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Sourdough or sourdough bread is a
bread Bread is a staple food prepared from a dough of flour (usually wheat) and water, usually by baking. Throughout recorded history and around the world, it has been an important part of many cultures' diet. It is one of the oldest human-made foo ...
made by the
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...
of
dough Dough is a thick, malleable, sometimes elastic paste made from grains or from leguminous or chestnut crops. Dough is typically made by mixing flour with a small amount of water or other liquid and sometimes includes yeast or other leavening ...
using wild
lactobacillaceae The ''Lactobacillaceae'' are a family of lactic acid bacteria. It is the only family in the lactic acid bacteria which includes homofermentative and heterofermentative organisms; in the ''Lactobacillaceae,'' the pathway used for hexose fermentati ...
and
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
.
Lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...
from fermentation imparts a sour taste and improves keeping qualities.


History

In the ''Encyclopedia of Food Microbiology'', Michael Gaenzle writes: "The origins of bread-making are so ancient that everything said about them must be pure speculation. One of the oldest sourdough breads dates from 3700 BCE and was excavated in
Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
, but the origin of sourdough
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...
likely relates to the origin of agriculture in the
Fertile Crescent The Fertile Crescent ( ar, الهلال الخصيب) is a crescent-shaped region in the Middle East, spanning modern-day Iraq, Syria, Lebanon, Israel, Palestine and Jordan, together with the northern region of Kuwait, southeastern reg ...
and
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medite ...
several thousand years earlier", which was confirmed a few years later by archeological evidence. ... "Bread production relied on the use of sourdough as a leavening agent for most of human history; the use of
baker's yeast Baker's yeast is the common name for the strains of yeast commonly used in baking bread and other bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentable s ...
as a leavening agent dates back less than 150 years."
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic ...
described the sourdough method in his '' Natural History'': Sourdough remained the usual form of leavening down into the European
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
until being replaced by
barm Barm is the foam or scum formed on the top of a fermenting liquid, such as beer, wine, or feedstock for spirits or industrial ethanol distillation. It is used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, h ...
from the
beer Beer is one of the oldest and the most widely consumed type of alcoholic drink in the world, and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from cer ...
brewing process, and after 1871 by purpose-cultured yeast. Bread made from 100% rye flour, popular in the northern half of Europe, is usually leavened with sourdough. Baker's yeast is not useful as a
leavening agent In cooking, a leavening agent () or raising agent, also called a leaven () or leavener, is any one of a number of substances used in doughs and batters that cause a foaming action (gas bubbles) that lightens and softens the mixture. An altern ...
for
rye bread Rye bread is a type of bread made with various proportions of flour from rye grain. It can be light or dark in color, depending on the type of flour used and the addition of coloring agents, and is typically denser than bread made from wheat ...
, as rye does not contain enough
gluten Gluten is a structural protein naturally found in certain cereal grains. Although "gluten" often only refers to wheat proteins, in medical literature it refers to the combination of prolamin and glutelin proteins naturally occurring in all grai ...
. The structure of rye bread is based primarily on the starch in the flour as well as other carbohydrates known as pentosans; however, rye amylase is active at substantially higher temperatures than wheat amylase, causing the structure of the bread to disintegrate as the starches are broken down during baking. The lowered pH of a sourdough starter, therefore, inactivates the amylases when heat cannot, allowing the carbohydrates in the bread to gel and set properly. In the southern part of Europe, where
panettone Panettone (, ; lmo, label=Milanese, panetton ) is an Italian type of sweet bread, and fruitcake, originally from Milan, usually prepared and enjoyed for Christmas and New Year in Western, Southern, and Southeastern Europe as well as in South ...
is still made with sourdough as leavening, sourdough has become less common in the 20th century; it has been replaced by the faster-growing baker's yeast, sometimes supplemented with longer fermentation rests to allow for some bacterial activity to build flavor. Sourdough fermentation has re-emerged as a major fermentation process in bread production in the past ten years although it is commonly used in conjunction with baker's yeast as leavening agent. French bakers brought sourdough techniques to
Northern California Northern California (colloquially known as NorCal) is a geographic and cultural region that generally comprises the northern portion of the U.S. state of California. Spanning the state's northernmost 48 counties, its main population centers incl ...
during the
California Gold Rush The California Gold Rush (1848–1855) was a gold rush that began on January 24, 1848, when gold was found by James W. Marshall at Sutter's Mill in Coloma, California. The news of gold brought approximately 300,000 people to California ...
, and it remains a part of the culture of
San Francisco San Francisco (; Spanish for " Saint Francis"), officially the City and County of San Francisco, is the commercial, financial, and cultural center of Northern California. The city proper is the fourth most populous in California and 17t ...
today. (The nickname remains in " Sourdough Sam", the
mascot A mascot is any human, animal, or object thought to bring luck, or anything used to represent a group with a common public identity, such as a school, professional sports team, society, military unit, or brand name. Mascots are also used as fic ...
of the
San Francisco 49ers The San Francisco 49ers (also written as the San Francisco Forty-Niners) are a professional American football team based in the San Francisco Bay Area. The 49ers compete in the National Football League (NFL) as a member of the league's National ...
.) 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 sourdough tradition was carried into
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U. ...
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. 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. However, freezing does not kill a sourdough starter; excessive heat does. Old hands came to be called "sourdoughs", a term that is still applied to any Alaskan or Klondike old-timer. 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 Barm is the foam or scum formed on the top of a fermenting liquid, such as beer, wine, or feedstock for spirits or industrial ethanol distillation. It is used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, h ...
from beer making, then, after the confirmation of
germ theory The germ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or "germs" can lead to disease. These small organisms, too small to be seen without magnification, invade ...
by
Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, microbial fermentation and pasteurization, the latter of which was named af ...
, 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 Real Bread Campaign calls these products sourfaux. 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.


Modern culture

Sourdough baking has a devoted community today. Many devotees share starters and tips via the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, p ...
. Hobbyists often proudly share their work on
social media Social media are interactive media technologies that facilitate the creation and sharing of information, ideas, interests, and other forms of expression through virtual communities and networks. While challenges to the definition of ''social me ...
. Sourdough cultures contain communities of living organisms, with a history unique to each individual starter, and bakers can feel an obligation to maintain them. "Many like to think that their sourdough is unique, or their creation, or one that’s been handed down for generations, or been over the Chilkoot Pass, et cetera. Because tarteris 'alive' some tender hearts feel an obligation to its permanent health and survival." The different yeasts present in the air in any region also enter sourdough, causing starters to change depending on location. Some devotees find interest in history. Sourdough expert Ed Wood isolated millennia-old yeast from an ancient Egyptian bakery near the
pyramids of Giza The Giza pyramid complex ( ar, مجمع أهرامات الجيزة), also called the Giza necropolis, is the site on the Giza Plateau in Greater Cairo, Egypt that includes the Great Pyramid of Giza, the Pyramid of Khafre, and the Pyramid of Men ...
, and many individual starters, such as Carl Griffith's 1847 starter, have been passed down through generations. "I like the throwback of traditional bread, the things our great grandmothers ate," writes professional baker Stacie Kearney. Some bakers describe starters generations old, though Griffith's seems exceptional. Sourdough baking became more popular during the
COVID-19 pandemic The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing global pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The novel virus was first identif ...
, as increased interest in home baking caused shortages of baker's yeast in stores, whereas sourdough can be propagated at home. Sourdough baking requires minimal equipment and simple ingredients – flour, salt, and water – but invites practice. Purism is a part of the appeal. 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." Many bakers feed their starters on elaborate schedules, and many name them. Some approach sourdough as science, attempting to optimize flavor and acidity with careful measurements, experimentation, and correspondence with professional microbiologists. Some lineages of starter are freely shared, and others can be purchased, but many prefer to cultivate their own. Some techniques for doing so are fiercely debated, such as the use of commercial yeast to jump-start a culture while capturing wild yeasts, or adding grapes or milk.


Preparation


Starter

The preparation of sourdough begins with a
pre-ferment A ferment (also known as bread starter) is a fermentation starter used in ''indirect'' methods of bread making. It may also be called mother dough. A ferment and a longer fermentation in the bread-making process have several benefits: there is ...
(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 A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
including wild
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
and
lactobacilli The ''Lactobacillaceae'' are a family of lactic acid bacteria. It is the only family in the lactic acid bacteria which includes homofermentative and heterofermentative organisms; in the ''Lactobacillaceae,'' the pathway used for hexose fermentati ...
. The purpose of the starter is to produce a vigorous leaven 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. Flour naturally contains a variety of yeasts and bacteria. When wheat flour comes into contact with water, the naturally occurring enzyme
amylase An amylase () is an enzyme that catalyses the hydrolysis of starch (Latin ') into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Foods that contain large amount ...
breaks down the
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human d ...
into the sugars
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
and
maltose } Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two ...
, which sourdough's natural yeast can metabolize. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a stable culture. This culture will cause a dough to rise. The bacteria ferment starches that the yeast cannot metabolise, and the by-products, chiefly maltose, are metabolised by the yeast, which produces carbon dioxide gas, leavening the dough. 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.


Refreshment of the starter

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. In San Francisco sourdough, the
ratio In mathematics, a ratio shows how many times one number A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in lan ...
is 40% of the total weight, which is roughly equivalent to 67% of the new-dough's weight. A high refreshment ratio keeps acidity of the refreshed dough relatively low. Acidity levels of below pH 4.0 inhibit lactobacilli and favour acid-tolerant yeasts. A starter prepared from scratch with a salted wheat-rye dough takes about 54 hours at to stabilise at a pH between 4.4 and 4.6. 4% salt inhibits ''L. sanfranciscensis'', while ''C. milleri'' can withstand 8%. A drier and cooler starter has less bacterial activity and more yeast growth, which results in the bacterial production of more
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compon ...
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. The yeasts produce mainly CO2 and ethanol. 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. 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." 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. Maintaining metabolically active sourdough with high leavening activity typically requires several refreshments per day, which is achieved in bakeries that use sourdough as sole leavening agents but not by amateur bakers that use the sourdough only weekly or even less frequently.


Local methods

Bakers have devised several ways of encouraging a stable culture of micro-organisms in the starter. Unbleached, unbromated flour contains more micro-organisms than more processed flours.
Bran Bran, also known as miller's bran, is the hard outer layers of cereal grain. It consists of the combined aleurone and pericarp. Corn (maize) bran also includes the pedicel (tip cap). Along with germ, it is an integral part of whole grains, ...
-containing (wholemeal) flour provides the greatest variety of organisms and additional minerals, though some cultures use an initial mixture of white flour and rye or whole wheat flour or "seed" the culture using unwashed organic grapes (for the wild yeasts on their skins). Grapes and grape
must Must (from the Latin ''vinum mustum'', "young wine") is freshly crushed fruit juice (usually grape juice) that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace and typically makes up 7–23% of t ...
are also sources of lactic acid bacteria, as are many other edible plants. Basil leaves are soaked in room-temperature water for an hour to seed traditional Greek sourdough. Using water from boiled potatoes is said to increase the activity of the bacteria by providing additional starch. 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. Some bakers recommend unchlorinated water for feeding cultures. Because a sourdough fermentation relies on microorganisms, using water without these agents may produce better results. Bottled drinking water is suitable; chlorine, but not
chloramines Chloramines refer to derivatives of ammonia and organic amines wherein one or more N-H bonds have been replaced by N-Cl bonds. Two classes of compounds are considered: inorganic chloramines and organic chloramines. Inorganic chloramines Inorgan ...
, can be removed from tap water by boiling it for a time, or simply by leaving it uncovered for at least 24 hours. Chlorine and chloramines can both be removed by activated carbon filters. Adding a small quantity of diastatic
malt Malt is germinated cereal grain that has been dried in a process known as " malting". The grain is made to germinate by soaking in water and is then halted from germinating further by drying with hot air. Malted grain is used to make beer, ...
provides
maltase Maltase (, '' alpha-glucosidase'', ''glucoinvertase'', ''glucosidosucrase'', ''maltase-glucoamylase'', ''alpha-glucopyranosidase'', ''glucosidoinvertase'', ''alpha-D-glucosidase'', ''alpha-glucoside hydrolase'', ''alpha-1,4-glucosidase'', ''al ...
and simple sugars to support the yeasts initially. 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. The original starter culture may be many years old. 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. For this reason, sourdough products inherently keep fresh for a longer time than other breads, and are good at resisting spoilage and mold without the additives required to retard spoiling of other types of bread. The flavour 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.


Baking

The starter must be fed 4 to 12 hours prior to being added to dough, by mixing flour and water to 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. The leaven is mixed with flour and water to make a final dough of the desired consistency. The starter weight is usually 13% to 25% of the total flour weight, though formulas may vary. The dough is shaped into loaves, left to rise, and then baked. A number of 'no knead' methods are available for sourdough bread. 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. Because the rise time of most sourdough starters is longer than that of breads made with baker's yeasts, sourdough starters are generally unsuitable for use in a
bread machine A bread making machine or breadmaker is a home appliance for baking bread. It consists of a bread pan (or "tin"), at the bottom of which are one or more built-in paddles, mounted in the center of a small special-purpose oven. This small oven is ...
. However, sourdough that has been proved over many hours, using a sourdough starter or mother dough, can then be transferred to the machine, utilizing only the baking segment of the bread-making program, bypassing 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 A baking stone is a portable cooking surface used in baking. It may be made of ceramic, stone or, more recently, salt. Food is put on the stone, which is then placed in an oven, though sometimes the stone is heated first. Baking stones are used mu ...
in the oven and misting of the dough to produce steam. Furthermore, ideal crust development requires loaves of shapes not achievable in a machine's loaf tin.


Biology and chemistry of sourdough

Sourdough is a stable culture of
lactic acid bacteria Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped (bacilli) or spherical ( cocci) bacteria that share common metabolic and physiological characteristics. These bacte ...
and
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
in a mixture of
flour Flour is a powder made by grinding raw grains, roots, beans, nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredient of bread, which is a staple food for many cult ...
and
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
. Broadly speaking, the yeast produces gas (
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is tr ...
) 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 Lactic acid fermentation is a metabolic process by which glucose or other six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactate, which is lactic acid ...
. During sourdough fermentation, many cereal enzymes, particularly phytases, proteases and pentosanases, are activated through acidification and contribute to biochemical changes during sourdough fermentation.


Lactic acid bacteria

Every starter consists of different lactic acid bacteria which is introduced to the starter through the environment, water, and flour used to create the starter. 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 is 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 aerotolerant anaerobes, which means that though they are anaerobes, they can multiply in the presence of oxygen. 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; heterofermentative lactic acid bacteria are usually associated with homofermentative (producing mainly one product) lactobacilli, particularly ''Lactobacillus'' and ''Companilactobacillus'' species.


Yeasts

The most common yeast species in sourdough are '' Kazachstania exigua (Saccharomyces exiguous)'', ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been ...
'', ''K. exiguus'' and ''K. humilis'' (previously '' Candida milleri'' or '' Candida humilis'').


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, have a pH range of 3.8 to 4.5, and are fermented in a temperature range of . '' Fructilactobacillus sanfranciscensis'' was named for its discovery in San Francisco sourdough starters, though it is not
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...
to San Francisco. ''F. sanfranciscensis'' and ''Limosilactobacillus pontis'' often highlight a lactic-acid bacterial flora that includes ''Limosilactobacillus fermentum'', ''Fructilactobacillus fructivorans'', ''Levilactobacillus brevis'', and ''Companilactobacillus paralimentarius''. The yeasts ''Saccharomyces exiguus'', ''Kasachstania humilis'', or ''Candida holmii'' usually populate sourdough cultures symbiotically with '' Fructilactobacillus sanfranciscensis''. The perfect yeast ''S. exiguus'' is related to the imperfect 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 anomaola'', ''C. peliculosa'', ''P. membranifaciens'', and ''C. valida''. See Table 183.6 There have been changes in the
taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
of yeasts in recent decades. ''F. sanfranciscensis'' requires maltose, 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 Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbe ...
. Wheat dough contains abundant starch and some polyfructosanes, which enzymes degrade to "maltose, fructose and little
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
." 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 (
fructan A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides. Fructans can be found in over 12% of the angiosperms including both monocots and dicots such as agave, artichokes, asparagus, leek ...
s)." 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. When ''L. sanfrancisensis'' reduces all available fructose, it stops producing acetic acid and begins producing ethanol. If the fermenting dough gets too warm, the yeasts slow down, producing less fructose. Fructose depletion is more of a concern in doughs with lower enzymatic activities. A Belgian study of
wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologica ...
and
spelt Spelt (''Triticum spelta''), also known as dinkel wheat or hulled wheat, is a species of wheat that has been cultivated since approximately 5000 BC. Spelt was an important staple food in parts of Europe from the Bronze Age to medieval times. No ...
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. ''S. cerevisiae'' has less tolerance to acetic acid than other sourdough yeasts. Continuously maintained, stable sourdough cannot be unintentionally contaminated by ''S. cerevisiae.''


Type II sourdough

In Type II sourdoughs,
baker's yeast Baker's yeast is the common name for the strains of yeast commonly used in baking bread and other bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentable s ...
or ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been ...
'' is added to leaven the dough; ''L. pontis'' and ''Limosilactobacillus panis'' in association with ''Lactobacillus'' species are dominant members of type II sourdoughs. They have a pH less than 3.5, and are fermented within a temperature range of for several days without feedings, which reduces the flora's activity. 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.


Type III sourdough

Type III sourdoughs are Type II sourdoughs subjected to a drying process, usually either spray or
drum drying Drum drying is a method used for drying out liquids from raw materials with a drying drum. In the drum-drying process, pureed raw ingredients are dried at relatively low temperatures over rotating, high-capacity drums that produce sheets of drum-dr ...
, and are mainly used at an industrial level as flavoring agents. They are dominated by "drying-resistant actic acid bacteriasuch as ''Pediococcus pentosaceus'', ''L. plantarum'', and ''L. brevis''." The drying conditions, time and heat applied, may be varied in order to influence
caramelization Caramelization is a process of browning of sugar used extensively in cooking for the resulting sweet nutty flavor and brown color. The brown colors are produced by three groups of polymers: caramelans (C24H36O18), caramelens (C36H50O25), and c ...
and produce desired characteristics in the baked product.


Types of bread

There are many breads that use techniques similar to that used in the making of sourdough bread. Danish
rugbrød () is a very common form of rye bread from Denmark. usually resembles a long brown extruded rectangle, no more than 12 cm high, and 30–35 cm wide, depending on the bread pan in which it is baked. The basic ingredient is rye flou ...
(''rye bread'') is a dense, dark bread best known from its use in the Danish
smørrebrød (; originally , "butter and bread") smørbrød "butter bread" ( Norwegian), is a traditional open-faced sandwich in the cuisines of Denmark, Norway and Sweden that usually consists of a piece of buttered rye bread (, a dense, dark brown bread), ...
(open-faced sandwiches). The Mexican '' birote salado'' started out in the city of
Guadalajara Guadalajara ( , ) is a metropolis in western Mexico and the capital of the state of Jalisco. According to the 2020 census, the city has a population of 1,385,629 people, making it the 7th largest city by population in Mexico, while the Guadalaj ...
as a short French
baguette A baguette (; ) is a long, thin type of bread of French origin that is commonly made from basic lean dough (the dough, though not the shape, is defined by French law). It is distinguishable by its length and crisp crust. A baguette has a dia ...
that replaces the
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
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 Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
and
milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modul ...
. 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 Pumpernickel (; ) is a typically heavy, slightly sweet rye bread traditionally made with sourdough starter and coarsely ground rye. It is sometimes made with a combination of rye flour and whole rye grains ("rye berries"). At one time it was ...
is traditionally made from a sourdough starter, although modern pumpernickel loaves often use commercial yeasts, sometimes spiked with
citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
or
lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...
to inactivate the amylases in the rye flour. Flemish desem bread (the word means 'starter') is a whole-wheat sourdough. In Azerbaijan, whole-wheat sourdough flatbreads are traditionally eaten. In Ethiopia,
teff ''Eragrostis tef'', also known as teff, Williams lovegrass or annual bunch grass, is an annual grass, a species of lovegrass native to the Horn of Africa, notably to both Eritrea and Ethiopia. It is cultivated for its edible seeds, also known ...
flour is fermented to make
injera Injera (, ; om, Biddeena; ) is a sour fermented pancake-like flatbread with a slightly spongy texture, traditionally made of teff flour. In Ethiopia, Eritrea, and some parts of Sudan, injera is the staple. Injera is central to the dining ...
. A similar variant is eaten in Somalia, Djibouti, and Yemen (where it is known as lahoh). In India,
idli Idli or idly () is a type of savoury rice cake, originating from the South India,popular as breakfast foods in Southern India and in Sri Lanka. The cakes are made by steaming a batter consisting of fermented black lentils (de-husked) and ric ...
s and dosa are made from a sourdough fermentation of rice and '' Vigna mungo''.


Possible fermentation effects

Sourdough bread has a relatively low
glycemic index The glycemic (glycaemic) index (GI; ) is a number from 0 to 100 assigned to a food, with pure glucose arbitrarily given the value of 100, which represents the relative rise in the blood glucose level two hours after consuming that food. The GI of ...
compared with other types of bread. The activity of cereal enzymes during sourdough fermentation hydrolyses phytates, which improves the absorption of some
dietary mineral In the context of nutrition, a mineral is a chemical element required as an essential nutrient by organisms to perform functions necessary for life. However, the four major structural elements in the human body by weight (oxygen, hydrogen, carbon, ...
s and vitamins, most of which are found in the bran. Sourdough fermentation reduces wheat components that may contribute to non-celiac wheat sensitivity and irritable bowel syndrome. 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 Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...
.


See also

* List of sourdough breads *
Biga Biga may refer to: Places * Biga, Çanakkale, a town and district of Çanakkale Province in Turkey * Sanjak of Biga, an Ottoman province * Biga Çayı, a river in Çanakkale Province * Biga Peninsula, a peninsula in Turkey, in the northwest par ...
, a pre-fermentation technique in Italian baking *
Herman cake Herman cake (often called Herman) is a 'friendship cake'. Similar to the Amish friendship bread, the starter is passed from person to person (like a chain letter) and continues to grow as it contains yeast and lactic acid bacteria. One starter ...
* History of California bread * Kyselo,
Czech Czech may refer to: * Anything from or related to the Czech Republic, a country in Europe ** Czech language ** Czechs, the people of the area ** Czech culture ** Czech cuisine * One of three mythical brothers, Lech, Czech, and Rus' Places * Czech ...
soup made from sourdough * List of microorganisms found in sourdough *
Salt-rising bread Salt-rising (or salt-risen) bread is a dense white bread that is traditional in the Appalachian Mountains, leavened by naturally occurring ''Clostridium perfringens'' and other bacteria rather than by yeast. Salt-rising bread is made from wheat f ...
*
Sour mash Sour mash (or sourmash) is a process used in the distilling industry that uses material from an older batch of mash to start the fermentation of a new batch, analogous to the making of sourdough bread with a starter. The term can also be used ...
*
Sour rye soup In West Slavic countries, as well as in Belarus, fermented cereals, such as rye, wheat, or oatmeal, are used to make soups. In Poland and parts of Belarus, rye is traditional for making żur; a variant made with wheat flour instead of rye is know ...
, Polish soup (''zurek'') made with rye flour soured in the same process that occurs in the forming of sourdough *
Injera Injera (, ; om, Biddeena; ) is a sour fermented pancake-like flatbread with a slightly spongy texture, traditionally made of teff flour. In Ethiopia, Eritrea, and some parts of Sudan, injera is the staple. Injera is central to the dining ...


Notes


References


External links

* {{Authority control Doughs Fermented foods