Two round loaves of naturally leavened (sourdough) bread
|Place of origin||Ancient Egypt|
|Main ingredient(s)||Flour, Lactobacillus culture, yeast|
Sourdough is a bread product made by a long fermentation of dough using naturally occurring lactobacilli and yeasts. 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.
Sourdough is a dough containing a Lactobacillus culture in symbiotic combination with yeasts. It is one of the principal means of biological leavening in bread baking, the others using cultivated forms of yeast. It is important in baking rye-based breads, where yeast does not produce comparable results. Compared to breads made with baker's yeast, it produces a mildly sour taste because of the lactic acid produced by the lactobacilli. The most commonly used yeast species in the production of sourdough are Kazachstania exigua, Saccharomyces cerevisiae, and Candida humilis.
The preparation of sourdough begins with a pre-ferment, (the "starter" or "levain", also known as the "chief", "chef", or "head"), made of flour and water. 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. The ratio of water to flour in the starter (the "hydration") varies and a starter may be a fluid batter or a stiff dough.
When wheat flour comes into contact with water, naturally occurring amylase enzymes break down the starch into maltose; the enzyme maltase converts the maltose sugar into glucose, which yeast can metabolize. Flour naturally contains a variety of yeasts and bacterial spores. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a balanced, symbiotic or stable culture. This culture will cause a dough to rise if the gluten has been developed sufficiently. The bacteria ferment sugars that the yeast cannot metabolise and their by-products are metabolised by yeast, which produces carbon dioxide gas, which leavens the dough.[note 1] Obtaining a satisfactory rise from sourdough takes longer than in a dough leavened with packaged 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 or 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.
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 dough to fresh dough is critical in the development and maintenance of a starter. This ratio is called "inoculation" or the "refreshment ratio". Higher refreshment ratios are associated with greater microbial stability in the sourdough. In San Francisco sourdough, the ratio is 40% of the total weight, which is roughly equivalent to 67% of the new-dough's weight. A high refreshment ratio keeps 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 27 °C (81 °F) to stabilise at a pH between 4.4 and 4.6.
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. 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 L. sanfranciscensis is the dominant bacterium requires a temperature between 25–30 °C (77–86 °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". 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 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.
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-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, 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. 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.
Bakers often make loaves with fermented dough from a previous batch (which they call "mother dough", "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 keep fresh for a long time and are good at resisting spoilage and mold.
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.
Finally, the starter is mixed with flour and water to make a 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.
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.
Biology and chemistry of sourdough
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 humilis or Saccharomyces exiguus usually populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis. 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.
There have been changes in the taxonomy of yeasts in recent decades. Lactobacillus species' phylogenetic groupings have also been undergoing reclassification, first being studied in 1991 by Collins, et al. In 1995, Hammes and Vogel phylogenetically grouped L. sanfranciscensis to L. casei-Pediococcus. In 2003, Hammes and Hertel grouped it to L. buchneri. In 2007, Dellaglio and Felis grouped it to L. fructivorans.
- Group A. Obligately homofermentative. They metabolise hexoses via the Embden–Meyerhof–Parnas (EMP) pathway to produce two molecules of lactic acid (C3H6O3), (>85%) but no carbon dioxide (CO2). They cannot tolerate oxygen. "They grow at 45 °C but not at 15 °C." "They are represented by L. delbrueckii and L. acidophilus."
- Group B. Facultatively heterofermentative. They metabolise hexoses to lactic acid, and pentoses to lactic and acetic acids. They can use oxygen and will "produce more oxidized fermentations (e.g. acetate) if O2 is present." They "grow at 15 °C and show variable growth at 45 °C." They are "represented by L. casei and L. plantarum."
- Group C. Obligately heterofermentative. They metabolise hexoses via the EMP pathway to produce lactic acid, acetic acid, and CO2; and pentoses via the phosphogluconate pathway to lactic and acetic acids. They are represented by L. fermentum, L. brevis, L. kefiri, and L. sanfranciscensis.
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. Type I sourdoughs have a pH range of 3.8 to 4.5 and are fermented in a 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. In Type II sourdoughs Saccharomyces cerevisiae is added to leaven the dough, L. pontis and L. panis highlight the flora. 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. This process was adopted by some in industry, in part, due to simplification of the multiple-step build typical of Type I traditional sourdoughs.
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. Continuously maintained, stable sourdough cannot be unintentionally contaminated by S. cerevisiae. 4% salt inhibits L. sanfranciscensis, while C. milleri can withstand 8%.
A Belgian study of wheat and spelt doughs refreshed once every 24 hours and fermented at 30 °C (86 °F) in a laboratory environment provides insight into the three-phase evolution of first-generation-to-stable sourdough ecosystems. 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 that the heterofermentative have outcompeted homofermentative lactobacilli.
In order to produce acetic acid, L. sanfrancisensis needs maltose and fructose. Wheat dough contains abundant starch and some polyfructosanes, which enzymes degrade to "maltose, fructose and little glucose." The terms "fructosan, glucofructan, sucrosyl fructan, polyfructan, and polyfructosan" are all used to describe a class of compounds that are "structurally and metabolically" related to sucrose, where "carbon is stored as sucrose and polymers of fructose (fructans)." Yeasts have the ability to free fructose from glucofructans which compose about 1-2% of the dough. Glucofructans are long strings of fructose molecules attached to a single glucose molecule. Sucrose can be considered the shortest glucofructan, with only a single fructose molecule attached. 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.
History of sourdough
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 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. 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.
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. A "sourdough" is primarily 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.
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.
In English-speaking countries, where wheat-based breads predominate, sourdough is no longer the standard method for bread leavening. It was gradually replaced, first by the use of barm from beer making, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts. Although sourdough bread was superseded in commercial bakeries in the 20th century, it has undergone a revival among artisan bakers.
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.
The 49er flapjack is a sourdough crepe which is popular in the United States, getting its name from the popularity of this style of pancake during the gold rush. During the Klondike gold rush of 1898, it was said that a real "Alaskan Sourdough" would just as soon spend a year in the hills without his rifle, as to tough it through without his bubbling sourdough pot. Since food was scarce, food provisions were more valuable than gold. In extreme cold, miners would put the dough ball under their clothes, next to their skin, or tuck it into their bedroll with them at night, anything to keep the yeast in it alive. The 49er is a signature menu item at The Original Pancake House, Walker Bros., and Good Day Cafe among other establishments. OHP advertises the crepe as "ooey, gooey, and chewy." Because it is similar to a Swedish pancake the 49er is sometimes served with lingonberry sauce, although most often it is rolled up with butter and powdered sugar, or served open faced and topped with maple syrup.
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 to 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.
- List of microorganisms found in sourdough
- History of California bread
- Klondike Gold Rush, where sourdough was a word for an experienced miner
- Salt-rising bread
- Żurek, Polish soup made with rye flour soured in the same process that occurs in the forming of sourdough
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- 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.
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- Modeling of Growth of Lactobacillus sanfranciscensis and Candida milleri... from Applied and Environmental Microbiology, July 1998, by Gänzle et al., Universität Hohenheim
- Sourdough at DMOZ