Sourdough#Starter

Sourdough is one of the most ancient forms of 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", and "Bread production relied on the use of sourdough as a leavening agent for most of human history; the use of baker's yeast as a leavening agent dates back less than 150 years".{{cite book |author=Gaenzle, Michael |editor1-last=Batt |editor1-first=Carl |title=Encyclopedia of Food Microbiology |date=1 April 2014 |edition=2nd |publisher=Academic Press |isbn=978-0123847300 |page=309 |chapter=Sourdough Bread}} Sourdough remained the usual form of leavening down into the European Middle Ages{{cite book |last1=Gobbetti |first1=Marco |last2=Gänzle |first2=Michael |title=Handbook on Sourdough Biotechnology |url=https://books.google.com/books?id=aRSEh1ce8XYC&pg=PA6 |date=2012 |publisher=Springer |isbn=978-1-4614-5425-0 |page=6}} 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.Peters, Erica J. San Francisco: A Food Biography. Rowman & Littlefield, 2013, p. 189. The "celebrated"{{cite book |last1=Davidson |first1=Alan |title=The Oxford Companion to Food |date=1999 |publisher=Oxford University Press |location=Oxford |isbn=978-0192115799 |pages=[https://archive.org/details/oxfordcompaniont00davi_0/page/756 756–757] |url=https://archive.org/details/oxfordcompaniont00davi_0/page/756 }} 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),{{Cite journal |last1=Zheng |first1=Jinshui |last2=Wittouck |first2=Stijn |last3=Salvetti |first3=Elisa |last4=Franz |first4=Charles M. A. P. |last5=Harris |first5=Hugh M. B. |last6=Mattarelli |first6=Paola |last7=O’Toole |first7=Paul W. |last8=Pot |first8=Bruno |last9=Vandamme |first9=Peter |last10=Walter |first10=Jens |last11=Watanabe |first11=Koichi |date=2020 |title=A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae |journal=International Journal of Systematic and Evolutionary Microbiology |volume=70 |issue=4 |pages=2782–2858 |doi=10.1099/ijsem.0.004107 |pmid=32293557 |issn=1466-5026 |doi-access=free |hdl=10067/1738330151162165141 |hdl-access=free}} alongside the sourdough yeast Kasachstania humilis (previously Candida milleri) found in the same cultures.

{{anchor|Alaska sourdough}}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. 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.{{cite journal |last=Fernald |first=Anya |date=November–December 2002 |title=Sourdough Baking |url=http://editore.slowfood.com/editore/riviste/slow/EN/34/sourdoughs.html |url-status=dead |journal=Slow - the International Herald of Tastes |issue=34 |archive-url=https://web.archive.org/web/20070928144452/http://editore.slowfood.com/editore/riviste/slow/EN/34/sourdoughs.html |archive-date=September 28, 2007 |access-date=June 18, 2010}} 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".{{Citation needed|date=January 2021}}

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,{{Cite web |title=The ale-barm method: Worthy of revival or just barmy bread? |url=https://www.bbc.co.uk/blogs/food/2011/07/the-ale-barm-method-worthy-of.shtml |website=BBC Food blog |language=en |access-date=2020-05-13}} then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts.{{Cite web |title=Biomedicine and Health: The Germ Theory of Disease |url=https://www.encyclopedia.com/science/science-magazines/biomedicine-and-health-germ-theory-disease |website=Encyclopedia.com |access-date=2020-05-13}} 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.{{Cite journal|last1=Gänzle|first1=Michael G.|last2=Zheng|first2=Jinshui|date=2019|title=Lifestyles of sourdough lactobacilli - Do they matter for microbial ecology and bread quality?|url=https://pubmed.ncbi.nlm.nih.gov/30172443/|journal=International Journal of Food Microbiology|volume=302|pages=15–23|doi=10.1016/j.ijfoodmicro.2018.08.019|issn=1879-3460|pmid=30172443|s2cid=52143236}}{{cite web |last1=Griggs |first1=Barbara |title=The rise and rise of sourdough bread |url=https://www.theguardian.com/lifeandstyle/2014/aug/12/rise-sourdough-bread-slow-fermented-health-benefits |newspaper=The Guardian |location=London |access-date=30 September 2016 |date=12 August 2014}} 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 it has pseudoscientifically speculated may be linked to celiac disease.{{Cite web |title=#Sourdough vs. #sourfaux |url=https://www.sustainweb.org/realbread/sourdough/ |access-date=2022-05-19 |website=Real Bread Campaign |language=en}}{{Cite news |date=2019-11-27 |title=Sourdough or sourfaux? Artisan bread label row erupts |language=en-GB |work=BBC News |url=https://www.bbc.com/news/business-50571461 |access-date=2022-05-19}}{{Cite web |title=What is #RealBread? {{!}} Real Bread Campaign |url=https://www.sustainweb.org/realbread/what_is_real_bread/ |access-date=2025-03-23 |website=www.sustainweb.org |language=en}}{{Cite web |title=Keep it for cakes Not #RealBread #sourdough {{!}} Real Bread Campaign |url=https://www.sustainweb.org/realbread/articles/feb21-baking-powder-soda-keep-it-for-cakes/ |access-date=2025-03-23 |website=www.sustainweb.org |language=en}}

File:Sourdough starter.jpg

Sourdough baking has a devoted community today. Many devotees share starters and tips via the Internet.{{Cite journal|last=Harris|first=Lynn|date=2003-08-01|title=Sourdough Culture|url=https://online.ucpress.edu/gastronomica/article/3/3/76/93471/Sourdough-Culture|journal=Gastronomica|language=en|volume=3|issue=3|pages=76–79|doi=10.1525/gfc.2003.3.3.76|issn=1529-3262|url-access=subscription}} Hobbyists often share their work on social media.{{Cite magazine|last=Nordhagen|first=Ari|date=2020-10-09|title=Sourdough Goes Viral: Lucky Lady Bread shares her starter during COVID-19 crisis|url=https://www.edibleinlandnw.com/print/sourdough-goes-viral-lucky-lady-bread-shares-her-starter-during-covid-19-crisis/|magazine=Edible Inland Northwest|language=en-US|access-date=2021-11-03|archive-date=2021-10-28|archive-url=https://web.archive.org/web/20211028120445/https://www.edibleinlandnw.com/print/sourdough-goes-viral-lucky-lady-bread-shares-her-starter-during-covid-19-crisis/|url-status=dead}}{{Cite news|last=Scott|first=Chey|date=2020-03-14|title=Homemade sourdough bread is seeing a quarantine-spurred resurgence; two local experts share their best bread-baking advice|language=en|work=Inlander|url=https://www.inlander.com/spokane/homemade-sourdough-bread-is-seeing-a-quarantine-spurred-resurgence-two-local-experts-share-their-best-bread-baking-advice/Content?oid=19607645|access-date=2021-10-28}} 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.

Some devotees find interest in history. Sourdough expert Ed Wood isolated millennia-old yeast from an ancient Egyptian bakery near the pyramids of Giza, and many individual starters, such as Carl Griffith's 1847 starter, have been passed down through generations.{{Cite news|last=Eaton|first=Lorraine|date=2012-05-02|title=For baker, old sourdough 'starter' still bubbles along|work=The Virginian-Pilot|url=https://www.pilotonline.com/food-drink/article_2aa16f6e-ced1-5258-9078-0843089a2d3b.html|access-date=2021-10-26}} "I like the throwback of traditional bread, the things our great grandmothers ate," writes professional baker Stacie Kearney. Some bakers describe starters that are generations old, though Griffith's seems exceptional.

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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. File:Mixing Sourdough starter into the flour.jpg

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.

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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.{{cite book |author=Jeffrey Hamelman |title=Bread: a baker's book of techniques and recipes |publisher=John Wiley |location=New York |year=2004 |pages=6–362 |isbn=978-0-471-16857-7}} 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.{{cite book |author=Reinhart, Peter |url=https://books.google.com/books?id=GHjNDAAAQBAJ&q=bread+baker%27s+apprentice |title=The Bread Baker's Apprentice: Mastering the Art of Extraordinary Bread |publisher=Ten Speed Press |location=Berkeley, Calif |year=2016 |page=244 |isbn=978-1-60774-865-6 |access-date=September 22, 2021}}

Flour naturally contains a variety of yeasts and bacteria.{{cite journal |journal=Cereal Chemistry |year=1978 |volume=55 |issue=6 |pages=889–898 |title=Microflora of wheat and wheat flour from six areas of the United States |author1=Rogers, R.F. |author2=Hesseltine, C.W. |name-list-style=amp |url=http://naldc.nal.usda.gov/download/25386/PDF |format=PDF |access-date=Feb 4, 2013 |archive-date=November 20, 2018 |archive-url=https://web.archive.org/web/20181120071612/https://naldc.nal.usda.gov/download/25386/PDF |url-status=dead }}{{cite book |title=Micro-Organisms in Foods 6 Microbial Ecology of Food Commodities |publisher=Kluwer Academic/Plenum Publishers |location=New York |year=2005 |pages=409–411 |isbn=978-0-387-28801-7 |url=https://books.google.com/books?id=Yy_oBodctoIC&pg=PA410 |access-date=2013-02-04}}See Table 8.9, bottom of page 410 When wheat flour comes into contact with water, the naturally occurring enzyme amylase breaks down the starch into the sugars glucose and maltose, which sourdough's natural yeast can metabolize.{{cite book|last1=Schlegel|first1=Hans G.|title=General Microbiology|date=1993|publisher=Cambridge University Press|isbn=978-0521439800|edition=7}} 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.{{#tag:ref|Michael Gänzle has said Markus Brandt estimated that, in a properly maintained sourdough of sufficient age, the yeasts and lactobacilli each contribute roughly 50% of the total CO₂. Gänzle pointed out that while there are fewer yeasts, they are larger.|group="note"}}

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.{{cite book |author=Peterson, James A. |title=Glorious French food: a fresh approach to the classics |publisher=J. Wiley |location=London |year=2002 |page=170 |isbn=978-0-471-44276-9 |url=https://books.google.com/books?id=EWSVpBheCnYC&pg=PT197 |access-date=2013-02-04 |quote=Because these natural yeasts are less aggressive and more genetically diverse than packaged yeasts, they give the dough a more complex flavor, partially because they allow for the competition of naturally occurring benevolent bacteria.}}{{cite book |author=Nicolette, M. Dumke |title=Easy Breadmaking for Special Diets: Use Your Bread Machine, Food Processor, Mixer, or Tortilla Maker to Make the Bread YOU Need Quickly and Easily |publisher=Allergy Adapt, Inc |year=2006 |page=95 |isbn=978-1-887624-11-4 |url=https://books.google.com/books?id=gRePix2Y4yAC&pg=PA95 |access-date=2013-02-04 |quote=In addition to the wild yeast being slower producers of the gas that makes bread rise, the lactobacilli take about twelve hours to develop the full flavor you want in your bread.}} In the presence of lactic acid bacteria, however, some sourdough yeasts have been observed to produce twice the gas of baker's yeast.{{Cite journal |last1=Häggman |first1=M. |last2=Salovaara |first2=H. |doi=10.1016/j.lwt.2007.02.001 |title=Microbial re-inoculation reveals differences in the leavening power of sourdough yeast strains |journal=LWT - Food Science and Technology |volume=41 |pages=148–154 |year=2008 |url=https://zenodo.org/record/1065934 }} 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.{{cite book |url=https://books.google.com/books?id=bKVCtH4AjwgC&pg=PA544 |author=McGee, Harold |title=On food and cooking: the science and lore of the kitchen |publisher=Scribner |location=New York |pages=544–546 |year=2004 |isbn=978-0-684-80001-1 |access-date=June 28, 2010}}

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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".{{cite book |url=https://archive.org/details/manualforarmyba00deptgoog |title=Manual for army bakers |publisher=Government Printing Office |location=Washington |year=1910 |page=[https://archive.org/details/manualforarmyba00deptgoog/page/n28 22] |access-date=Aug 13, 2011}} As long as this starter culture is fed flour and water regularly, it will remain active.{{cite web |url=http://www222.pair.com/sjohn/blueroom/sour.htm |author=S. John Ross |title=Sourdough Bread: How To Begin (easy sourdough for the beginner or novice) |access-date=June 17, 2011 |archive-date=December 11, 2018 |archive-url=https://web.archive.org/web/20181211153409/http://www222.pair.com/sjohn/blueroom/sour.htm |url-status=dead }}{{cite book |url=https://archive.org/details/completesourdoug00holm |url-access=registration |author1=Don Holm |author2=Myrtle Holm |title=The Complete Sourdough Cookbook |publisher=Caxton Press |location=Caldwell, Idaho |year=1972 |page=[https://archive.org/details/completesourdoug00holm/page/40 40] |isbn=978-0-87004-223-2 |access-date=June 28, 2010}}{{cite book |author=Khachatourians, George G. |title=Food Biotechnology: Microorganisms |publisher=Wiley-Interscience |location=New York |year=1994 |pages=799–813 |isbn=978-0-471-18570-3}}

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.{{cite journal |vauthors=Valcheva R, Korakli M, Onno B, etal |title=Lactobacillus hammesii sp. nov., isolated from French sourdough |journal=Int. J. Syst. Evol. Microbiol. |volume=55 |issue=Pt 2 |pages=763–7 |date=March 2005 |pmid=15774659 |doi=10.1099/ijs.0.63311-0 |url=http://ijs.sgmjournals.org/content/55/2/763.long |quote=... maintained by back slopping or rafraîchi ... in terms of ratio (sourdough/dough),...|doi-access=free }}{{cite web |url=http://www.thefreshloaf.com/node/5381/sourdough-rise-time-table |title=Sourdough Rise Time Table |publisher=The Fresh Loaf |date=2008-03-28 |access-date=2016-09-15}} Higher refreshment ratios are associated with greater microbial stability in the sourdough. In San Francisco sourdough, the ratio{{cite book |author=Panel on the Applications of Biotechnology to Traditional Fermented Foods, National Research Council |title=Applications of Biotechnology in Traditional Fermented Foods |year=1992 |publisher=The National Academies Press |url=http://www.nap.edu/openbook.php?record_id=1939&page=14 |isbn=9780309046855 |quote=This can be achieved by the sourdough process, in which some portion of one batch of fermented dough is used to inoculate another batch. This practice is also referred to as "back-slopping" or inoculum enrichment. The resulting starters are active and should not be stored but used in a continuous manner. |access-date=June 28, 2012}} 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 favor acid-tolerant yeasts.{{Citation needed|date=January 2021}}

A starter prepared from scratch with a salted wheat-rye dough takes about 54 hours at {{convert|27|C|F}} to stabilise at a pH between 4.4 and 4.6.{{cite book |author=Calvel, Raymond |url=https://books.google.com/books?id=xe0HePwpQrwC&q=primary+culture+levain&pg=PA89 |title=The taste of bread |publisher=Aspen Publishers |location=Gaithersburg, Md |year=2001 |pages=89–90 |isbn=978-0-8342-1646-4 |access-date=June 28, 2010}} 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 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.{{cite web |url=http://www.faqs.org/faqs/food/sourdough/faq/section-1.html |title=Section - 22. What is the Microbiology of San Francisco Sourdough? |access-date=2013-02-23 |quote=...yeasts do not produce appreciable amounts of either lactic or acetic acids, their main metabolites are ethanol and CO2.}} 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.{{cite book |author=Simpson, Benjamin K. |title=Food Biochemistry and Food Processing |date=2012 |publisher=John Wiley & Sons, Inc. |location=Oxford, UK |isbn=978-0-8138-0874-1 |page=667 |edition=2nd |url=https://books.google.com/books?id=EfAm-G5sYuoC&pg=PA666 |access-date=2014-11-16}} A dry, cool starter produces a sourer loaf than a wet, warm one.{{cite web |url=http://www.thefreshloaf.com/node/10375/lactic-acid-fermentation-sourdough |title=Lactic Acid Fermentation in Sourdough |publisher=The Fresh Loaf |date=2009-01-19 |access-date=2016-09-15}} 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.{{Citation needed|date=January 2021}}

A stable culture in which F. sanfranciscensis is the dominant bacterium requires a temperature between {{convert|25|-|30|C|F}} 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."Wikibooks:Cookbook:Sourdough Starter In this process, the ratio of yeasts to lactobacilli may be altered.{{cite book |author1=Nanna A. Cross |author2=Corke, Harold |author3=Ingrid De Leyn |author4=Nip, Wai-Kit |title=Bakery products: science and technology |url=https://archive.org/details/bakeryproductssc00huiy |url-access=limited |publisher=Blackwell |location=Oxford |year=2006 |page=[https://archive.org/details/bakeryproductssc00huiy/page/n546 551] |isbn=978-0-8138-0187-2}} 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.{{cite journal|first1=Duygu |last1=Gocmen |first2=Ozan |last2=Gurbuz |first3=Ayşegul Yıldırım |last3=Kumral |first4=Adnan Fatih |last4=Dagdelen |first5=Ismet |last5=Sahin |year=2007|title=The effects of wheat sourdough on glutenin patterns, dough rheology and bread properties|url=http://www.aseanfood.info/Articles/11023732.pdf|journal=European Food Research and Technology|volume=225|pages=821–830|doi=10.1007/s00217-006-0487-6|access-date=Aug 5, 2012|number=5–6|s2cid=83885854|archive-date=December 31, 2013|archive-url=https://web.archive.org/web/20131231233202/http://www.aseanfood.info/Articles/11023732.pdf|url-status=dead}}

Faster starter processes, requiring fewer refreshments, have been devised, sometimes using commercial sourdough starters as inoculants.{{cite journal|last1=Siragusa |first1=S |last2=Di Cagno |first2=R |last3=Ercolini |first3=D |last4=Minervini |first4=F |last5=Gobbetti |first5=M |last6=De Angelis |first6=M |date=February 2009|title=Taxonomic structure and monitoring of the dominant population of lactic acid bacteria during wheat flour sourdough type I propagation using Fructilactobacillus sanfranciscensis (formerly Lactobacillus sanfranciscensis) starters|journal=Appl. Environ. Microbiol.|volume=75|issue=4|pages=1099–109|doi=10.1128/AEM.01524-08|pmc=2643576|pmid=19088320}} 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.{{citation needed|date=November 2020}}

Bakers have devised several ways of encouraging a stable culture of microorganisms in the starter. Unbleached, unbromated flour contains more microorganisms than more processed flours. Bran-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 are also sources of lactic acid bacteria,{{cite book |author=Gottfried Unden |title=Biology of Microorganisms on Grapes, in Must and in Wine |publisher=Springer |location=Berlin |year=2009 |page=6 |isbn=978-3-540-85462-3 |url=https://books.google.com/books?id=6CR1vQUMw-sC&pg=PA6 |access-date=Dec 28, 2011}}{{cite book |author=Huis in ʻt Veld, J. H. J. |author2=Konings, Wilhelmus Nicolaas |author3=Kuipers, Otto |name-list-style=amp |title=Lactic acid bacteria: genetics, metabolism, and applications: proceedings of the Sixth Symposium on lactic acid bacteria: genetics, metabolism and applications, 19–23 September 1999, Veldhoven, The Netherlands |publisher=Kluwer |location=Bruxelles |year=1999 |page=319 |isbn=978-0-7923-5953-1 |url=https://books.google.com/books?id=t_anz9sEexEC&pg=PA319 |access-date=2011-01-17 |quote=Table 1. Specific enumeration of lactic acid bacteria in cabernet sauvignon fermenting must (CFU/ml) (Lonvaud-Funel et al. 1991)}} as are many other edible plants.{{cite journal|vauthors=Felis GE, Dellaglio F|date=September 2007|title=Taxonomy of Lactobacilli and Bifidobacteria|url=http://www.horizonpress.com/ciim/v/v8/05.pdf|journal=Curr Issues Intest Microbiol|volume=8|issue=2|pages=44–61|pmid=17542335}}{{cite journal |vauthors=Mundt JO, Hammer JL |title=Lactobacilli on plants |journal=Appl Microbiol |volume=16 |issue=9 |pages=1326–30 |date=September 1968 |doi=10.1128/AEM.16.9.1326-1330.1968 |pmid=5676407 |pmc=547649}} 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.{{Citation needed|date=January 2021}}

The piped drinking water supplied in most urban areas is treated by chlorination or chloramination, adding small amounts of substances that inhibit potentially dangerous microorganisms but are harmless to animals. Some bakers recommend unchlorinated water for feeding cultures.{{rp|353}} 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, 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.{{cite book |last1=Maher |first1=John |title=Replacement of Renal Function by Dialysis: A Text Book of Dialysis |date=1989 |publisher=Kluwer Academic Publishers |isbn=978-0898384147 |page=192 |edition=Third |url=https://books.google.com/books?id=T845qZmgqroC&pg=PA192 |access-date=2014-06-11}}

Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially.{{cite book |url=https://books.google.com/books?id=H-WRU06X-4kC&q=diastatic+malt&pg=RA1-PA32 |author=Reinhart, Peter |title=Crust & Crumb: Master Formulas For Serious Bakers |publisher=Ten Speed Press |location=Berkeley, Calif |year=1998 |page=32 |isbn=978-1-58008-003-3 |access-date=June 28, 2010}}

Bakers often make loaves with fermented dough from a previous batch (which they call "mother dough",{{refn|group=note|The term mother dough sometimes refers to a yeast sponge,{{cite book |author=Esposito, Mary Ann |title=Ciao Italia in Tuscany: traditional recipes from one of Italy's most famous regions |publisher=St. Martin's Press |location=New York |year=2003 |page=94 |isbn=978-0-312-32174-1 |url=https://books.google.com/books?id=37_Lt9Ch7nwC&pg=PA94 |access-date=Aug 13, 2010}}{{cite book |author=Christina Tosi |title=Momofuku Milk Bar |publisher=Crown Publishing Group |isbn=978-0307720498 |year=2011 |url=https://books.google.com/books?id=xohBTmnvEX4C&pg=PA222 |access-date=2014-12-02}} so one must look at the ingredients and process to understand if it is a multi-refreshment sourdough or instead a sponge made from only fresh ingredients.}} "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.{{cite journal |url=http://www.dzumenvis.nic.in/Physiology/pdf/Biochemistry%20and%20physiology%20of%20sourdough%20lactic.pdf |title=Biochemistry and physiology of sourdough lactic acid bacteria |first1=M. |last1=Gobbetti |first2=M. |last2=De Angelis |first3=A. |last3=Corsetti |first4=R. |last4=Di Cagno |journal=Trends in Food Science & Technology |volume=16 |year=2005 |issue=1–3 |pages=57–69 |doi=10.1016/j.tifs.2004.02.013}}

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.{{Citation needed|date=January 2021}}

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.{{cite journal |url=http://www.lallemand.com/BakerYeastNA/eng/PDFs/LBU%20PDF%20FILES/2_11PAIN.PDF |publisher=Lallemand Inc. |title=Pain au Levain Production |journal=Baking Update |volume=2 |issue=11 |access-date=Dec 9, 2011}}{{cite journal |author1=Thiele, C. |author2=Gänzle, M. G. |author3=Vogel, R. F. |title=Contribution of Sourdough Lactobacilli, Yeast, and Cereal Enzymes to the Generation of Amino Acids in Dough Relevant for Bread Flavor |journal=Cereal Chemistry |volume=79 |issue=1 |pages=45–51 |date=January–February 2002 |doi=10.1094/CCHEM.2002.79.1.45 |url=http://www.aaccnet.org/cerealchemistry/articles/2002/1203-06R.pdf |access-date=2012-02-02 |archive-date=2012-03-24 |archive-url=https://web.archive.org/web/20120324135649/http://www.aaccnet.org/cerealchemistry/articles/2002/1203-06R.pdf |url-status=dead }}{{cite web |url=http://www.wraithnj.com/breadpics/rise_time_table/bread_model_bwraith.htm |title=Calculated sourdough rise times at various temperatures. |access-date=2012-08-03}} 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. 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.{{Citation needed|date=January 2021}}

File:Homemade Sourdough Boule.jpg

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.{{Cite web |title=Light Sourdough Bread |url=https://www.zojirushi.com/app/recipe/light-sourdough-bread |access-date=2025-03-23 |website=Zojirushi RECIPES |language=en}} 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.

{{See also|Lactic acid fermentation}}

File:Masa madre.jpg

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.{{Cite journal|last=Gänzle|first=Michael G|date=2015|title=Lactic metabolism revisited: metabolism of lactic acid bacteria in food fermentations and food spoilage|url=http://www.sciencedirect.com/science/article/pii/S2214799315000508|journal=Current Opinion in Food Science|series=Food Microbiology • Functional Foods and Nutrition|language=en|volume=2|pages=106–117|doi=10.1016/j.cofs.2015.03.001|issn=2214-7993|url-access=subscription}}{{Cite journal|last1=Gänzle|first1=Michael G.|last2=Vermeulen|first2=Nicoline|last3=Vogel|first3=Rudi F.|date=2007|title=Carbohydrate, peptide and lipid metabolism of lactic acid bacteria in sourdough|url=https://pubmed.ncbi.nlm.nih.gov/17008155/|journal=Food Microbiology|volume=24|issue=2|pages=128–138|doi=10.1016/j.fm.2006.07.006|issn=0740-0020|pmid=17008155}} During sourdough fermentation, many cereal enzymes, particularly phytases, proteases and pentosanases, are activated through acidification and contribute to biochemical changes during sourdough fermentation.

{{main|Lactic acid bacteria}}

Every starter consists of different lactic acid bacteria which are introduced to the starter through the environment, water, and flour used to create the starter.{{cite journal |last1=Reese |first1=Aspen T |last2=Maden |first2=Anne A |last3=Joossens |first3=Marie |last4=Lacaze |first4=Guylaine |last5=Dunee |first5=Robert |title=Influences of Ingredients and Bakers on the Bacteria and Fungi in Sourdough Starters and Bread |journal=mSphere |date=February 26, 2020 |volume=5 |issue=1 |doi=10.1128/mSphere.00950-19 |pmid=31941818 |pmc=6968659 }} 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.{{cite journal |last1=Bengar |first1=Sneh Punia |last2=Suri |first2=Shweta |last3=Trif |first3=Monica |last4=Ozogul |first4=Fatih |title=Organic acids production from lactic acid bacteria: A preservation approach |journal=Food Bioscience |year=2022 |volume=46 |page=101615 |doi=10.1016/j.fbio.2022.101615 |s2cid=246920460 }}

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.{{cite journal |last1=Wink |first1=Debra |title=Fermentations in Sourdough |date=February 2017 |url=https://members.bbga.org/resources/Documents/2017%20Formulas/Bornarth,Wink/2017,BornarthWink-FermentationsSourdough.pdf}}

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.{{cite journal |last1=Papadimitriou |first1=Konstantinos |last2= Alegría |first2=Ángel |last3=Bron |first3=Peter |last4=de Angelis |first4=Maria |last5=Gobbetti |first5=Marco |last6=Kleerebezem |first6=Michiel |last7=Lemos |first7=José |last8=Linares |first8=Daniel |last9=Ross |first9=Paul |last10=Stanton |first10=Catherine |last11=Turroni |first11=Francesca |last12=van Sinderen |first12=Douwe |last13=Varmanen |first13=Pekka |last14=Ventura |first14=Marco |last15=Zúñiga |first15=Manuel |last16=Tsakalidou |first16=Effie |last17=Kok |first17=Jan |title=Stress physiology of lactic acid bacteria |journal=Microbiology and Molecular Biology Reviews |date=July 2016 |volume=80 |issue=3 |pages=837–890 |doi=10.1128/mmbr.00076-15 |pmid=27466284 |pmc=4981675 |url=https://doi.org/10.1128/mmbr.00076-15}}

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.{{Citation needed|date=January 2021}}

The most common yeast species in sourdough are Kazachstania exigua (Saccharomyces exiguus), Saccharomyces cerevisiae, K. exiguus and K. humilis (previously Candida milleri or Candida humilis).{{Cite journal|last1=De Vuyst|first1=Luc|last2=Harth|first2=Henning|last3=Van Kerrebroeck|first3=Simon|last4=Leroy|first4=Frédéric|date=2016|title=Yeast diversity of sourdoughs and associated metabolic properties and functionalities|url=https://pubmed.ncbi.nlm.nih.gov/27470533/|journal=International Journal of Food Microbiology|volume=239|pages=26–34|doi=10.1016/j.ijfoodmicro.2016.07.018|issn=1879-3460|pmid=27470533}}{{Cite book |pages=477–526 |title=Fermented Bread |author=Weibiao Zhou |author2=Nantawan Therdthai |journal=Handbook of Plant-Based Fermented Food and Beverage Technology |edition=2 |editor1=Y.H. Hui |editor2=E. Özgül Evranuz |publisher=CRC Press |year=2012 |isbn=978-1439849040}}

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.{{cite book |url=https://books.google.com/books?id=FFjF4HykWv0C&pg=PA179 |author1=Golden, David M. |author2=Jay, James M. |author3=Martin J. Loessner |title=Modern food microbiology |publisher=Springer |location=Berlin |year=2005 |page=179 |isbn=978-0-387-23180-8 |access-date=June 28, 2010}} 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 {{convert|20| to |30|C|F}}. Fructilactobacillus sanfranciscensis was named for its discovery in San Francisco sourdough starters, though it is not endemic 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.{{cite journal |vauthors=De Vuyst L, Schrijvers V, Paramithiotis S, etal |title=The biodiversity of lactic acid bacteria in Greek traditional wheat sourdoughs is reflected in both composition and metabolite formation |journal=Appl. Environ. Microbiol. |volume=68 |issue=12 |pages=6059–69 |date=December 2002 |pmid=12450829 |pmc=134406 |doi=10.1128/aem.68.12.6059-6069.2002|bibcode=2002ApEnM..68.6059D }}{{cite journal |vauthors=Arendt EK, Ryan LA, Dal Bello F |title=Impact of sourdough on the texture of bread |journal=Food Microbiol. |volume=24 |issue=2 |pages=165–74 |date=April 2007 |pmid=17008161 |doi=10.1016/j.fm.2006.07.011 |url=http://www.aseanfood.info/Articles/11017456.pdf |access-date=June 28, 2010 |archive-date=April 28, 2021 |archive-url=https://web.archive.org/web/20210428084611/http://www.aseanfood.info/Articles/11017456.pdf |url-status=dead }} The yeasts Saccharomyces exiguus, Kasachstania humilis, or Candida holmii usually populate sourdough cultures symbiotically with Fructilactobacillus sanfranciscensis.{{cite journal |vauthors=Gänzle MG, Ehmann M, Hammes WP |title=Modeling of Growth of Lactobacillus sanfranciscensis and Candida milleri in Response to Process Parameters of Sourdough Fermentation |journal=Appl. Environ. Microbiol. |volume=64 |issue=7 |pages=2616–23 |date=July 1998 |pmid=9647838 |pmc=106434 |doi= 10.1128/AEM.64.7.2616-2623.1998|bibcode=1998ApEnM..64.2616G |url=}} 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 anomaola, C. peliculosa, P. membranifaciens, and C. valida.{{cite book |author=Yiu H. Hui |title=Handbook of food science, technology, and engineering |publisher=Taylor & Francis |location=Washington, DC |year=2006 |pages=183–9–183–11 |isbn=978-0-8493-9849-0 |url=https://books.google.com/books?id=rTjysvUxB8wC&pg=PA547 |access-date=Dec 20, 2011}} See Table 183.6{{cite book |author1=Gotthard Kunze |author2=Satyanarayana, T. |title=Yeast Biotechnology: Diversity and Applications |publisher=Springer |location=Berlin |year=2009 |page=180 |isbn=978-1-4020-8291-7 |url=https://books.google.com/books?id=jLFmiervaqMC&pg=PA180 |access-date=2012-01-25|bibcode=2009ybda.book.....S }} There have been changes in the taxonomy of yeasts in recent decades. F. sanfranciscensis requires maltose,{{cite journal |vauthors=Neubauer H, Glaasker E, Hammes WP, Poolman B, Konings WN |title=Mechanism of maltose uptake and glucose excretion in Lactobacillus sanfrancisco |journal=J Bacteriol |year=1994 |pages=3007–12 |volume=176 |issue=10 |pmid=8188601 |pmc=205458|doi=10.1128/jb.176.10.3007-3012.1994 }} while C. milleri is maltase negative and thus cannot consume maltose.{{cite journal |last=Decock |first=Pieter |author2=Cappelle, Stefan |title=Bread technology and sourdough technology |journal=Trends in Food Science & Technology |date=January–March 2005 |volume=16 |issue=1–3 |pages=113–120 |doi=10.1016/j.tifs.2004.04.012 |url=http://www.aseanfood.info/Articles/11015364.pdf |access-date=Dec 17, 2011 |archive-date=November 12, 2020 |archive-url=https://web.archive.org/web/20201112034123/http://www.aseanfood.info/articles/11015364.pdf |url-status=dead }}{{cite journal |title=Utilisation of maltose and glucose by lactobacilli isolated from sourdough |journal=FEMS Microbiology Letters |volume=109 |issue=2–3 |pages=237–242 |year=1993 |issn=0378-1097|author1=Stolz, Peter |author2=Böcker, Georg |author3=Vogel, Rudi F. |author4=Hammes, Walter P. |doi=10.1016/0378-1097(93)90026-x|doi-access=free }}{{cite journal |vauthors=Sugihara TF, Kline L, Miller MW |title=Microorganisms of the San Francisco sour dough bread process. I. Yeasts responsible for the leavening action |journal=Appl Microbiol |volume=21 |issue=3 |pages=456–8 |date=March 1971 |doi=10.1128/AEM.21.3.456-458.1971 |pmid=5553284 |pmc=377202 }}{{cite journal |vauthors=Kline L, Sugihara TF |title=Microorganisms of the San Francisco sour dough bread process. II. Isolation and characterization of undescribed bacterial species responsible for the souring activity |journal=Appl Microbiol |volume=21 |issue=3 |pages=459–65 |date=March 1971 |doi=10.1128/AEM.21.3.459-465.1971 |pmid=5553285 |pmc=377203 }}{{cite journal |author1=Daeschel, M.A. |author2=Andersson, R.E. |author3=Fleming, H.P. |year=1987 |title=Microbial ecology of fermenting plant materials |journal=FEMS Microbiology Letters |volume=46 |issue=3 |pages=357–367 |doi=10.1111/j.1574-6968.1987.tb02472.x |quote=The bacterium Lactobacillus sanfrancisco ferments maltose, but not glucose. Some glucose is provided by the action of the maltose phosphorylase pathway which is then fermented by the acid-tolerant yeast, Saccharomyces exiguus, which cannot use maltose. The yeast in turn provides growth stimulants for the bacterium. |doi-access=free }} C. milleri can grow under conditions of low pH and relatively high acetate levels, a factor contributing to sourdough flora's stability.{{cite book|author1=Lorenz, Klaus J.|url=https://books.google.com/books?id=eZjIfud742wC&pg=PA23|title=Handbook of dough fermentations|author2=Kulp, Karel|publisher=Marcel Dekker, Inc|year=2003|isbn=978-0-8247-4264-5|location=New York|pages=23–50|access-date=Dec 15, 2011}}

In order to produce acetic acid, F. sanfrancisensis needs maltose and fructose.{{cite journal |author=Gobbetti, M., A. Corsetti |title=Lactobacillus sanfrancisco a key sourdough lactic acid bacterium: a review |journal=Food Microbiology |volume=14 |pages=175–187 |number=2 |year=1997 |url=http://comenius.susqu.edu/biol/312/lactobacillussanfranciscoakeysourdoughlacticacidbacteriumareview.pdf |access-date=Mar 1, 2013|doi=10.1006/fmic.1996.0083 }} Wheat dough contains abundant starch and some polyfructosanes, which enzymes degrade to "maltose, fructose and little glucose."{{cite journal |author=Vogel, Rudi F. |title=Microbial ecology of cereal fermentations |journal=Food Technology and Biotechnology |volume=35 |number=1 |year=1997 |url=http://ftb.pbf.hr/index.php/ftb/article/view/147/147 |access-date=Feb 27, 2013}} 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)."{{ cite book |chapter=Fructans |author1=C.J. Pollock |author2=N.J. Chatterton |publisher=Academic Press Inc. |location=San Diego, California |volume=14 |year=1980 |title=The Biochemistry of plants: a comprehensive treatise: Carbohydrates |editor1=P.K. Stumpf |editor2=E.E. Conn, J. Preiss |isbn=978-0-12-675414-8 |chapter-url=https://books.google.com/books?id=4y4Dlp4wqeEC&pg=PA109 |pages=109–140 |access-date=Feb 28, 2013}} 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.{{cite book |author1=Scott, Alan |author2=Daniel Wing |url=https://books.google.com/books?id=VA6y1EMnkpYC&pg=PA34 |title=The Bread Builders: Hearth Loaves and Masonry Ovens |publisher=Chelsea Green Publishing Company |location=White River Junction (VT) |year=1999 |pages=34–230 |isbn=978-1-890132-05-7 |access-date=June 28, 2010}}

A Belgian study of wheat and spelt doughs refreshed once every 24 hours and fermented at {{convert|30|C|F}} 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.{{cite journal |vauthors=Van der Meulen R, Scheirlinck I, Van Schoor A, etal |title=Population dynamics and metabolite target analysis of lactic acid bacteria during laboratory fermentations of wheat and spelt sourdoughs |journal=Appl. Environ. Microbiol. |volume=73 |issue=15 |pages=4741–50 |date=August 2007 |pmid=17557853 |pmc=1951026 |doi=10.1128/AEM.00315-07|bibcode=2007ApEnM..73.4741V }} 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.{{Cite journal|last1=Ripari|first1=Valery|last2=Gänzle|first2=Michael G.|last3=Berardi|first3=Enrico|date=2016|title=Evolution of sourdough microbiota in spontaneous sourdoughs started with different plant materials|url=https://pubmed.ncbi.nlm.nih.gov/27240218/|journal=International Journal of Food Microbiology|volume=232|pages=35–42|doi=10.1016/j.ijfoodmicro.2016.05.025|issn=1879-3460|pmid=27240218|s2cid=21591819 }}

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.{{cite web |url=http://samartha.net/SD/docs/DW-post1-4.html |title=Dan Woods long posts 1–4 |access-date=Dec 15, 2011 |author=Wing, Gänzle |archive-date=November 20, 2018 |archive-url=https://web.archive.org/web/20181120071611/http://samartha.net/SD/docs/DW-post1-4.html |url-status=dead }}

In Type II sourdoughs, baker's yeast or Saccharomyces cerevisiae{{cite book |author1=Nanna A. Cross |author2=Corke, Harold |author3=Ingrid De Leyn |author4=Nip, Wai-Kit |url=https://books.google.com/books?id=TXYIrkrtDw0C&pg=PA370 |title=Bakery products: science and technology |publisher=Blackwell |location=Oxford |year=2006 |page=370 |isbn=978-0-8138-0187-2 }} 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 {{convert|30| to |50|C|F}} for several days without feedings, which reduces the flora's activity.{{cite book |author1=Ercolini, Danilo |author2=Cocolin, Luca |url=https://books.google.com/books?id=il9WEQPUfk8C&q=3.5+50&pg=PA119 |title=Molecular techniques in the microbial ecology of fermented foods |publisher=Springer |location=Berlin |year=2008 |page=119 |isbn=978-0-387-74519-0 |access-date=June 28, 2010}} This process was adopted by some in industry, in part, due to simplification of the multiple-step build typical of Type I sourdoughs.{{cite book |url=https://books.google.com/books?id=mnh6aoI8iF8C&q=Type+II+sourdough&pg=PA364 |author1=Yiu H. Hui |author2=Stephanie Clark |title=Handbook of food products manufacturing |publisher=Wiley |location=New York |year=2007 |page=364 |isbn=978-0-470-12524-3 |access-date=June 28, 2010}}

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 sourdoughs are Type II sourdoughs subjected to a drying process, usually either spray or drum drying, and are mainly used at an industrial level as flavoring agents. They are dominated by "drying-resistant [lactic acid bacteria] such as Pediococcus pentosaceus, L. plantarum, and L. brevis." The drying conditions, time and heat applied, may be varied in order to influence caramelization and produce desired characteristics in the baked product.

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.{{cite book |author=Smith, Jim Q. |title=Technology of reduced additive foods |publisher=Blackwell Science |edition=Second |location=Oxford |year=2004 |page=204 |isbn=978-0-632-05532-6 |url=https://books.google.com/books?id=u1-YidnFGYUC&pg=PA204 |access-date=2013-02-28 |quote=When baker's yeast became available, the immediate need for the dough resting time of several hours disappeared. The industrialisation of bread-making was introduced and consequently the production time was dramatically reduced. Dough conditioners and enzymes became necessary to secure the required dough characteristics.}}

{{See also|List of sourdough breads}}

File:Homemade Sourdough Bread with Oil.jpg

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).{{cite web |url=http://denmark.dk/en/lifestyle/food-drink/recipes-baking-that-dark-sour-bread/ |title=Recipes: Baking that dark, sour bread (Rugbrød) -The official website of Denmark |publisher=Denmark.dk |access-date=2016-09-15 |archive-date=2016-09-17 |archive-url=https://web.archive.org/web/20160917081320/http://denmark.dk/en/lifestyle/food-drink/recipes-baking-that-dark-sour-bread |url-status=dead }}{{cite web |url=http://www.epicurious.com/articlesguides/blogs/editor/2013/11/discovering-danish-rye-bread-rugbrod-smorrebrod.html |title=Discovering Danish Rye Bread |publisher=Epicurious.com |date=2013-11-15 |access-date=2016-09-15}} The Mexican birote salado started out in the city of Guadalajara as a short French baguette{{Citation needed|date=January 2023}} that replaces the yeast with a sourdough fermentation process, yielding a bread that is crunchy outside but soft and savory inside.{{cite web |url=https://thetequilafiles.com/2012/06/19/birote-bread-the-unique-taste-of-jalisco/ |title=Birote bread: the unique taste of Jalisco |date=2012-06-19 |access-date=2017-10-21}} 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,{{cite web |title=How to Bake Traditional German-Style Pumpernickel at Home |url=http://www.sourdoughlibrary.org/homemade-traditional-pumpernickel/ |publisher=Sourdough Library |access-date=30 September 2016}} 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.{{cite book |last1=Robertson |first1=Laurel |last2=Flinders |first2=Carol |last3=Godfrey |first3=Bronwen |title=The Laurel's Kitchen Bread Book: A Guide to Whole-Grain Breadmaking |url=https://books.google.com/books?id=YcAbb07yfbMC&pg=PA111 |year=2011 |publisher=Random House |isbn=978-0-307-76116-3 |pages=111–131}} Whole-wheat sourdough flatbreads are traditionally eaten in Azerbaijan.{{cite web |url=http://azer.com/aiweb/categories/magazine/ai104_folder/104_articles/104_food_north_south.html |title=10.4. Forgotten Foods Comparison of the Cuisines of Northern and Southern Azerbaijan by Pirouz Khanlou |publisher=Azer.com |access-date=2016-09-15}} In Ethiopia, teff flour is fermented to make injera.{{cite web |title=Recipe: Ethiopian Injera |url=http://www.exploratorium.edu/cooking/bread/recipe-injera.html |publisher=The Accidental Scientist |access-date=30 September 2016}} A similar variant is eaten in Somalia, Djibouti, and Yemen (where it is known as lahoh).{{cite web |title=Lahoh Sana'ani |url=http://www.shebayemenifood.com/content/lahoh-sanaani |publisher=Sheba Yemeni Foods |access-date=30 September 2016 |date=18 May 2012 |quote=Lahoh is a sourdough flatbread which is eaten in Yemen Somalia, Djibouti, and Ethiopia.}} In India, idlis and dosa are made from a sourdough fermentation of rice and black gram.{{cite book |last=Steinkraus |first=Keith |title=Handbook of Indigenous Fermented Foods, Second Edition |url=https://books.google.com/books?id=VKlHKIvrogUC&pg=PA149 |year=1995 |publisher=CRC Press |isbn=978-0-8247-9352-4 |page=149}}

Sourdough bread has a relatively low glycemic index compared with other types of bread.{{cite journal| vauthors=Stamataki NS, Yanni AE, Karathanos VT| title=Bread making technology influences postprandial glucose response: a review of the clinical evidence. | journal=Br J Nutr | year= 2017 | volume= 117 | issue= 7 | pages= 1001–1012 | pmid=28462730 | doi=10.1017/S0007114517000770 | type=Review | doi-access=free }}{{cite journal|pmc=4210917|year=2014|last1=d'Alessandro|first1=A.|title=Mediterranean diet pyramid: A proposal for Italian people|journal=Nutrients|volume=6|issue=10|pages=4302–4316|last2=De Pergola|first2=G.|pmid=25325250|doi=10.3390/nu6104302|doi-access=free}}{{Cite journal|last1=Gobbetti|first1=Marco|last2=De Angelis|first2=Maria|last3=Di Cagno|first3=Raffaella|last4=Calasso|first4=Maria|last5=Archetti|first5=Gabriele|last6=Rizzello|first6=Carlo Giuseppe|date=2019|title=Novel insights on the functional/nutritional features of the sourdough fermentation|url=https://pubmed.ncbi.nlm.nih.gov/29801967/|journal=International Journal of Food Microbiology|volume=302|pages=103–113|doi=10.1016/j.ijfoodmicro.2018.05.018|issn=1879-3460|pmid=29801967|s2cid=44105613}} The activity of cereal enzymes during sourdough fermentation hydrolyses phytates, which improves the absorption of some dietary minerals 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.{{Cite journal|last1=Loponen|first1=Jussi|last2=Gänzle|first2=Michael G.|date=2018|title=Use of Sourdough in Low FODMAP Baking|journal=Foods|volume=7|issue=7|page=96|doi=10.3390/foods7070096|issn=2304-8158|pmc=6068548|pmid=29932101|doi-access=free}}{{Cite journal|last1=Huang|first1=Xin|last2=Schuppan|first2=Detlef|last3=Rojas Tovar|first3=Luis E.|last4=Zevallos|first4=Victor F.|last5=Loponen|first5=Jussi|last6=Gänzle|first6=Michael|date=2020|title=Sourdough Fermentation Degrades Wheat Alpha-Amylase/Trypsin Inhibitor (ATI) and Reduces Pro-Inflammatory Activity|journal=Foods|volume=9|issue=7|page=943|doi=10.3390/foods9070943|issn=2304-8158|pmc=7404469|pmid=32708800|doi-access=free}} 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.{{cite journal|pmc=3231922|year=2011|last1=Arendt|first1=E. K.|title=Medical nutrition therapy: Use of sourdough lactic acid bacteria as a cell factory for delivering functional biomolecules and food ingredients in gluten free bread|journal=Microbial Cell Factories|volume=10|issue=Suppl 1|pages=S15|last2=Moroni|first2=A.|last3=Zannini|first3=E.|pmid=21995616|doi=10.1186/1475-2859-10-S1-S15 |doi-access=free }}{{cite journal|pmid=26980564|year=2017|last1=Axel|first1=C.|title=Mold spoilage of bread and its biopreservation: A review of current strategies for bread shelf life extension|journal=Critical Reviews in Food Science and Nutrition|volume=57|issue=16|pages=3528–3542|last2=Zannini|first2=E.|last3=Arendt|first3=E. K.|doi=10.1080/10408398.2016.1147417|s2cid=43288325}}

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".{{Cite journal |last1=D’Amico |first1=Vera |last2=Gänzle |first2=Michael |last3=Call |first3=Lisa |last4=Zwirzitz |first4=Benjamin |last5=Grausgruber |first5=Heinrich |last6=D’Amico |first6=Stefano |last7=Brouns |first7=Fred |date=2023-07-20 |title=Does sourdough bread provide clinically relevant health benefits? |journal=Frontiers in Nutrition |language=English |volume=10 |doi=10.3389/fnut.2023.1230043 |doi-access=free |pmid=37545587 |issn=2296-861X|pmc=10399781 }}

In northern Europe, where bread made from 100% rye flour is popular, bread is typically leavened with sourdough rather than baker’s yeast because rye lacks sufficient gluten to support yeast leavening. Instead, rye bread’s structure relies on the starch in the flour and other carbohydrates known as pentosans. However, rye amylase remains active at higher temperatures than wheat amylase, which can cause the bread’s structure to weaken as the starches break down during baking. 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.{{cite book |author1=Scott, Alan |author2=Daniel Wing |url=https://books.google.com/books?id=VA6y1EMnkpYC&pg=PA34 |title=The Bread Builders: Hearth Loaves and Masonry Ovens |publisher=Chelsea Green Publishing Company |location=White River Junction (VT) |year=1999 |pages=34–230 |isbn=978-1-890132-05-7 |access-date=June 28, 2010}}

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.{{Cite journal|last1=Gänzle|first1=Michael G.|last2=Zheng|first2=Jinshui|date=2019|title=Lifestyles of sourdough lactobacilli - Do they matter for microbial ecology and bread quality?|url=https://pubmed.ncbi.nlm.nih.gov/30172443/|journal=International Journal of Food Microbiology|volume=302|pages=15–23|doi=10.1016/j.ijfoodmicro.2018.08.019|issn=1879-3460|pmid=30172443|s2cid=52143236}}

{{Portal|Food}}

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• List of sourdough breads
• Biga, a pre-fermentation technique in Italian baking
• Herman cake
• History of California bread
• Kyselo, Czech soup made from sourdough
• List of microorganisms found in sourdough
• Salt-rising bread
• Sour mash
• Sour rye soup, Polish soup (zurek) made with rye flour soured in the same process that occurs in the forming of sourdough
• Injera

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