Sourdough is a bread made by the fermentation of dough using wild lactobacillaceae and yeast. Lactic acid from fermentation imparts a sour taste and improves keeping qualities. In the Encyclopedia of Food Microbiology, Michael Gaenzle writes: “The origins of bread-making are so sourdough bread bowl that everything said about them must be pure speculation. Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer brewing process, and after 1871 by purpose-cultured yeast.
Europe, is usually leavened with sourdough. Baker’s yeast is not useful as a leavening agent for rye bread, as rye does not contain enough gluten. French bakers brought sourdough techniques to Northern California during the California Gold Rush, and it remains a part of the culture of San Francisco today. The sourdough tradition was carried into Alaska and the Yukon territories of Canada during the Klondike Gold Rush of 1898. Conventional leavenings such as yeast and baking soda were much less reliable in the conditions faced by the prospectors. In English-speaking countries, where wheat-based breads predominate, sourdough is no longer the standard method for bread leavening.
Manufacturers of non-sourdough breads make up for the lack of yeast and bacterial culture by introducing into their dough an artificially-made mix known as bread improver or flour improver. Sourdough baking has a devoted community today. Many devotees share starters and tips via the Internet. Hobbyists often proudly share their work on social media.
Some devotees find interest in history. Sourdough baking became more popular during the COVID-19 pandemic, as increased interest in home baking caused shortages of baker’s yeast in stores, whereas sourdough can be propagated at home. Purism is a part of the appeal. Flour naturally contains a variety of yeasts and bacteria. Obtaining a satisfactory rise from sourdough takes longer than a dough leavened with baker’s yeast because the yeast in a sourdough is less vigorous. In the presence of lactic acid bacteria, however, some sourdough yeasts have been observed to produce twice the gas of baker’s yeast.
As it ferments, sometimes for several days, the volume of the starter is increased by periodic additions of flour and water, called “refreshments”. As long as this starter culture is fed flour and water regularly, it will remain active. The ratio of fermented starter to fresh flour and water is critical in the development and maintenance of a starter. This ratio is called the refreshment ratio. Higher refreshment ratios are associated with greater microbial stability in the sourdough. A drier and cooler starter has less bacterial activity and more yeast growth, which results in the bacterial production of more acetic acid relative to lactic acid. Conversely, a wetter and warmer starter has more bacterial activity and less yeast growth, with more lactic acid relative to acetic acid.