You just can eat the sour breads in San Francisco! You will not make, nor will you eat this kind of taste in another part of the world! This happen because lactobacillus sanfrancisco is specie of lactic acid bacteria that make the bread sour! This bacteria acts in yeast and it is just found in SF because here there is natural air-conditioning to this bacteria developed itself. Did you understand how much is it important??
I went to Boudin at 160, on Jefferson St and this location there are Bistro Boudin and Museum & Bakery Tour! There are another locations.
See more information on website http://www.boudinbakery.com/ ;)