Boudin's sourdough bread's unique tangy taste and chewy center has been a hit with San Francisco residents since the bakery first opened in 1849. A big part of brand's success has been due to its unyielding consistency—a feat only accomplished because the company is still using the same yeast culture that Isidore Boudin collected 160 years ago.
Sourdough differs from conventional breads in that unlike it's fast-rising counterparts, sourdough does not employ cultivated baker's yeast for its leavening process. Instead, sourdough uses naturally-occurring symbiotic Lactobacillus-yeast cultures, much like what is used to make rye breads. However, a side effect of using Lactobacillus is that it produces lactic acid during the initial fermentation process, which imparts sourdough's sour taste.
Isidore Boudin immigrated to the US and settled in San Francisco at the start of the Gold Rush era. A baker by trade, he started Boudin's Bakery in 1849 and set about collecting natural airborne yeast—a process that involves mixing flour and water and letting it stand outside for a while. The available strains of airborne yeasts vary by region, however San Francisco's moist, temperate seaside climate turned out to be home to a yeast of exceptional quality, some of which Boudin was able to collect.
To make sourdough bread, you start off with a "head" of stiff dough (either wheat or white flour mixed with warm water) which is first left outside to collect wild yeast spores then allowed to ferment into an active yeast culture wherein naturally occurring enzymes in the flour break down starch into maltose which is further broken down into glucose that the yeast consume. Once the yeast rises—essentially reaching a point of equilibrium between yeast and flour—it can be used as "mother dough," a perpetually renewing yeast monoculture.
Since wild yeasts that make sourdough tend to be less vigorous than cultures baker's yeasts, they take much longer to ferment and cause bread to rise. Mother dough allows bakers to pre-ferment their dough before baking. This not only increases the shelf life of loaves but also gives the yeast more time to ferment and interact with the flour, which increases the complexity of the bread's taste. This mother dough is the secret to the consistent taste of Boudin's bread: since the strain of Lactobacillus-yeast cultures has not changed in the last 160 years, neither has the exquisite taste of the sourdough made from it. [Boudin's Bakery via 7x7 - Wiki]