[3] Sourdough remained the usual form of leavening down into the European Middle Ages[4] until being replaced by barm from the beer brewing process, and after 1871 by purpose-cultured yeast.

The lowered pH of a sourdough starter inactivates the amylases when heat alone cannot, allowing the carbohydrates to gel and set properly in the bread structure.

During the 2010s, sourdough fermentation regained popularity as a major method in bread production, often used alongside baker’s yeast as a leavening agent.

[8] The sourdough tradition was carried into the Department of Alaska in the United States and the Yukon Territory in Canada during the Klondike Gold Rush of 1898.

Experienced miners and other settlers frequently carried a pouch of starter either around their neck or on a belt; these were fiercely guarded to keep from freezing.

[10] The significance of the nickname's association with Yukon culture was immortalized in the writings of Robert Service, particularly his collection of "Songs of a Sourdough".

It was gradually replaced, first by the use of barm from beer making,[11] then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts.

[18][19] Sourdough cultures contain communities of living organisms, with a history unique to each individual starter, and bakers can feel an obligation to maintain them.

[20] "I like the throwback of traditional bread, the things our great grandmothers ate," writes professional baker Stacie Kearney.

[19] Sourdough baking requires minimal equipment and simple ingredients – flour, salt, and water – but invites practice.

As described by one enthusiast, "If you take flour, water, (wild) yeast and salt, and play around with time and temperature, what comes out of the oven is something utterly transformed."

Some approach sourdough as science, attempting to optimize flavor and acidity with careful measurements, experimentation, and correspondence with professional microbiologists.

[17] The preparation of sourdough begins with a pre-ferment (the "starter" or "leaven", also known as the "chief", "chef", "head", "mother" or "sponge"), a fermented mixture of flour and water, containing a colony of microorganisms including wild yeast and lactobacilli.

[36] As it ferments, sometimes for several days, the volume of the starter is increased by periodic additions of flour and water, called "refreshments" or referred to as a "feeding".

[citation needed] A stable culture in which F. sanfranciscensis is the dominant bacterium requires a temperature between 25–30 °C (77–86 °F) and refreshments every 24 hours for about two weeks.

[32] The intervals between refreshments of the starter may be reduced in order to increase the rate of gas (CO2) production, a process described as "acceleration.

[citation needed] The piped drinking water supplied in most urban areas is treated by chlorination or chloramination, adding small amounts of substances that inhibit potentially dangerous micro-organisms but are harmless to animals.

[59] Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially.

Because of their pH level and the presence of antibacterial agents, such cultures are stable and able to prevent colonization by unwanted yeasts and bacteria.

[53][64][65] Using a smaller ratio of cold un-feed starter in the range of 5% to 10% can also create good sourdough loaves, however, the fermentation time will be longer and can result in improved flavor.

Due to the length of time sourdough bread takes to proof, many bakers may refrigerate their loaves prior to baking.

[citation needed] Sourdough is a stable culture of lactic acid bacteria and yeast in a mixture of flour and water.

[71] Major lactic acid bacteria in sourdough are heterofermentative (producing more than one product) organisms and convert hexoses by the phosphoketolase pathway to lactate, CO2 and acetate or ethanol;[66] heterofermentative lactic acid bacteria are usually associated with homofermentative (producing mainly one product) lactobacilli, particularly Lactobacillus and Companilactobacillus species.

[58][74][75][6] The yeasts Saccharomyces exiguus, Kasachstania humilis, or Candida holmii[74] usually populate sourdough cultures symbiotically with Fructilactobacillus sanfranciscensis.

[27][28][29][30][31] C. milleri can grow under conditions of low pH and relatively high acetate levels, a factor contributing to sourdough flora's stability.

[5] A Belgian study of wheat and spelt doughs refreshed once every 24 hours and fermented at 30 °C (86 °F) in a laboratory environment provides insight into the three-phase evolution of first-generation-to-stable sourdough ecosystems.

The drying conditions, time and heat applied, may be varied in order to influence caramelization and produce desired characteristics in the baked product.

[87][88] The Mexican birote salado started out in the city of Guadalajara as a short French baguette[citation needed] that replaces the yeast with a sourdough fermentation process, yielding a bread that is crunchy outside but soft and savory inside.

[96][97][98] The activity of cereal enzymes during sourdough fermentation hydrolyses phytates, which improves the absorption of some dietary minerals[98] and vitamins, most of which are found in the bran.

[98][99][100] Sourdough fermentation and lactic acid bacteria may be useful to improve the quality of gluten-free breads, such as by enhancing texture, aroma, and shelf life.