Sourdough

Sourdough bread

Two round loaves of naturally leavened (sourdough) bread
Type Bread
Main ingredients Flour, water, sourdough culture, salt
Cookbook: Sourdough bread  Media: Sourdough bread

Sourdough bread is made by the fermentation of dough using naturally-occurring lactobacilli and yeast. Sourdough bread has a mildly sour taste not present in most breads made with baker's yeast and better inherent keeping qualities than other breads, due to the lactic acid produced by the lactobacilli.

Introduction

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.[1]

History

Writing in the Encyclopedia of Food Microbiology Michael Gaenzle writes "The origins of bread-making are so ancient that everything said about them must be pure speculation. One of the oldest sourdough breads dates from 3700 BCE and was excavated in Switzerland, but the origin of sourdough fermentation likely relates to the origin of agriculture in the Fertile Crescent several thousand years earlier... 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."[2]

Pliny the Elder described the sourdough method in his Natural History:[3][4]

Generally however they do not heat it up at all, but only use the dough kept over from the day before; manifestly it is natural for sourness to make the dough ferment[3]

Sourdough remained the usual form of leavening down into the European Middle Ages[5] until being replaced by barm from the beer brewing process, and then later purpose-cultured yeast.

Bread made from 100 percent rye flour, 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.[6] In the southern part of Europe, where panettone was originally made with sourdough,[5] 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.[7] The "celebrated"[1] San Francisco sourdough is a white bread characterized by a pronounced sourness, and indeed the strain of lactobacillus in sourdough starters is named Lactobacillus sanfranciscensis, alongside the sourdough yeast Candida milleri found in the same cultures.[1]

The sourdough tradition was carried into Alaska and the western Canadian territories 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 old-timer.[8]

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.[9]

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.[10]

Preparation

How to make and maintain firm sourdough

Starter

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.[11] The purpose of the starter is to produce a vigorous leaven and to develop the flavour of the bread. In practice there are several kinds of starters, as the ratio of water to flour in the starter (hydration) varies. A starter may be a liquid batter or a stiff dough.

Flour naturally contains a variety of yeasts and bacterial spores.[12][13] When wheat flour comes into contact with water, the naturally occurring enzyme amylase breaks down the starch into the sugars glucose, sucrose, galactose and raffinose, which sourdough's natural yeast can metabolize. Amylase also breaks down starch into maltose, which the yeast cannot metabolize.[14] With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a stable culture.[11][15] This culture will cause a dough to rise if the gluten has been developed sufficiently.[11] The bacteria ferment maltose that the yeast cannot metabolise, and the byproducts are metabolised by the yeast which produces carbon dioxide gas, leavening the dough.[16][17][18][19][20][note 1]

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.[22][23] In the presence of lactic acid bacteria, however, some sourdough yeasts have been observed to produce twice the gas of baker's yeast.[24] 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.[25]

Refreshment of the starter

Recently refreshed sourdough.

As it ferments, sometimes for several days, the volume of the starter is increased by periodic additions of flour and water, called "refreshments".[26] As long as this starter culture is fed flour and water regularly it will remain active.[27][28][29]

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.[30][31] Higher refreshment ratios are associated with greater microbial stability in the sourdough. In San Francisco sourdough, the ratio[32] 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.[29] 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.[33] 4% salt inhibits L. sanfranciscensis, while C. milleri can withstand 8%.[34]

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.[35] The yeasts produce mainly CO2 and ethanol.[36] 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.[37] A dry, cool starter produces a sourer loaf than a wet, warm one.[35] 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.[21]

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."[38] In this process, the ratio of yeasts to lactobacilli may be altered.[39] 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.[40]

Faster starter processes, requiring fewer refreshments, have been devised, sometimes using commercial sourdough starters as inoculants.[41] 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.[29][42]

Local methods

Bakers have devised several ways of encouraging a stable culture of micro-organisms in the starter. Unbleached, unbromated flour contains more micro-organisms than more processed flours. Bran-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,[43][44] as are many other edible plants.[45][46] Basil leaves are soaked in room-temperature water for an hour to seed traditional Greek sourdough.[47] Using water from boiled potatoes is said to increase the activity of the bacteria by providing additional starch.

The piped drinking water supplied in most urban areas is treated by chlorination or chloramination, adding small amounts of substances that inhibit potentially dangerous micro-organisms but are harmless to animals. Some bakers recommend unchlorinated water for feeding cultures.[11]: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[48] and other methods.[49]

Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially.[50]

Bakers often make loaves with fermented dough from a previous batch (which they call "mother dough",[note 2] "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.[53]

The flavour of sourdough bread varies from place to place according to the method used, the hydration of the starter and the final dough, the refreshment ratio, the length of the fermentation periods, ambient temperature, humidity, and elevation, all of which contribute to the microbiology of the sourdough.

Baking

The starter 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.[42][54][55] 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. However, sourdough that has been proved over many hours, using a sourdough starter or mother dough, can then be transferred to the machine, utilizing only the baking segment of the bread-making program, bypassing timed mechanical kneading by the machine's paddle. This may be convenient for single loaf production, but the complex blistered and slashed crust characteristics of oven-baked sourdough bread cannot be achieved in a bread making machine, as this usually requires the use of a baking stone in the oven and misting of the dough to produce steam. Furthermore, ideal crust development requires loaves of shapes not achievable in a machine's loaf tin.

Biology and chemistry of sourdough

Sourdough starter made with flour and water refreshed for three or more days

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 produces 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 byproducts of lactic acid fermentation.

Lactic acid bacteria

Main article: Lactic acid bacteria

Lactic acid bacteria are aerotolerant anaerobes, which means that though they are anaerobes, they can multiply in the presence of oxygen. Hammes and Vogel in 1995 distinguished three metabolic groups of lactic acid bacteria:[56][57]

The phylogenetic groupings of Lactobacillus species have been undergoing reclassification, first being studied in 1991 by Collins, et al.[45] In 1995, Hammes and Vogel phylogenetically grouped L. sanfranciscensis to L. casei-Pediococcus.[57] In 2003, Hammes and Hertel grouped it to L. buchneri. In 2007, Dellaglio and Felis grouped it to L. fructivorans.[45]

Yeasts

The most common yeast species in sourdough are Kazanchastania exigua (Saccharomyces exiguous), Saccharomyces cerevisiae, Candida milleri, and Candida humilis.[60]

Type I sourdough

Traditional San Francisco sourdough is a Type I sourdough.[61] Type I sourdoughs are generally firm doughs,[60] 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). Lactobacillus sanfranciscensis was named for its discovery in San Francisco sourdough starters, though it is not endemic to San Francisco. Lactobacillus sanfranciscensis and L. pontis often highlight a lactic-acid bacterial flora that includes L. fermentum, L. fructivorans, L. brevis, and L. paralimentarius.[47][61][62] The yeasts Saccharomyces exiguus, Candida milleri, or Candida holmii[61] usually populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis.[34] 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.[59][63] There have been changes in the taxonomy of yeasts in recent decades.[59][63] L. sanfranciscensis requires maltose,[64] while C. milleri is maltase negative and thus cannot consume maltose.[16][17][18][19][20] C. milleri can grow under conditions of low pH and relatively high acetate levels, a factor contributing to sourdough flora's stability.[56]

In order to produce acetic acid, L. sanfrancisensis needs maltose and fructose.[65] Wheat dough contains abundant starch and some polyfructosanes, which enzymes degrade to "maltose, fructose and little glucose."[66] 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)."[67] 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.[56] 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.[6]

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.[15]

Investigations of wheat sourdough found that S. cerevisiae died off after two refreshment cycles.[56] S. cerevisiae has less tolerance to acetic acid than other sourdough yeasts.[59] Continuously maintained, stable sourdough cannot be unintentionally contaminated by S. cerevisiae.[21]

Type II sourdough

In Type II sourdoughs, baker's yeast or Saccharomyces cerevisiae[68] is added to leaven the dough; L. pontis and L. panis highlight the flora.[60][61][62] They 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.[69][70] This process was adopted by some in industry, in part, due to simplification of the multiple-step build typical of Type I sourdoughs.[71]

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.[60]

Type III sourdough

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, Lactobacillus 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.[60]

Types of sourdough bread

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).[72][73] 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,[74] 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.[75] In Azerbaijan whole-wheat sourdough flatbreads are traditionally eaten.[76] In Ethiopia, teff flour is fermented to make injera.[77] A similar variant is eaten in Somalia, Djibouti, and Yemen (where it is known as lahoh).[78] In India idlis and dosa are made from a sourdough fermentation of rice and vigna mungo.[79]

Possible benefits

Sourdough does not require much gluten, in contrast to factory-made bread which often has gluten added. People with gluten intolerance may therefore find sourdough bread more comfortable to eat.[9] Sourdough bread produces a smaller surge in blood glucose and insulin than other types of bread,[9][80] and there is limited evidence that it is suitable even for coeliac patients.[9][81]

See also

Notes

  1. ↑ 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.[21]
  2. ↑ The term mother dough sometimes refers to a yeast sponge,[51][52] 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.

References

  1. 1 2 3 Davidson, pages 756–757
  2. ↑ Gaenzle, Michael (1 April 2014). "Sourdough Bread". In Batt, Carl. Encyclopedia of Food Microbiology (2nd ed.). Academic Press. p. 309. ISBN 978-0123847300.
  3. 1 2 Tannahill, Reay (1973). Food in History. Stein and Day. pp. 68–69. ISBN 0-8128-1437-1.
  4. ↑ Pliny the Elder (1938). Natural History. Loeb Classics. p. 1.255.
  5. 1 2 Gobbetti, Marco; Gänzle, Michael (2012). Handbook on Sourdough Biotechnology. Springer. p. 6. ISBN 978-1-4614-5425-0.
  6. 1 2 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.
  7. ↑ Peters, Erica J. San Francisco: A Food Biography. Rowman & Littlefield, 2013, p. 189.
  8. ↑ Fernald, Anya (November–December 2002). "Sourdough Baking" (34). Slow - The International Herald of Tastes. Retrieved June 18, 2010.
  9. 1 2 3 4 Griggs, Barbara (12 August 2014). "The rise and rise of sourdough bread". The Guardian. Retrieved 30 September 2016.
  10. ↑ 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.
  11. 1 2 3 4 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."
  12. ↑ Rogers, R.F. & 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.
  13. ↑ 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
  14. ↑ 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: 456–8. PMC 377202Freely accessible. PMID 5553284.
  15. 1 2 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 1951026Freely accessible. PMID 17557853.
  16. 1 2 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.
  17. 1 2 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.
  18. 1 2 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 377202Freely accessible. PMID 5553284. Retrieved Dec 20, 2011.
  19. 1 2 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 377203Freely accessible. PMID 5553285. Retrieved Dec 20, 2011.
  20. 1 2 Daeschel, M.A.; Andersson, R.E.; Fleming, H.P. (1987). "Microbial ecology of fermenting plant materials" (PDF). 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.
  21. 1 2 3 Wing, Gänzle. "Dan Woods long posts 1–4". Retrieved Dec 15, 2011.
  22. ↑ 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.
  23. ↑ 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.
  24. ↑ Häggman, M.; Salovaara, H. (2008). "Microbial re-inoculation reveals differences in the leavening power of sourdough yeast strains". LWT - Food Science and Technology. 41: 148–154. doi:10.1016/j.lwt.2007.02.001.
  25. ↑ 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.
  26. ↑ Manual for army bakers. Washington: Government Printing Office. 1910. p. 22. Retrieved Aug 13, 2011.
  27. ↑ S. John Ross. "Sourdough Bread: How To Begin (easy sourdough for the beginner or novice)". Retrieved June 17, 2011.
  28. ↑ Don Holm; Myrtle Holm (1972). The Complete Sourdough Cookbook. Caldwell, Idaho: Caxton Press. p. 40. ISBN 0-87004-223-8. Retrieved June 28, 2010.
  29. 1 2 3 Khachatourians, George G. (1994). Food Biotechnology: Microorganisms. New York: Wiley-Interscience. pp. 799–813. ISBN 0-471-18570-1.
  30. ↑ 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),...
  31. ↑ "Sourdough Rise Time Table". The Fresh Loaf. 2008-03-28. Retrieved 2016-09-15.
  32. ↑ 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.
  33. ↑ Calvel, Raymond (2001). The taste of bread. Gaithersburg, Md: Aspen Publishers. pp. 89–90. ISBN 0-8342-1646-9. Retrieved June 28, 2010.
  34. 1 2 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 106434Freely accessible. PMID 9647838.
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  36. ↑ "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.
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