Sourdough

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Sourdough
Two round loaves of naturally leavened (sourdough) bread
TypeBread
Place of originAncient Egypt
Main ingredientsFlour, Lactobacillus culture, yeast

Sourdough is a type of bread produced by a long fermentation of dough using naturally occurring yeasts and lactobacilli. In comparison with breads made quickly with cultivated yeast, it usually has a mildly sour taste because of the lactic acid produced by the lactobacilli.

The preparation of sourdough begins with a starter made from a mixture of flour and water. As it ferments, sometimes for several days, the starter is refreshed with flour and water, then mixed into the bread dough, which is kneaded and allowed to rise slowly before baking.

Sourdough may have originated in Ancient Egypt around 1500 BC and was probably the first leavened bread. It was the usual form of leavening until the European Middle Ages,[1] when it was replaced by barm from the beer brewing process, and then in the 19th-20th century by baker's yeast. It was historically important in the production of rye-based breads, where yeast does not produce comparable results. It was the main type of bread made in Northern California during the California Gold Rush and it remains a part of the culture of San Francisco today. The sourdough tradition was carried into Alaska and the western Canadian territories during the Klondike Gold Rush. Many breads around the world use similar techniques. Although sourdough bread was superseded in commercial bakeries in the 20th century, it has undergone a revival among artisan bakers.

Introduction

Sourdough is a dough containing a Lactobacillus culture, usually in symbiotic combination with yeasts. It is one of two principal means of biological leavening in bread baking, along with the use of cultivated forms of yeast (Saccharomyces). It is of particular importance in baking rye-based breads, where yeast does not produce comparable results. In comparison with yeast-based breads, it produces a distinctively tangy or sour taste, mainly because of the lactic acid produced by the lactobacilli; the actual medium, known as "starter" or levain, is in essence an ancestral form of pre-ferment. 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 beermaking, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts. However, some form of natural leaven is still used by many specialty bakeries.

Sourdough starter is traditionally made with a small amount of old dough, preferably saved from a prior batch. This is traditionally called mother dough or chef, or in more modern usage, seed sour. First-generation starter or spontaneous seed may be created by storing new dough in a warm place and allowing sufficient time for it to sour. This small amount of old-dough starter contains the culture, and its weight is increased by additions of new dough and mixing or kneading followed by rest or leavening periods.[2] A small amount of the resulting dough is then saved to use as starter sour for the next batch.[3] As long as this starter culture is fed flour and water weekly, it can stay at room temperature indefinitely.[4][5][6]

Sourdough bread is made by combining the increased amount of starter with another new-dough addition, along with any other desired ingredients to make the final dough. The starter comprises about 13 to 25% of the final dough, though particular formulas vary.[7][8] This final dough may be divided and shaped, then allowed to rise, followed by baking.

It is not uncommon for a baker's starter dough to have years of history, from many hundreds of previous batches. As a result, each bakery's sourdough has a distinct taste. The combination of starter processes, refreshment ratios and rest times, culture and air temperature, humidity, and elevation also makes each batch of sourdough different.

Biology and chemistry of sourdough

Sourdough starter made with flour and water refreshed for three or more days
See also: Lactic acid fermentation

A sourdough is a stable symbiotic culture of lactic acid bacteria (LAB) and yeast in a mixture of flour and water. Typically, the LAB metabolises sugars that the yeast cannot metabolise and the yeast metabolises the products of the LAB fermentation. Broadly speaking, the yeast produces the gas that leavens the dough and the LAB produces lactic acid, which contributes flavor.

The yeasts Candida milleri or Saccharomyces exiguus usually populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis.[9] The perfect yeast S. exiguus is related to the imperfect yeasts C. milleri and C. holmii; while Torulopsis holmii, Torula holmii, and S. rosei are synonyms used more frequently 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.[10][11]

There have been changes in the taxonomy of yeasts in recent decades.[10][11] Lactobacillus species' phylogenetic groupings have also been undergoing reclassification, first being studied in 1991 by Collins, et al.[12] In 1995, Hammes and Vogel phylogenetically grouped L. sanfranciscensis to L. casei-Pediococcus.[13] In 2003, Hammes and Hertel grouped it to L. buchneri. In 2007, Dellaglio and Felis grouped it to L. fructivorans.[12]

LAB are anaerobic, which means they can multiply in the absence of oxygen. Hammes and Vogel in 1995 distinguished three metabolic groups of LAB:[13][14]

  • Group A. Obligately homofermentative. They metabolise hexoses via the Embden–Meyerhof–Parnas (EMP) pathway to produce two molecules of lactic acid (C3H6O3), (>85%)[13][14] but no carbon dioxide (CO2). They cannot tolerate oxygen. "They grow at 45 °C but not at 15 °C."[15] "They are represented by L. delbrueckii and L. acidophilus."[15]
  • Group B. Facultatively heterofermentative. They metabolise hexoses to lactic acid,[12] and pentoses to lactic and acetic acids.[13][14] They can use oxygen and will "produce more oxidized fermentations (e.g. acetate) if O2 is present."[15] They "grow at 15 °C and show variable growth at 45 °C."[15] They are "represented by L. casei and L. plantarum."[15]
  • Group C. Obligately heterofermentative. They metabolise hexoses via the EMP pathway to produce lactic acid, acetic acid, and CO2;[12] and pentoses via the phosphogluconate pathway to lactic and acetic acids.[10][13][14] They are represented by L. fermentum, L. brevis, L. kefiri, and L. sanfranciscensis.[12][15]

Lactobacillus sanfranciscensis was named for its discovery in San Francisco sourdough starters, although it is not endemic to San Francisco. In general, San Francisco sourdough is the same as a Type I sourdough.[16] Type I sourdoughs have a pH range of 3.8 to 4.5 and are fermented in a room-temperature range of 20 to 30 °C (68 to 86 °F); Saccharomyces exiguus leavens the dough, Lactobacillus sanfranciscensis and L. pontis highlight a lactic-acid bacterial flora that includes L. fermentum, L. fructivorans, L. brevis, and L. paralimentarius.[16][17][18] In Type II sourdoughs Saccharomyces cerevisiae[19] is added to leaven the dough, L. pontis and L. panis highlight the flora.[16][17] These sourdoughs have a pH less than 3.5 and are fermented within a temperature range of 30 to 50 °C (86 to 122 °F) for several days without feedings, which reduces the flora's activity.[20][21] This process was adopted by some in industry, in part, due to simplification of the multiple-step build typical of Type I traditional sourdoughs.[22]

Preparation of sourdough products

The sourdough starter

The preparation of sourdough products begins with a starter made from a mixture of flour and water. The term "starter culture" is not clearly defined and in practice there are several kinds of starter.

Fresh flour naturally contains a variety of yeasts and bacterial spores. When wheat flour comes into contact with water, naturally occurring amylase enzymes break down the starch into the disaccharide maltose; maltase converts the sugar into glucose, which yeast can metabolize.[23] L. sanfranciscensis prefers to consume maltose, while C. milleri is maltase negative.[24][25][26][27] The mixture develops a balanced, symbiotic culture after repeated feedings.

A starter consists of water and bread flour and is developed over several days. As it ferments, it is regularly refreshed with additional flour and water. In this process, the ratio of yeasts to lactobacilli may be altered.[3] A term encountered in the literature is "acceleration", which means decreasing the time intervals between refreshements of the starter in order to increase the rate of gas (CO2) production.[28] The ratio of water to flour in the starter varies according to the practice of the bakery, either a relatively fluid batter or a stiffer dough. Firm starters (such as the Flemish Desem starter) are often more resource-intensive, traditionally being buried in a large container of flour to prevent drying out.

The yeast and bacteria in the culture will cause a wheat-based dough, if the gluten has been developed sufficiently, to retain gas, to leaven or rise. Obtaining a satisfactory rise from sourdough takes longer than in a dough leavened with packaged 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.[29]

The amount of seed sour typically used during refreshment can vary from 2-20%. However, higher figures correlate to increased microbial stability. In San Francisco sourdough, back-slopping[30] is 40% based upon total dough weight, and may be expressed as a refreshment ratio of old:new dough,[31] or 66⅔ percent. This keeps the pH of the refreshed dough relatively low.[6] Below pH 4.0, lactobacilli are inhibited, this selects for acid-tolerant yeasts. Dutch wheat sourdough investigations found that, even though S. cerevisiae exerted infection pressure on sourdough's microbial ecosystem, it had died off after two refreshment cycles.[14] Continuously maintained, stable sourdough cannot be unintentionally contaminated by S. cerevisiae.[32] 4% salt inhibits L. sanfranciscensis, while C. milleri can withstand 8%.[9]

A 10-day Belgian study of wheat and spelt doughs refreshed once every 24 hours and fermented at 30 °C (86 °F) in a laboratory environment provides insight into the three-phase evolution of first-generation-to-stable sourdough ecosystems. 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 L. 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 the heterofermentative have outcompeted homofermentative lactobacilli.[33]

A Type I primary-culture levain (spontaneous seed sour) is prepared from a salted wheat-rye dough; the process takes about 54 hours at 27 °C (81 °F) to build to a pH range of 4.4 to 4.6.[34] If the objective is to create a stable L. sanfranciscensis-highlighted culture, fermentation temperatures in the range of 25–30 °C (77–86 °F) and once-daily dough refreshments for about two weeks are adequate. Refreshment intervals longer than three days acidify the dough and may change the microbial ecosystem.[32]

The flour-water mixture can also be inoculated from a previously maintained culture. The culture is stable because of its ability to prevent colonization by other yeasts and bacteria as a result of its acidity and other antibacterial agents. As a result, many sourdough bread varieties tend to be relatively resistant to spoilage and mold.

Faster starter processes, requiring fewer refreshments, have also been devised, sometimes using commercial sourdough starters as inoculants.[35] These starters generally fall into two types. One is made from traditionally maintained and stable starter doughs, often dried, in which the ratios of micro-organisms are uncertain. Another is made from micro-organisms 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.[6][7]

Several methods are used to produce a stable culture of micro-organisms in the starter. Unbleached, unbromated flour contains more micro-organisms 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,[36][37] as are many other edible plants.[12][38] Basil leaves are soaked in room-temperature water for an hour to seed traditional Greek sourdough.[18] Using water from boiled potatoes is said to increase the activity of the bacteria by providing additional starch. Some bakers recommend unchlorinated water for feeding cultures. Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially.[39]

Making bakery products from a sourdough starter

A sourdough starter may be used in different ways. Traditionally, a certain amount of sourdough starter (in which the flour is 20 to 25 percent by weight of the flour in the final dough) is mixed into the bread dough, and the bread is kneaded and allowed to rise as normal. The process is similar to that using baker's yeast, although the rise time of most sourdough starters is longer than that baker's yeasts. For this reason, many sourdough starters are unsuitable for use in a bread machine. When using a particularly liquid starter with a high concentration of lactobacillus or acetic acid bacteria, the large amount of lactic and acetic acids produced needs to be managed carefully,[clarification needed] since the acid can break down the gluten in the bread dough; this becomes less of a concern in a stiffer starter, where the yeast usually predominates.

History of sourdough

File:Sourdoughbread.jpg
Sourdough bread

Sourdough likely originated in Ancient Egyptian times around 1500 BC and was likely the first form of leavening available to bakers. Sourdough remained the usual form of leavening down into the European Middle Ages[1] until being replaced by barm from the beer brewing process, and then later purpose-cultured yeast.

Bread made from 100 percent rye flour, which is very popular in the northern half of Europe, is usually leavened with sourdough. Baker's yeast is not useful as a leavening agent for rye bread, as rye does not contain enough gluten. 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 cooking. 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.[40] In the southern part of Europe, where baguette and even panettone were originally made with wheat flour and rye flour, sourdough has become less common in recent times; 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 was the main bread made in Northern California during the California Gold Rush, and it remains a part of the culture of San Francisco today. The bread became so common that "sourdough" became a general nickname for the gold prospectors. The nickname remains in "Sourdough Sam", the mascot of the San Francisco 49ers. A 'Sourdough' is also a nickname used in the North (Yukon/Alaska) for someone having spent an entire winter north of the Arctic Circle and refers to their tradition of protecting their Sourdough during the coldest months by keeping it close to their body.[41]

The sourdough tradition was carried into Alaska and the western Canadian territories during the Klondike Gold Rush. 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 old-timer.[42]

San Francisco sourdough is the most famous sourdough bread made in the U.S. today. In contrast to sourdough production in other areas of the country, the San Francisco variety has remained in continuous production since 1849, with some bakeries (e.g., Boudin Bakery among others) able to trace their starters back to California's Gold Rush period. It is a white bread characterized by a pronounced sourness (not all varieties are as sour as San Francisco sourdough), so much so that the dominant strain of lactobacillus in sourdough starters was named Lactobacillus sanfranciscensis. Sourdough also became popular because of its ability to combine well with seafoods and soups such as cioppino, clam chowder, and chili.

Sourdough has not enjoyed the popularity it once had since bread became mass-produced. However, many restaurant chains, such as Cracker Barrel, keep it as a menu staple. Manufacturers make up for the lack of yeast and bacterial culture by introducing into their dough an artificially-made mix known as bread improver.

Types of sourdough bread

There are many breads that use techniques similar to that used in the making of sourdough bread.

Baking soda (and sometimes baking powder) may be added to a sourdough-type starter. This neutralizes the acid in the starter and generates carbon dioxide in the process, providing a lift to the dough or batter in a manner similar to Irish soda bread. This method is used in kitchens where the starter is kept off-balance with a high acid level. It is common in Alaska.

Amish Friendship Bread uses a sourdough starter that includes sugar and milk. It is also leavened with baking powder and baking soda, making like a quick bread. An Amish sourdough is fed with sugar and potato flakes every 3–5 days.

German Pumpernickel is traditionally made from a sourdough starter, although modern pumpernickel loaves often use commercial yeasts, sometimes spiked with citric acid or lactic acid to inactivate the amylases in the rye flour.

The Flemish Desem bread is a popular form of whole-wheat sourdough, cultured in a dryish medium.

Other recipes use starters that are not natural leavens. The Italian Biga and French Poolish add sourdough-like flavors to breads by allowing the yeast for ferment for at least half a day. Unlike a true sourdough, these recipes usually start with commercial yeast, and the production of lactobacillus is incidental.

In Azerbaijan, whole-wheat sourdough flatbreads are traditionally eaten.[43]

In Ethopia, teff flour is used to make Injera, a similar variant is eaten in Somalia (where it is called canjeelo or lahooh) and Yemen (where it is known as lahoh)

See also

References

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  37. ^ Huis in ʻt Veld, J. H. J.; Konings, Wilhelmus Nicolaas; Kuipers, Otto (1999). 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. Bruxelles: Kluwer. p. 319. ISBN 0-7923-5953-4. Retrieved 2011-01-17. Table 1. Specific enumeration of lactic acid bacteria in cabernet sauvignon fermenting must (CFU/ml) (Lonvaud-Funel et al. 1991){{cite book}}: CS1 maint: multiple names: authors list (link)
  38. ^ Mundt JO, Hammer JL (1968). "Lactobacilli on plants". Appl Microbiol. 16 (9): 1326–30. PMC 547649. PMID 5676407. {{cite journal}}: Unknown parameter |month= ignored (help)
  39. ^ Reinhart, Peter (1998). Crust & Crumb: Master Formulas For Serious Bakers. Berkeley, Calif: Ten Speed Press. p. 32. ISBN 1-58008-003-0. Retrieved 2010 June 28. {{cite book}}: Check date values in: |accessdate= (help)
  40. ^ Scott, Alan; Daniel Wing (1999). The Bread Builders: Hearth Loaves and Masonry Ovens. White River Junction (VT): Chelsea Green Publishing Company. p. 34. ISBN 1-890132-05-5. Retrieved 2010 June 28. {{cite book}}: Check date values in: |accessdate= (help)CS1 maint: multiple names: authors list (link)
  41. ^ http://www.etymonline.com/index.php?term=sourdough
  42. ^ Fernald, Anya (2002). "Sourdough Baking" (34). Slow - The International Herald of Tastes. Retrieved 2010 June 18. {{cite journal}}: Check date values in: |accessdate= (help); Cite journal requires |journal= (help); Unknown parameter |month= ignored (help)
  43. ^ Forgotten Foods Comparison of the Cuisines of Northern and Southern Azerbaijan by Pirouz Khanlou
  44. ^ "Sourdough - Definition from the Merriam-Webster Dictionary". Retrieved 2010 June 24. {{cite web}}: Check date values in: |accessdate= (help)

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