Sourdough

Sourdough is one of the most ancient forms of leavening bread. It was the standard method of breadmaking for most of human history until the Middle Ages, when it was replaced by barm. Barm, in turn, was replaced in the late 19th and early 20th centuries by industrially produced baker's yeast. The Encyclopedia of Food Microbiology states: "One of the oldest sourdough breads dates from 3700 BCE and was excavated in Switzerland, but the origin of sourdough fermentation likely relates to the origin of agriculture in the Fertile Crescent and Egypt several thousand years earlier", Pliny the Elder described several methods of sourdough leavening to make bread. 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. 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 nickname remains in "Sourdough Sam", the mascot of the San Francisco 49ers.) Sourdough has long been associated with the 1849 gold prospectors, though they were more likely to make bread with commercial yeast or baking soda. The "celebrated" San Francisco sourdough is a white bread characterized by a pronounced sourness, and indeed the strain of Lactobacillus in sourdough starters is named Fructilactobacillus sanfranciscensis (previously Lactobacillus sanfranciscensis), alongside the sourdough yeast Kasachstania humilis (previously Candida milleri) found in the same cultures. 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". In English-speaking countries, where wheat-based breads predominate, sourdough is no longer the standard method for bread leavening. It was gradually replaced, first by the use of barm from beer making, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts. Although sourdough bread was superseded in commercial bakeries in the 20th century, it has undergone a revival among artisan bakers and, more recently, in industrial bakeries. In countries where there is no legal definition of sourdough bread, the dough for some products named or marketed as such is leavened using baker's yeast or chemical raising agents as well as, or instead of, a live sourdough starter culture. The term sourfaux has been applied to such breads by the Real Bread Campaign, an initiative that has advised against the consumption of most modern breads (sourdough and otherwise) because of such preparation techniques plus the inclusion of leavening agents, such as baking soda and baking powder, which may be linked to celiac disease. ==Preparation==

Starter Sourdough leavening requires a pre-ferment (the starter or leaven, variously known as the chief, chef, head, mother, or sponge), a live mixture of flour and water, containing a colony of microorganisms including wild yeast and lactobacilli. The purpose of the starter is to produce a vigorous leavening and to develop the flavour of the bread. In practice there are several kinds of starters, as the ratio of water to flour in the starter (hydration) varies. A starter may be a liquid batter or a stiff dough. To initially create the pre-ferment, water and flour are mixed and left in a warm place for a week or two. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a stable culture. 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. In San Francisco sourdough, the ratio is 40% of the total weight, which is roughly equivalent to 67% of the new-dough's weight. A high refreshment ratio keeps the pH of the refreshed dough relatively low (making it more acidic). 4% salt inhibits L. sanfranciscensis, while C. milleri can withstand 8%. High amounts of lactic acid are desired in rye and mixed-rye fermentations, while relatively higher amounts of acetic acid are desired in wheat fermentations. A dry, cool starter produces a sourer loaf than a wet, warm one. Firm starters (such as the Flemish Desem starter, which may be buried in a large container of flour to prevent drying out) tend to be more resource-intensive than wet ones. Intervals between refreshments A stable culture in which F. sanfranciscensis is the dominant bacterium requires a temperature between and refreshments every 24 hours for about two weeks. Refreshment intervals of longer than three days acidify the dough and may change the microbial ecosystem. In this process, the ratio of yeasts to lactobacilli may be altered. Generally, if once-daily refreshment-intervals have not been reduced to several hours, the percentage amount of starter in the final dough should be reduced to obtain a satisfactory rise during proof. Faster starter processes, requiring fewer refreshments, have been devised, sometimes using commercial sourdough starters as inoculants. These starters generally fall into two types. One is made from traditionally maintained and stable starter doughs, often dried, in which the ratios of microorganisms are uncertain. Another is made from microorganisms carefully isolated from petri dishes, grown into large, homogeneous populations in fermentors, and processed into combined baker's products with numerically defined ratios and known quantities of microorganisms well suited to particular bread styles. as are many other edible plants. Basil leaves are soaked in room-temperature water for an hour to seed traditional Greek sourdough. Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially. The flavor of sourdough bread varies from place to place according to the method used, the hydration of the starter and the final dough, the refreshment ratio, the length of the fermentation periods, ambient temperature, humidity, and elevation, all of which contribute to the microbiology of the sourdough. Mixing and shaping the bread The starter is typically fed 4 to 12 hours prior to being added to dough. Feeding means mixing flour and water into the starter. This creates an active leaven, which should grow in size and is ready to use when it is bubbly and floats in water. A portion of the leaven is mixed with flour and water to make a final dough of the desired consistency. The starter weight is usually 13–25% of the total flour weight, though formulas may vary. Using a smaller ratio of cold unfed starter (called sourdough discard) in the range of 5% to 10% can also create good sourdough loaves; the fermentation time will be longer and can result in improved flavor. A number of 'no knead' methods are available for sourdough bread. The dough is shaped into loaves, left to proof (the final resting period before baking), and then baked. Due to the length of time sourdough bread takes to proof, many bakers may refrigerate their loaves prior to baking. This process is known as retardation to slow down the proofing process. This process has the added benefit of developing a richer flavoured bread. 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. The acidic conditions in sourdough, along with the bacteria also producing enzymes that break down proteins, result in weaker gluten and may produce a denser finished product. Because the rise time of most sourdough starters is longer than that of breads made with baker's yeasts, traditional sourdough starters are generally unsuitable for use in a bread machine. However, bread machine manufacturers may offer recipes specifically optimized for their devices, with the starter made directly in the machine's pan using a dedicated setting and then supplemented later with ingredients such as apple cider vinegar. Also, sourdough that has been proofed over many hours, using a sourdough starter or mother dough, may be transferred to a bread machine to be used only with the baking segment of the bread-making program, bypassing the timed mechanical kneading by the machine's paddle. This may be convenient for single loaf production, but the complex blistered and slashed crust characteristics of oven-baked sourdough bread cannot be achieved in a bread making machine, as this usually requires the use of a baking stone in the oven and misting of the dough to produce steam. In sourdough baking, one or more deep primary cuts are often made to allow controlled expansion during oven spring, and additional shallow slashes are added for decorative effect. Once baked, sourdough products keep fresh for a longer time than many other breads, and are good at resisting spoilage and mold without the additives required to retard spoiling of other types of bread. Continuing the cycle Bakers often make loaves with fermented dough from a previous batch (which they call mother dough, mother sponge, chef, or seed sour), rather than making a new starter every time they bake. There are two ways to continue the sourdough cycle after the bread dough is mixed. The first simply reserves a portion of the starter culture for another day. 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, and the original starter culture may be many years old. Flour, water, and any other ingredients (e.g., sugar, milk, apples, potatoes) are stirred in to the reserved portion. A portion of this starter is used on subsequent days for baking more bread, while always saving some for future use. The other approach is the French-style levain technique. Instead of preserving a portion of the sourdough starter, it preserves a portion of raw, unbaked bread dough. The next day, the dough is put into a bowl with the other ingredients for a loaf of bread. Before shaping and baking the new loaf, a portion is pulled out and set aside so that it will be available to leaven the next batch of dough. ==Biology and chemistry==

Sourdough is a stable culture of lactic acid bacteria and yeast in a mixture of flour and water. Broadly speaking, the yeast produces gas (carbon dioxide) which leavens the dough, and the lactic acid bacteria produce lactic acid, which contributes flavor in the form of sourness. The lactic acid bacteria metabolize sugars that the yeast cannot, while the yeast metabolizes the by-products of lactic acid fermentation. During sourdough fermentation, many cereal enzymes, particularly phytases, proteases and pentosanases, are activated through acidification and contribute to biochemical changes during sourdough fermentation. The lactic acid bacteria are a group of gram-positive bacteria capable of converting carbohydrate substrates into organic acids and producing a wide range of metabolites. Organic acids, including propionic, formic, acetic acid, and lactic acid, create an unfavorable environment for the growth of spoilage and pathogenic microorganisms. Lactic acid bacteria commonly found in sourdough include Leuconostoc, Pediococcus, Weissella and other genera. But by far, the most prevalent species belong to the very large and diverse genus, Lactobacillus. Lactic acid bacteria are a group comprising aerotolerant anaerobes, meaning anaerobes that can multiply in the presence of oxygen, and microaerophiles, meaning microbes that multiply at levels of oxygen lower than atmospheric. 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; Type I sourdough Traditional sourdoughs used as sole leavening agent are referred to as Type I sourdough; examples include sourdoughs used for San Francisco Sourdough Bread, Panettone, and rye bread. Type I sourdoughs are generally firm doughs, The yeast S. exiguus is related to the yeasts C. milleri and C. holmii. Torulopsis holmii, Torula holmii, and S. rosei are synonyms used prior to 1978. C. milleri and C. holmii are physiologically similar, but DNA testing established them as distinct. Other yeasts reported found include C. humilis, C. krusei, Pichia anomala, C. peliculosa, P. membranifaciens, and C. valida. There have been changes in the taxonomy of yeasts in recent decades. while C. milleri is maltase negative and thus cannot consume maltose. C. milleri can grow under conditions of low pH and relatively high acetate levels, a factor contributing to sourdough flora's stability. In order to produce acetic acid, F. sanfrancisensis needs maltose and fructose. Wheat dough contains abundant starch and some polyfructosanes, which enzymes degrade to "maltose, fructose and little glucose." The terms "fructosan, glucofructan, sucrosyl fructan, polyfructan, and polyfructosan" are all used to describe a class of compounds that are "structurally and metabolically" related to sucrose, where "carbon is stored as sucrose and polymers of fructose (fructans)." Yeasts have the ability to free fructose from glucofructans which compose about 1–2% of the dough. Glucofructans are long strings of fructose molecules attached to a single glucose molecule. Sucrose can be considered the shortest glucofructan, with only a single fructose molecule attached. A Belgian study of wheat and spelt doughs refreshed once every 24 hours and fermented at in a laboratory environment provides insight into the three-phase evolution of first-generation-to-stable sourdough ecosystems. In the first two days of refreshment, atypical genera Enterococcus and Lactococcus bacteria highlighted the doughs. During days 2–5, sourdough-specific bacteria belonging to the genera Lactobacillus, Pediococcus, and Weissella outcompete earlier strains. Yeasts grew more slowly and reached population peaks near days 4–5. By days 5–7, "well-adapted" Lactobacillus strains such as L. fermentum and Lactiplantibacillus plantarum had emerged. At their peaks, yeast populations were in the range of about 1–10% of the lactobacilli populations or 1:10–1:100. One characteristic of a stable dough is that the heterofermentative have outcompeted homofermentative lactobacilli. F. sanfranciscensis has typically not been identified in spontaneous sourdoughs, even after multiple cycles of back-slopping; it was rapidly introduced in wheat sourdoughs, however, when plant materials were used to start the fermentation. Investigations of wheat sourdough found that S. cerevisiae died off after two refreshment cycles. Type II sourdough In Type II sourdoughs, baker's yeast or Saccharomyces cerevisiae is added to leaven the dough; L. pontis and Limosilactobacillus panis in association with Lactobacillus species are dominant members of Type II sourdoughs. This process was adopted by some in industry, in part, due to simplification of the multiple-step build typical of Type I sourdoughs. In Type II sourdoughs, yeast growth is slowed or stopped due to higher fermentation temperatures. These doughs are more liquid and once fermented may be chilled and stored for up to a week. They are pumpable and used in continuous bread production systems. ==Types of bread==

There are many breads that use techniques similar to that used in the making of sourdough bread. Danish rugbrød (rye bread) is a dense, dark bread best known from its use in the Danish smørrebrød (open-faced sandwiches). The Mexican birote salado started out in the city of Guadalajara as a short French baguette that replaces the yeast with a sourdough fermentation process, yielding a bread that is crunchy outside but soft and savory inside. Amish friendship bread uses a sourdough starter that includes sugar and milk. It is also leavened with baking powder and baking soda. An Amish sourdough is fed with sugar and potato flakes every 3–5 days. German pumpernickel is traditionally made from a sourdough starter, although modern pumpernickel loaves often use commercial yeasts, sometimes spiked with citric acid or lactic acid to inactivate the amylases in the rye flour. Flemish desem bread (the word means 'starter') is a whole-wheat sourdough. Whole-wheat sourdough flatbreads are traditionally eaten in Azerbaijan. In Ethiopia, teff flour is fermented to make injera. A similar variant is eaten in Somalia, Djibouti, and Yemen (where it is known as lahoh). In India, idlis and dosa are made from a sourdough fermentation of rice and black gram. ==Possible fermentation effects==

Sourdough bread has a relatively low glycemic index compared with other types of bread. The activity of cereal enzymes during sourdough fermentation hydrolyses phytates, which improves the absorption of some dietary minerals 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. A review of the health effects of sourdough published in 2023 concluded that: "...although a broad range of sourdough related health benefits are praised in publications, social media and by bakers, a sound evidence base for measurable effects on health related clinical endpoints has not been established". == Modern culture ==

Sourdough baking has a devoted community today. Many devotees share starters and tips via the Internet. Hobbyists often share their work on social media. Sourdough cultures contain communities of living organisms, with a history unique to each individual starter, and bakers can feel an obligation to maintain them. The different yeasts present in the air in any region also enter sourdough, causing starters to change depending on location. "I like the throwback of traditional bread, the things our great grandmothers ate," writes professional baker Stacie Kearney. In southern Europe, sourdough remains traditional for certain breads like panettone. However, in the 20th century, sourdough became less common, replaced by the faster-acting baker's yeast. Sometimes, this yeast is supplemented with longer fermentation periods to allow some bacterial activity, which adds flavor. During the 2010s, sourdough fermentation regained popularity as a major method in bread production, often used alongside baker's yeast as a leavening agent. ==See also==