every 4 hours or so. You should be able to smell the natural yeast and bacteria at work. Cover and then refrigerate. It should not stink; it should smell like sourdough.
I have used and recommend Cook's Illustrated; No Knead Bread V2 recipe to make a rustic sourdough loaf.
My local pizza restaurant and a local bakery will sell sourdough dough, the bakery requires a min 5 lb order, pizza place is ~4$, enough for two large pies.