Place of origin
|Flour, Lactobacillus culture, yeast|
|Cookbook:Sourdough bread Sourdough bread|
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. Because a sourdough fermentation relies on microorganisms, using either bottled water, or tap water boiled for sufficient time then cooled to room temperature, will avoid or boil off some chlorine in the tap water which may kill, slow down, or adversely select the microorganisms being grown. Boiling tap water for a short time will not eliminate chloramines, but it will degas the water. Chlorine and chloramines can both be removed by activated carbon filters.
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
Sourdough is a stable culture of lactic acid bacteria (LAB) and yeast in a mixture of flour and water. Broadly speaking, the yeast produces gas (carbon dioxide) which leavens the dough, and the LAB produces lactic acid which contributes flavor. The LAB metabolizes sugars that the yeast cannot, and the yeast metabolizes byproducts of LAB fermentation.
Type I Sourdough
Traditional San Francisco sourdough is 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). The LAB Lactobacillus sanfranciscensis was named for its discovery in San Francisco sourdough starters, though it is not endemic to San Francisco. Saccharomyces exiguus leavens the dough. L. sanfranciscensis prefers to consume maltose, while C. milleri is maltase negative and cannot consume maltose. The yeasts Candida humilis or Saccharomyces exiguus populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis. Lactobacillus sanfranciscensis and L. pontis highlight a lactic-acid bacterial flora that includes L. fermentum, L. fructivorans, L. brevis, and L. paralimentarius. The perfect yeast S. exiguus is related to the imperfect yeasts C. milleri and C. holmii. Torulopsis holmii, Torula holmii, and S. rosei are synonyms used prior to 1978. C. milleri and C. holmii are physiologically similar, but DNA testing established them as distinct. Other yeasts reported found include C. humilis, C. krusei, Pichia anomaola, C. peliculosa, P. membranifaciens, and C. valida.
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.
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.
Type II Sourdough
In Type II sourdoughs, baker's yeast or 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 sourdoughs.
- 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. 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.
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.
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 starter 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 (OPH), Walker Bros., and Good Day Cafe among other establishments. OPH 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
- Kyselo, Czech soup made from sourdough.
- 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.
- Weibiao Zhou; Nantawan Therdthai (2012). "Fermented Bread". In Y.H. Hui, E. Özgül Evranuz. Handbook of Plant-Based Fermented Food and Beverage Technology (2 ed.) (CRC Press): 479. ISBN 1439849048.
- Jeffrey Hamelman (2004). Bread: a baker's book of techniques and recipes. New York: John Wiley. pp. 6–362. ISBN 0-471-16857-2. Pg. 6: "It is woefully common at this point in the mix of inexperienced bakers to conclude that the dough is too wet and to add flour. What seems to be excessively loose dough early in the mix, however, will soon transform once the second phase of mixing--gluten development--is accomplished. Extra flour added early on has ruined many a dough. Only by feeling the dough throughout the mixing process can we understand--through our hands--the considerable change from loose and shaggy to firm, elastic, and developed." Pg. 352: "Once water is added to the flour, the life cycle of the incipient culture is begun. After 24 hours in a warm room, the flour-water paste will show signs of having risen. The evidence of the rise indicates the presence of gas within the bowl; the presence of gas means that metabolism is under way--yes, there is life in the bowl: A little colony of microorganisms has begun taking up residence. At first things are tentative and fragile, there is little strength, the culture is vulnerable to intrusion by non-bread-friendly yeasts and bacteria. Soon, a natural selection will take place, and if all goes well, beneficial strains of bacteria will dominate the culture. They will work to create an environment that favors their own perpetuation, the synergy between yeasts and bacteria sufficient to ward off the incursion of competing strains. The baker, by providing food and water in sufficient quantity, at proper intervals, and keeping the developing culture in a favorable temperature zone, does his or her part to aid in the growth of the sourdough. Soon, the culture is strong enough to make bread, and unfathomable flavors follow, almost as if by magic." Pg. 356: "The words sourdough and levain are often used interchangeably in the United States. This, however, is not the case in Europe. In Germany, the word for sourdough is Sauerteig, and it always refers to a culture of rye flour and water. In France, the world for sourdough is levain, which refers to a culture that is made entirely, or almost so, of white flour. (The desem method of sourdough production, originally from Belgium, utilizes a whole-wheat culture, maintained in a cool environment, and almost always the bread is made without the addition of baker's yeast.) While outwardly these methods are different, there are a number of similarities between them. Most important is that each is a culture of naturally occurring yeasts and bacteria that have the capacity to both leaven bread and provide it with flavor."
- Rosada, Didier (1997) Advanced Sourdough. Minneapolis: National Baking Center.
- Rogers, R.F.; and Hesseltine, C.W. (1978). "Microflora of wheat and wheat flour from six areas of the United States" (PDF). Cereal Chemistry 55 (6): 889–898. Retrieved Feb 4, 2013.
- Micro-Organisms in Foods 6 Microbial Ecology of Food Commodities. New York: Kluwer Academic/Plenum Publishers. 2005. pp. 409–411. ISBN 0-387-28801-5. Retrieved 2013-02-04.See Table 8.9, bottom of page 410
- Van der Meulen R, Scheirlinck I, Van Schoor A, et al. (August 2007). "Population dynamics and metabolite target analysis of lactic acid bacteria during laboratory fermentations of wheat and spelt sourdoughs". Appl. Environ. Microbiol. 73 (15): 4741–50. doi:10.1128/AEM.00315-07. PMC 1951026. PMID 17557853.
- Decock, Pieter; Cappelle, Stefan (January–March 2005). "Bread technology and sourdough technology" (PDF). Trends in Food Science & Technology 16 (1-3): 113–120. doi:10.1016/j.tifs.2004.04.012. Retrieved Dec 17, 2011.
- Stolz, Peter; Böcker, Georg; Vogel, Rudi F.; Hammes, Walter P. (1993). "Utilisation of maltose and glucose by lactobacilli isolated from sourdough". FEMS Microbiology Letters 109 (2-3): 237–242. doi:10.1016/0378-1097(93)90026-x. ISSN 0378-1097.
- Sugihara TF, Kline L, Miller MW (March 1971). "Microorganisms of the San Francisco sour dough bread process. I. Yeasts responsible for the leavening action" (PDF). Appl Microbiol 21 (3): 456–8. PMC 377202. PMID 5553284. Retrieved Dec 20, 2011.
- Kline L, Sugihara TF (March 1971). "Microorganisms of the San Francisco sour dough bread process. II. Isolation and characterization of undescribed bacterial species responsible for the souring activity" (PDF). Appl Microbiol 21 (3): 459–65. PMC 377203. PMID 5553285. Retrieved Dec 20, 2011.
- Daeschel, M.A.; Andersson, R.E.; Fleming, H.P. (1987). "Microbial ecology of fermenting plant materials". FEMS Microbiology Letters 46: 357–367. doi:10.1111/j.1574-6968.1987.tb02472.x. Retrieved Nov 23, 2012. "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."
- Wing, Gänzle. "Dan Woods long posts 1-4". Retrieved Dec 15, 2011.
- Peterson, James A. (2002). Glorious French food: a fresh approach to the classics. London: J. Wiley. p. 170. ISBN 0-471-44276-3. Retrieved 2013-02-04. "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."
- Nicolette, M. Dumke (2006). Easy Breadmaking for Special Diets: Use Your Bread Machine, Food Processor, Mixer, or Tortilla Maker to Make the Bread YOU Need Quickly and Easily. Allergy Adapt, Inc. p. 95. ISBN 1-887624-11-2. Retrieved 2013-02-04. "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."
- Häggman, M.; Salovaara, H. (2008). "Microbial re-inoculation reveals differences in the leavening power of sourdough yeast strains". LWT - Food Science and Technology 41: 148. doi:10.1016/j.lwt.2007.02.001.
- McGee, Harold (2004). On food and cooking: the science and lore of the kitchen. New York: Scribner. pp. 544–546. ISBN 0-684-80001-2. Retrieved June 28, 2010.
- Manual for army bakers. Washington: Government Printing Office. 1910. p. 22. Retrieved Aug 13, 2011.
- S. John Ross. "Sourdough Bread: How To Begin (easy sourdough for the beginner or novice)". Retrieved June 17, 2011.
- Don Holm, Myrtle Holm (1972). The Complete Sourdough Cookbook. Caldwell, Idaho: Caxton Press. p. 40. ISBN 0-87004-223-8. Retrieved June 28, 2010.
- Khachatourians, George G. (1994). Food Biotechnology: Microorganisms. New York: Wiley-Interscience. pp. 799–813. ISBN 0-471-18570-1.
- Valcheva R, Korakli M, Onno B, et al. (March 2005). "Lactobacillus hammesii sp. nov., isolated from French sourdough". Int. J. Syst. Evol. Microbiol. 55 (Pt 2): 763–7. doi:10.1099/ijs.0.63311-0. PMID 15774659. "... maintained by back slopping or rafraîchi ... in terms of ratio (sourdough/dough),..."
- The Fresh Loaf
- Panel on the Applications of Biotechnology to Traditional Fermented Foods, National Research Council (1992). Applications of Biotechnology in Traditional Fermented Foods. The National Academies Press. ISBN 9780309046855. Retrieved June 28, 2012. "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."
- Calvel, Raymond (2001). The taste of bread. Gaithersburg, Md: Aspen Publishers. pp. 89–90. ISBN 0-8342-1646-9. Retrieved June 28, 2010.
- "Lactic acid fermentation in sourdough", Deborah Wink
- "Section - 22. What is the Microbiology of San Francisco Sourdough?". Retrieved 2013-02-23. "...yeasts do not produce appreciable amounts of either lactic or acetic acids, their main metabolites are ethanol and CO2."
- "Cookbook:Sourdough Starter - Wikibooks, open books for an open world". Retrieved 2011-12-09.
- Nanna A. Cross; Corke, Harold; Ingrid De Leyn; Nip, Wai-Kit (2006). Bakery products: science and technology. Oxford: Blackwell. p. 551. ISBN 0-8138-0187-7.
- Duygu Gocmen, Ozan Gurbuz, Ayşegul Yıldırım Kumral, Adnan Fatih Dagdelen and Ismet Sahin (2007). "The effects of wheat sourdough on glutenin patterns, dough rheology and bread properties" (PDF). European Food Research and Technology 225 (5-6): 821–830. doi:10.1007/s00217-006-0487-6. Retrieved Aug 5, 2012.
- Siragusa S, Di Cagno R, Ercolini D, Minervini F, Gobbetti M, De Angelis M (February 2009). "Taxonomic structure and monitoring of the dominant population of lactic acid bacteria during wheat flour sourdough type I propagation using Lactobacillus sanfranciscensis starters". Appl. Environ. Microbiol. 75 (4): 1099–109. doi:10.1128/AEM.01524-08. PMC 2643576. PMID 19088320. Retrieved 2012-01-31.
- "Pain au Levain Production" (PDF). Baking Update (Lallemand Inc.) 2 (11). Retrieved Dec 9, 2011.
- Gottfried Unden (2009). Biology of Microorganisms on Grapes, in Must and in Wine. Berlin: Springer. p. 6. ISBN 3-540-85462-2. Retrieved Dec 28, 2011.
- 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)"
- Felis GE, Dellaglio F (September 2007). "Taxonomy of Lactobacilli and Bifidobacteria". Curr Issues Intest Microbiol 8 (2): 44–61. PMID 17542335.
- Mundt JO, Hammer JL (September 1968). "Lactobacilli on plants". Appl Microbiol 16 (9): 1326–30. PMC 547649. PMID 5676407.
- De Vuyst L, Schrijvers V, Paramithiotis S, et al. (December 2002). "The biodiversity of lactic acid bacteria in Greek traditional wheat sourdoughs is reflected in both composition and metabolite formation". Appl. Environ. Microbiol. 68 (12): 6059–69. doi:10.1128/aem.68.12.6059-6069.2002. PMC 134406. PMID 12450829. Retrieved Dec 21, 2011.
- Maher, John (1989). Replacement of Renal Function by Dialysis: A Text Book of Dialysis (Third ed.). Kluwer Academic Publishers. p. 192. ISBN 0898384141. Retrieved 2014-06-11.
- Reinhart, Peter (1998). Crust & Crumb: Master Formulas For Serious Bakers. Berkeley, Calif: Ten Speed Press. p. 32. ISBN 1-58008-003-0. Retrieved June 28, 2010.
- Thiele, C.; Gänzle, M. G.; Vogel, R. F. (January–February 2002). "Contribution of Sourdough Lactobacilli, Yeast, and Cereal Enzymes to the Generation of Amino Acids in Dough Relevant for Bread Flavor". Cereal Chemistry 79 (1): 45–51. doi:10.1094/CCHEM.2002.79.1.45. Retrieved 2012-02-02.
- "Calculated sourdough rise times at various temperatures.". Retrieved 2012-08-03.
- Golden, David M.; Jay, James M.; Martin J. Loessner (2005). Modern food microbiology. Berlin: Springer. p. 179. ISBN 0-387-23180-3. Retrieved June 28, 2010.
- Gänzle MG, Ehmann M, Hammes WP (July 1998). "Modeling of Growth of Lactobacillus sanfranciscensis and Candida milleri in Response to Process Parameters of Sourdough Fermentation". Appl. Environ. Microbiol. 64 (7): 2616–23. PMC 106434. PMID 9647838.
- Arendt EK, Ryan LA, Dal Bello F (April 2007). "Impact of sourdough on the texture of bread" (PDF). Food Microbiol. 24 (2): 165–74. doi:10.1016/j.fm.2006.07.011. PMID 17008161. Retrieved June 28, 2010.
- Yiu H. Hui (2006). Handbook of food science, technology, and engineering. Washington, DC: Taylor & Francis. pp. 183–9–183–11. ISBN 0-8493-9849-5. Retrieved Dec 20, 2011. See Table 183.6
- Gotthard Kunze; Satyanarayana, T. (2009). Yeast Biotechnology: Diversity and Applications. Berlin: Springer. p. 180. ISBN 1-4020-8291-6. Retrieved 2012-01-25.
- Gobbetti, M., A. Corsetti (1997). "Lactobacillus sanfrancisco a key sourdough lactic acid bacterium: a review". Food microbiology 14 (2). Retrieved Mar 1, 2013.
- Vogel, Rudi F. (1997). "Microbial ecology of cereal fermentations". Food Technology and Biotechnology 35 (1). Retrieved Feb 27, 2013.
- C.J. Pollock; N.J. Chatterton (1980). "Fructans". In P.K. Stumpf, E.E. Conn, J. Preiss. The Biochemistry of plants: a comprehensive treatise: Carbohydrates 14. San Diego, California: Academic Press Inc. pp. 109–140. ISBN 0-12-675414-4. Retrieved Feb 28, 2013.
- Lorenz, Klaus J.; Kulp, Karel (2003). Handbook of dough fermentations. New York: Marcel Dekker, Inc. pp. 23–50. ISBN 0-8247-4264-8. Retrieved Dec 15, 2011.
- Scott, Alan; Daniel Wing (1999). The Bread Builders: Hearth Loaves and Masonry Ovens. White River Junction (VT): Chelsea Green Publishing Company. pp. 34–230. ISBN 1-890132-05-5. Retrieved June 28, 2010.
- Hammes, W.P.; Vogel, R.F. (1995). Holzapfel, W. H.; Wood, Brian J. B., ed. The Genera of lactic acid bacteria. London: Blackie Academic & Professional. pp. 19–35. ISBN 0-7514-0215-X. Retrieved Dec 25, 2011.
- Nanna A. Cross; Corke, Harold; Ingrid De Leyn; Nip, Wai-Kit (2006). Bakery products: science and technology. Oxford: Blackwell. p. 370. ISBN 0-8138-0187-7.
- Sadeghi, A. (2008). "The Secrets of Sourdough; A Review of Miraculous Potentials of Sourdough in Bread Shelf Life". Biotechnology(Faisalabad) 7 (3): 413–417. doi:10.3923/biotech.2008.413.417.
- Ercolini, Danilo; Cocolin, Luca (2008). Molecular techniques in the microbial ecology of fermented foods. Berlin: Springer. p. 119. ISBN 0-387-74519-X. Retrieved June 28, 2010.
- Yiu H. Hui, Stephanie Clark (2007). Handbook of food products manufacturing. New York: Wiley. p. 364. ISBN 0-470-12524-1. Retrieved June 28, 2010.
- "Todar's Online Textbook of Bacteriology: Lactic Acid Bacteria". Retrieved 2012-07-18.
- Peters, Erica J. San Francisco: A Food Biography. Rowman & Littlefield, 2013, p. 189.
- Fernald, Anya (November–December 2002). Sourdough Baking (34). Slow - The International Herald of Tastes. Retrieved June 18, 2010.
- Smith, Jim Q. (2004). Technology of reduced additive foods (Second ed.). Oxford: Blackwell Science. p. 204. ISBN 0-632-05532-4. Retrieved 2013-02-28. "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."
- Forgotten Foods Comparison of the Cuisines of Northern and Southern Azerbaijan by Pirouz Khanlou
- "Sourdough - Definition from the Merriam-Webster Dictionary". Retrieved June 24, 2010.
- Marco Gobbetti, Michael Gänzle (Editors) (2012) Handbook on Sourdough Biotechnology
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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