Sourdough Bread

The sour taste of Sourdough Bread actually comes not from the yeast, but from a lactobacillus, with which the yeast lives in symbiosis. The lactobacillus feeds on the byproducts of the yeast fermentation, and in turn makes the culture go sour by excreting lactic acid, which protects it from spoiling (since most microbes are unable to survive in an acid environment). All breads used to be sourdoughs, and the leavening process was not understood until the 19th century, when with the advance of microscopes, scientists were able to discover the microbes that make the dough rise. Since then, strains of yeast have been selected and cultured mainly for reliability and quickness of fermentation. 

Billions of cells of these strains are then packaged and marketed as "Baker's Yeast". Bread made with baker's yeast is not sour because of the absence of the lactobacillus. Bakers around the world quickly embraced baker's yeast for it made baking simple and so allowed for more flexibility in the bakery's operations. It made baking quick as well, allowing bakeries to make fresh bread from scratch as often as three times a day. While European bakeries kept producing sourdough breads, in the U.S., sourdough baking was widely replaced by baker's yeast, and only recently has that country (or parts of it, at least) seen the rebirth of sour-vinegar dough in artisan bakeries.

Sourdough breads are most often made with a sourdough starter (not to be confused with the starter method discussed above). A sourdough starter is a culture of yeast and lactobacillus. It is essentially a dough-like or pancake-like flour/water mixture in which the yeast and lactobacilli live. A starter can be maintained indefinitely by periodically discarding a part of it and refreshing it by adding fresh flour and water. (When refrigerated, a starter can go weeks without needing to be fed.) 

There are starters owned by bakeries and families that are several human generations old, much revered for creating a special taste or texture. Starters can be obtained by taking a piece of another starter and growing it, or they can be made from scratch. There are hobbyist groups on the web who will send their starter for a stamped, self-addressed envelope, and there are even mail-order companies that sell different starters from all over the world. An acquired starter has the advantage to be more proven and established (stable and reliable, resisting spoiling and behaving predictably) than from-scratch starters.

There are other ways of sourdough baking and culture maintenance. A more traditional one is the process that was followed by peasant families throughout Europe in past centuries. The family (usually the woman was in charge of bread making) would bake on a fixed schedule, perhaps once a week. The starter was saved from the previous week's dough. The starter was mixed with the new ingredients, the dough was left to rise, then a piece of it was saved (to be the starter for next week's bread). The rest was formed into loaves which were marked with the family sign (this is where today's decorative slashing of bread loaves originates from), and taken to the communal oven to bake. These communal ovens over time evolved into what we know today as bakeries, when certain people specialized in bread baking, and with time enhanced the process so far as to be able to mass produce cheap bread for everyone in the village.

San Francisco sourdoughs

The most famous sourdough bread made in the U.S. is the San Francisco Sourdough, which in contrast to the majority of the country has remained in continuous production for nearly 150 years, with some bakeries able to trace their starters back to California's territorial period. It is a white bread, characterized by a pronounced sourness (not all sourdoughs are as sour as the San Francisco Sourdough), so much so that the dominant strain of lactobacillus in sourdough starters was named Lactobacillus sanfrancisco.