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BREAD | Sourdough Bread
Sourdough Bread MG Ga¨nzle, University of Alberta, Edmonton, AB, Canada Ó 2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by Brian J. B. Wood, volume 1, pp. 295–301, Ó 1999, Elsevier Ltd.
Introduction Sourdough bread-making encompasses dough fermentation with yeast and lactic acid bacteria. The use of sourdough in baking is an ancient craft that is currently undergoing a revival of interest. The technology and microbiology of the constituent processes are examined, and the diversity of the processes is illustrated. Connections with other traditional fermentations of cereals and legumes are noted.
History 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 BC 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. Sourdough fermentation starts spontaneously if a mixture of flour and water is left in a warm place for a few hours, and satisfactory bread can be made from such a ferment. Sourdough fermentation to obtain porridges or beverages may have been the original process, out of which the production of bread would develop fairly easily. Bread production relied on the use of sourdough as leavening agent for most of human history; the use of baker’s yeast as a leavening agent dates back less than 150 years. Hieroglyphs in early Egypt as well as the analysis of bread from that time demonstrates that bread production certainly used sourdough fermentation. More detailed descriptions of sourdough fermentations were provided in the first century by Pliny the Elder in the Natural History; here, the use of backslopped, acidified dough as well as the use of yeast from winemaking are described. In early Egypt as well as the Roman Empire, bread was produced at a large, essentially industrial scale. In Europe, sourdough fermentation remained the main process for dough leavening until the use of excess brewer’s yeast became common in the fifteenth century. The dedicated production of yeast for use as leavening agent started in the late nineteenth century and all, but replaced the use of sourdough for production of wheat bread. Nevertheless, sourdough breads continued to play a significant part in the market in much of Europe, particularly in countries where rye bread is common, including Scandinavia, Germany, eastern Europe, and the former Soviet Union, as well as in parts of the Middle East. In the United States, sourdough bread was vital to the pioneers traveling west in slow-moving wagon parties, with no means of preserving yeast for baking. Sourdough starters are relatively easy to maintain, and if all else failed, another starter could be prepared from flour and water. It was so important a part of the survival kit of the adventurers seeking gold in
Encyclopedia of Food Microbiology, Volume 1
Alaska and the Yukon in 1898 that they became known as ‘sourdoughs,’ as featured in the poems of Robert Service (1957) and the novels of Jack London. In North America, sourdough bread is usually associated with San Francisco, California, where the tradition and practice of sourdough bread production survived in numerous small-craft bakeries in the century after the California gold rush. It reemerged in the 1980s with San Francisco sourdough bread on sale throughout the United States. In some cases, bakers use sourdough technology without realizing that they are doing so. The use of sponge dough, extended fermentation of a part of the dough after addition of baker’s yeast, is commonly used to improve the quality of wheat bread and soda crackers. If the fermentation time extends to more than 8–12 h, a lactic microbiota invariably develops, resulting in moderate acidification of the dough. The growth of lactic acid bacteria in sponge dough exerts a decisive influence on product quality, but is often not adequately controlled by starter cultures or process parameters. In these cases, minor changes in the recipe or the process (e.g., a different supplier of baker’s yeast) can lead to unexpected and entirely undesirable consequences for product quality. Likewise, the practice of overnight soaking of whole grains used in bread recipes to ensure full uptake of water by the grains is commonly associated with lactic fermentation.
Contemporary Use of Sourdough and Pattern of Consumption The sourdough process is the original type of bread-making, but it is easy for a consumer in the Anglo-Saxon world to assume that sourdough bread has been replaced in all but a few specialist cases by baker’s yeast-leavened bread. The expanding interest in the San Francisco bread is seen as a rather new phenomenon. However, Scandinavia, Germany, the Low Countries, eastern Europe, and the countries of the former Soviet Union all maintained thriving baking industries based on sourdough technology. This continued use of sourdough is, in part, related to the use of rye in bread production because rye flour requires acidification for optimal bread quality. Sourdough use in these countries also reflects the demand of consumers for bread variety and quality. To a British visitor, the variety of breads on offer in these countries can seem most bewildering – or stimulating, if forewarned and interested in this subject. Mediterranean countries also maintained the use of sourdough as leavening agent for specialty products with unmatched quality. The best example of this are the Italian Panettone and Colomba, sweet breads associated with the Christmas and Easter festivities, respectively. Both are produced with sourdough as sole leavening agent, which is labeled as lievito naturale or natural yeast on the ingredient list.
http://dx.doi.org/10.1016/B978-0-12-384730-0.00045-8
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In addition to the use of sourdough as leavening agent, which retains its place in bread production, current industrial practice predominantly employs sourdough or sourdough products as ingredient to achieve dough acidification, and as baking improver. In these cases, sourdough fermentation is used in combination with baker’s yeast as leavening agent, although the use of sourdough can substantially reduce the amount of yeast in the recipe. The most basic form of these dough fermentations is sponge dough, which typically includes a significant contribution of lactic acid bacteria to biochemical conversions in dough. Other processes are derived from traditional fermentations, and current fermentation equipment allows the automated fermentation of sourdough at a scale that is compatible with industrial bread production. This development was pioneered in the Soviet Union in the early twentieth century as a result of the (forced) industrialization of bread production and was replicated independently in Western countries more than 50 years later. The large-scale sourdough fermentation requires liquid, pumpable sourdoughs, and is typically fermented to higher levels of acidity to reduce the amount of sourdough in the final recipe. Dough consistency and acidity levels alter fermentation microbiota, favoring lactic acid bacteria over yeasts. Moreover, fermentation control does not allow maintaining metabolic activity of sourdough microbiota at a level that achieves dough leavening without baker’s yeast. Other benefits of sourdough fermentation, however, are achieved with these processes combining sourdough fermentation with baker’s yeast. Because sourdough fermentation allows obtaining improved bread quality and product diversification without adding ingredients or additives, up to 50% of (industrial) bread production in European countries currently includes sourdough or sourdough products. Dried or pasteurized sourdough products provide a third avenue to the use of sourdough in baking. Dried sourdough has been produced by specialized suppliers to the baking industry since the 1970s and has surpassed the economic importance of sourdough starter cultures. Drying or pasteurization inactivates fermentation microbiota but also stabilizes the product, providing a long shelf life and allowing distribution without refrigeration. Drying facilitates transportation as water is removed; drying at high temperatures (e.g., drum drying) also generates flavor compounds through the Maillard reaction. Pasteurization of sourdough retains fermentation flavors and partially gelatinizes the starch with concomitant improvements of dough hydration in the final recipe. In sourdough fermentations performed at artisanal or industrial bakeries, the composition of fermentation microbiota results from the raw materials and the choice of fermentation parameters. In contrast, fermentation for production of dried sourdough allows for control of fermentation microbiota by direct inoculation of pure cultures with desired properties. The array of products ranges from dried sourdoughs to achieve the desired level of acidity to products that are fermented and dried to achieve a high level of flavor volatiles or hydrocolloids. The quality and flavor intensity of bread produced with stabilized sourdough does not quite match that of the ‘originals’ produced by traditional sourdough fermentation. When compared to straight dough processes, however, stabilized sourdoughs offer significant opportunities
for improved bread quality, shelf life, and product diversification without the use of additives.
Raw Materials and Methods of Production A process with a rich history, a widespread geographic distribution, and significant variations in terms of integration in contemporary industrial processes will have many variations. The following sections outline the basic process of sourdough fermentation, to which variations can be linked. The raw materials are flour and water. In continental Europe, much of the sourdough bread is made with rye flour, but the North American and Mediterranean markets are predominantly devoted to wheat-flour sourdough, often using white flour. The increasing production of gluten-free bread in North America and Europe has resulted in the commercialization of glutenfree sourdough on the basis of corn, sorghum, or rice flours. The remarkable diversity of processes and raw materials is matched by a corresponding diversity of lactic acid bacteria isolated from sourdough; sourdough is the only known source for more than a dozen Lactobacillus species. Even if the perspective is limited to the traditional sourdoughs, a remarkable diversity of fermentation procedures is recorded. However, all of these processes rely on continuous propagation to maintain yeasts and lactic acid bacteria in a continuous state of growth and high metabolic activity. These traditional processes select for a fermentation microbiota that shows remarkable convergence across different countries or continents and a high stability over time.
Traditional Sourdough Fermentation Preparation and Maintenance of the Starter Sourdough starter can be initiated by mixing flour and water and leaving the mixture in a warm place overnight for spontaneous fermentation. After 12–24 h, visible fermentation has occurred, and the dough will possess a sour, alcoholic odor. The conditions favor yeasts and lactic acid bacteria that dominate the fermentation rapidly, but the outcome of spontaneous fermentations can be quite variable. Bakers control the fermentation by continuous propagation, also referred to as refreshments or back-slopping – a portion of fully fermented sourdough is used to inoculate the next batch. The inoculation of each new batch with sourdough containing actively fermenting organisms results in more rapid fermentation than would otherwise be the case. It also selects for fast-growing organisms. The yeasts and lactic acid bacteria grow synergistically, and this process, ensuring constant reselection, results in the emergence of a very stable consortium of organisms. Following initial spontaneous fermentation, a stable fermentation microbiota consisting of heterofermentative lactic acid bacteria and yeasts is established after about 10 refreshments. Sourdoughs are regularly refreshed for very long periods of time: some are known to be over a century old, with stable fermentation microbiota documented over a period of more than 20 years. However, a price must be paid for these advantages, and it is expressed in the form of labor. Sourdoughs for use as leavening agent are refreshed every 6–12 h and thus require a labor-intensive process that does not
BREAD j Sourdough Bread lend itself to much automation. Modified fermentation protocols that combine sourdough fermentation with leavening by baker’s yeast reduce the demands of the traditional procedures, but do not afford the same stability of fermentation microbiota.
Examples for Traditional Sourdough Processes There are at least as many protocols for traditional sourdough propagation as there are bakers using sourdough as leavening agent. Two representative examples for sourdough propagation, sourdough for production of Panettone and rye bread, are shown in Table 1. Panettone is sweet sourdough bread without pronounced acidity; likewise, plain white wheat bread without pronounced acidic taste can be produced with sourdough as sole leavening agent. Dough propagation for rye bread production aims to achieve a more or less pronounced acidic taste in the final product. Wheat sourdough for production of San Francisco sourdough bread is also propagated to achieve distinct bread acidity. Despite substantial variations in sourdough processes, there are several common principles for the use of sourdough as leavening agent: (1) Processes are based on continuous propagation; (2) Sourdough is refreshed two or three times before the bread dough is prepared. This usually corresponds to 2–4 refreshment steps per day; (3) Dough propagation is done in the temperature range of 20–30 C. Sourdough does not keep well during storage; storage at refrigeration temperature rapidly reduces the metabolic activity of sourdough microbiota. Freezing of sourdough inactivates sourdough yeasts and much of the leavening activity of sourdough. Thus, the use of sourdough as leavening agent requires continuous refreshment of the dough even when the bakery is not producing bread. Experienced bakers adjust the fermentation time, temperature, dough yields, and level of inoculum to ensure sufficient activity of the fermentation microbiota, and the desired balance between lactic acid bacteria and yeasts, corresponding Table 1
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to the balance between dough acidity and leavening. As a general rule, low temperatures and firm dough favor growth of yeasts over the growth of lactic acid bacteria. Referring to the two examples shown in Table 1, the wheat sourdough is propagated at lower dough yield and at a lower temperature when compared to the rye sourdough, resulting in a higher contribution of sourdough yeasts to the overall metabolic activity, higher leavening activity, and lower acidity. Likewise, the second stage of the rye sourdough propagation shown in Table 1 is conducted at a lower temperature and dough yield to promote yeast growth. Yeasts and lactobacilli in traditional sourdoughs grow optimally at 28 and 32 C, respectively with temperature maxima of 35 and 40 C, respectively. Fermentation temperatures of more than 30 C thus favor growth of lactobacilli over the growth of yeasts and result in sourdoughs with higher acidity and reduced leavening activity. It is an apparent paradox that low temperatures also favor formation of acetic acid in sourdough. Acetic acid is produced almost exclusively by heterofermentative lactobacilli. However, acetic acid formation by heterofermentative lactobacilli is dependent on the availability of fructose (see below), and thus on invertase activity of sourdough yeasts to release fructose from fructooligosaccharides present in wheat and rye flours. Salt is generally not included in the sourdough propagation steps, but added to the final bread dough. The addition of 1% NaCl to sourdough does not fundamentally alter the microbial ecology of the dough, but is sufficient to significantly reduce the growth rate of obligate heterofermentative lactobacilli. Higher salt concentrations (2–5%) shift fermentation microbiota in favor of homofermentative lactobacilli; these sourdoughs are suitable for dough acidification but not for leavening.
Processes to Combine Sourdough Fermentation with Baker’s Yeast Fermentation combining sourdough fermentation with leavening by baker’s yeast eliminates the need to maintain
Examples for propagation of sourdough for use as leavening agent Wheat sourdough for panettone production (Italy)
1st stage 2nd stage 3rd stage
% starter 3.5 10 30
% flour 4.3 13 50
% water 2.2 7.0 20
% total 10 30 100
DYa 150 150 150
Sourdough used for production of ca. 350% Panettoneb in one or two stages of dough preparation
Fermentation temp./time 24–26 C, 4 h 24–26 C, 4 h 24–26 C, 4 h Starter culture (madre) for inoculation of next dough is stored at 15–16 C for 12 h
Rye sourdough for rye bread production (Germany)
1st stage 2nd stage 3rd stage
% starter 0.4 3 28
% flour 1.45 16 35
% water 1.45 9 37
% total 3.1 28 100
DY 200 160 180
Sourdough used for production of ca. 250% bread in one stagec
Fermentation temp./time 25–26 C, 6 h 23–27 C, 8 h 28–31 C, 3 h Starter culture (Anstellsauer) used for inoculation without prolonged storage
DY, dough yield (g dough/100 g flour). A typical Panettone recipe consists of 38% wheat flour, 20% water, 9% sugar, 9% butter, 2% egg yolk, skim milk powder, dried or candied fruit, emulsifiers, and flavors. Rye or mixed rye-wheat bread, final dough yield between 160 and 170 depending on the flour(s) used.
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b c
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metabolic activity of sourdough microbiota at a level that generates sufficient CO2 to leaven the dough. Metabolism of sourdough lactic acid bacteria and yeasts, however, remains sufficient to produce dough acidification and to attain other beneficial effects of sourdough on bread quality. Sourdough propagation is derived from traditional procedures but with a reduced number of refreshments – one or two refreshments per day – is commonly used in combination with baker’s yeast. Sponge dough or ‘poolish’ are a second example. In sponge dough fermentations, 10–20% of the flour used in the bread dough is fermented with addition of baker’s yeast for several hours or overnight. If fermentation times exceed 8–12 h, lactic acid bacteria grow to high cell counts and the pH drops to values of less than 4.5 while the leavening activity is entirely attributable to baker’s yeast. Large-scale and in some cases continuous fermentation systems rely on long fermentation times – 12 h to several days – to achieve high levels of acidity and to obtain sourdough that remains stable for several hours or days of refrigerated storage. Dried or pasteurized sourdoughs are produced on the basis of the same principle; the defining difference is the stabilization step to allow shipment from the sourdough producer to the bakery.
Microbiology The diversity of sourdough fermentation processes is matched by the diversity of lactic acid bacteria and yeasts that are found in sourdough. Over 100 species of lactic acid bacteria have been identified, predominantly lactobacilli, but Weissella and Leuconostoc are also frequently found, and species of the genera Lactococcus, Enterococcus, and Pediococcus are occasionally identified. The diversity of lactic acid bacteria can be categorized to some extent by differentiation between different sourdough processes. Traditional sourdough used for leavening is categorized as Type I sourdough. The frequent refreshments needed to sustain a high metabolic activity selects for fast-growing organisms. In Type I sourdoughs, yeasts and lactobacilli are found in a numerical ratio of about 1:100. In most cases, Lactobacillus sanfranciscensis (previously Lactobacillus sanfrancisco or Lactobacillus brevis ssp. lindneri) is the dominating lactic acid bacterium and occurs together with Candida humilis (syn. of Candida milleri) or Kazachstania exiguus (syn. Saccharomyces exiguus, anamorph Candida holmii, prev. Torulopsis holmii). Sourdough yeasts are more acid tolerant than Saccharomyces cerevisiae. The species L. sanfranciscensis was described with isolates from San Francisco Sourdough as type strains. However, strains of the species dominate Type I sourdoughs worldwide and have no specific association with the Bay Area in the United States. The dominance of this species in a majority of Type I sourdoughs is explained by its rapid growth; L. sanfranciscensis grows optimally between 28 and 32 C and a pH of 5.0–6.0, conditions matching Type I sourdough fermentations. Moreover, its metabolism is highly adapted to maltose and sucrose, the most abundant carbohydrate sources in wheat and rye sourdoughs. Coexistence with C. humilis or K. exiguus relies on the lack of competition for nutrients – L. sanfranciscensis preferentially uses maltose or sucrose and peptides, while sourdough yeasts preferentially metabolize glucose and amino acids. Moreover, yeast invertase
hydrolyzes fructo-oligosaccharides, which are not accessible to lactic metabolism, to release fructose, which stimulates growth of obligate heterofermentative lactobacilli (see below). In Type I sourdoughs, L. sanfranciscensis is sometimes replaced by related hetofermentative lactic acid bacteria (e.g., L. brevis, Lactobacillus hammesii, Lactobacillus rossiae, or W. confusa), and often associated with the homofermentative Lactobacillus plantarum or Lactobacillus paralimentarius. Sourdoughs with long fermentation times, often at elevated temperature, that are fermented to achieve high levels of acidity are categorized as Type II sourdoughs. Owing to the more diverse fermentation conditions when compared to Type I sourdoughs, more diverse microbiota are encountered in different processes. Infrequent refreshments and high levels of acidity select for acid tolerant and typically thermophilic lactobacilli. Lactobacillus reuteri, Lactobacillus pontis, Lactobacillus amylovorus, and Lactobacillus fermentum frequently dominate Type II sourdoughs. Comparable to L. sanfranciscensis, L. reuteri, L. fermentum, and L. pontis are heterofermentative lactobacilli that preferentially metabolize maltose and sucrose. In contrast to L. sanfranciscensis, these species generally convert arginine to ornithine and glutamine to g-aminobutyrate. Both conversions contribute to the acid tolerance of these species. Type II sourdough microbiota show remarkable overlap with Lactobacillus species in intestinal microbiota of humans and animals, and the intestinal origin of Type II lactobacilli was shown for sourdough isolates of L. reuteri. Sponge doughs that are started by addition of baker’s yeast are categorized as Type 0 sourdoughs. The microbiota of sponge doughs that are not started by back-slopping of mature sourdough is dependent on lactic acid bacteria from the bakery environment, the raw materials, or those present in baker’s yeast. Baker’s yeast is probably the most significant source of lactic acid bacteria present in sponge doughs, but yeast from different suppliers may be contaminated with different levels and types of lactic acid bacteria. Lactobacillus sakei, L. plantarum, and Pediococcus species were isolated from sponge doughs in France, Germany, and the United States.
Biochemistry of Sourdough Fermentation In contrast to most other food fermentations, obligately heterofermentative lactic acid bacteria are numerically dominant in most sourdoughs. Heterofermentative metabolism converts hexoses via the phosphoketolase pathway to lactate, ethanol or acetate, and CO2. Heterofermentative lactobacilli contribute to the leavening power of sourdough, and experimental sourdough fermentations have demonstrated that sufficient leavening can be achieved by L. sanfranciscensis in pure culture. The competitiveness of heterofermentative lactobacilli in sourdough is attributable to the efficient metabolism of maltose and sucrose. Utilization of these disaccharides is not repressed by glucose and is preferred over glucose metabolism by many sourdough lactobacilli, including L. sanfranciscensis and L. reuteri. Maltose and sucrose metabolism by maltose phosphorylase and sucrose phosphorylase generates glucose-1phosphate without expenditure of ATP and thus increases the energy yield of hexose metabolism. The effective utilization of fructose as electron acceptor to achieve cofactor regeneration is a second important contributor to the competitiveness of
BREAD j Sourdough Bread heterofermentative lactobacilli. Hexose metabolism via the phosphoketolase pathway generates acetyl-phosphate as energy-rich metabolic intermediate, which is reduced to ethanol as end product with concomitant oxidation of two reduced cofactors, NADH, to NADþ. If fructose is present, heterofermentative lactic acid bacteria generally reduce fructose to mannitol with concomitant oxidation of NADH to NADþ. This allows the conversion of acetyl-phosphate to acetate, coupled to synthesis of ATP from ADP and a further increase of metabolic efficiency. The use of fructose as electron acceptor is preferred over the use of fructose as carbon source by heterofermentative lactobacilli. Virtually all strains of L. sanfranciscensis reduce fructose to mannitol, but many strains do not use fructose as carbon source. A majority of sourdough lactobacilli are not capable of oligo- or polysaccharide metabolism and rely on cereal- and yeast-derived amylases and invertase, respectively, to release maltose and fructose from starch and fructo-oligosaccharides, respectively. Likewise, sourdough lactobacilli generally do not exhibit extracellular protease activity and rely on cereal enzymes to provide peptides, which are taken up by oligopeptide or dipeptide transporters.
Technological Effects of Sourdough Fermentation on Bread Quality Sourdough fermentation affects all aspects of bread quality, volume, texture, microbial shelf life and staling, and taste and aroma. Principal effects can be attributed to acid production by lactic acid bacteria and to the increased fermentation time allowing for increased activity of cereal amylases, proteases, phytases, and pentosanases. Dough acidification is particularly important in rye bread, a fact that contributed to the continued use of sourdough in countries where rye bread has a substantial share of the bread market. The importance of dough acidification in rye baking is attributable to two factors. First, rye flour lacks structure-forming gluten proteins; dough hydration and gas retention are largely dependent on pentosans. The partial hydrolysis of pentosans by cereal pentosanases during sourdough fermentation increases their solubility and water binding, and improves the volume and texture of the bread. A solubilization of water-insoluble pentosans also occurs in wheat sourdough, but is of secondary importance due to the presence of polymeric gluten proteins. Second, rye flour has a higher amylase activity and rye starch has a lower gelatinization temperature when compared to wheat. This results in a small temperature range during which active amylase and gelatinized starch coexist. During heating of the crumb in the baking process, the amylase activity of rye flour is sufficient to compromise or destroy the crumb structure unless amylase is inhibited by low pH. The low pH also enhances the activity of cereal proteases and phytases. Minerals in wheat and rye flours are mainly bound in insoluble complexes with phytate. Dough acidification allows for optimal activity of cereal phytase (pH 5.0–5.5) and solubilizes the insoluble phytates (pH < 5.0). Phytate hydrolysis during sourdough fermentation reduces the binding of minerals and increases their bioavailability. It is doubtful if this is significant for a consumer eating a reasonably varied diet. However there is an increasing concern that even the American
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diet is only marginally adequate in terms of trace minerals, and so a reduction in phytate may be significant. There is no evidence that phytases from lactic acid bacteria contribute to phytate degradation during sourdough fermentation. Proteases in wheat and rye flours are optimally active in the pH range of 3.5–4.5. Sourdough fermentation thus allows for high protease activity, and the amino acid concentration increases 5–10-fold during fermentation. Amino acids are important precursors for flavor formation by lactic and yeast metabolism, or in thermal reactions during baking. Excess proteolysis in wheat sourdoughs, however, compromises the gluten network and results in a reduced bread volume. Microbial metabolites with specific effect on bread quality particularly include acetic acid, ornithine, glutamate, and exopolysaccharides. Acetic acid is produced by heterofermentative lactic acid bacteria if fructose is available as electron acceptor. Acetic acid is volatile and thus influences bread odor as well as taste. Moreover, acetic acid has a stronger antimicrobial activity than lactic acid and contributes to the extended microbial shelf life of sourdough bread. However, acetic acid in concentrations that prevents fungal bread spoilage has such a strong impact on bread flavor that the bread is inacceptable to most consumers. Ornithine is the product of arginine conversion by L. reuteri, L. pontis, L. rossiae, and other sourdough lactobacilli. During baking, ornithine reacts to 2-acetyl-1-pyrroline, the character impact compound of wheat crust odor. Experimental strategies to specifically augment the ornithine content of sourdough also increased the pleasant, roasty crust odor. Glutamine is the most abundant amino acid in wheat and rye proteins; individual gliadins contain up to 50% glutamine. Sourdough lactobacilli convert glutamine to glutamate, which imparts umami taste. Glutamate addition to levels matching microbial glutamate accumulation improved the sensory properties of bread. All sourdough lactobacilli are capable of glutamine conversion, although the extent of conversion is strain specific. The conversion of amino acids by yeast metabolism results in formation of flavor volatiles; for example, methylbutanal and phenylethanol are formed from leucine or isoleucine and phenylalanine, respectively. Exopolysaccharide formation by lactobacilli in sourdough is based on glucansucrase or fructansucrase activity. These enzymes are extracellular or cell wall associated and convert sucrose to polymeric fructans (fructansucrases) or glucans (glucansucrases). Polymers produced by lactobacilli in sourdough include the fructans inulin and levan, and the glucans dextran, reuteran, or mutan. The frequency of exopolysaccharides producing sourdough strains is high, and any given sourdough likely contains at least one exopolysaccharideproducing strain. The amount of exopolysaccharides produced during sourdough fermentation is dependent on the strain employed, sucrose concentration, and process conditions. In experimental and industrial sourdough fermentations, exopolysaccharides accumulate to more than 20 g kg1 dough. This quantity is sufficient to improve the volume and texture of sourdough bread and to delay bread staling. For example, the long shelf life of Panettone was attributed to dextran formation by Leuconostoc spp. during sourdough fermentation. Sucrose conversion by glucansucrases and fructansucrases, however, also releases fructose and increases acetic acid formation by heterofermentative lactobacilli. Most Weissella strains are an
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exception among heterofermentative lactic acid bacteria as they do not employ fructose as electron acceptor to support acetate formation and dextran-producing Weissella spp. are highly suitable to improve bread volume and texture. Sourdough fermentation delays the spoilage of bread by rope-forming bacilli and molds. Moderate acidification of bread that is achieved in most sourdough breads is sufficient to prevent ropy spoilage. Experimental sourdough fermentations were also shown to delay the growth of fungal spores on bread. This antifungal effect is strain specific. Active antifungal metabolites remain to be identified; known antifungal metabolites of lactobacilli are not produced to inhibitory concentrations during sourdough fermentation. The antifungal effect of sourdough is likely attributable to a combination of several microbial metabolites and substrate-derived antimicrobial compounds.
Sourdough Starter Cultures Sourdough starter cultures have been available since 1910 and were among the first commercially available starter cultures. Initial culture preparations were based on cereal substrate – essentially refrigerated sourdough with mixed strain composition containing yeasts and lactic acid bacteria and a shelf life of a few weeks. Culture preparations on cereal substrate have retained their relevance to date, owing to their superior activity upon refreshment at the bakery when compared to dried cultures. Recent developments include the commercialization of rice- or sorghum-based gluten-free starter cultures for use in gluten-free baking. Freeze-dried, pure culture preparations for sourdough fermentations have also been available for several decades. However, freeze-dried cultures fail to develop sufficient metabolic activity in straight dough processes; their use requires one or more refreshment in the bakery. Moreover, freeze-dried cultures of lactic acid bacteria cannot replace traditional sourdoughs that contain sourdough yeast as well as lactic acid bacteria. Dried or pasteurized sourdough products do not contain relevant numbers of viable lactic acid bacteria and are used as baking improver rather than as starter culture.
Related Cereal Fermentations Numerous other cereal fermentations exist worldwide that are highly related to sourdough fermentation in terms of fermentation conditions and microbial ecology, but are used to produce beverages or porridges rather than bread. A detailed description of these fermentations is beyond the scope of this article, but a few examples are presented to indicate that very diverse products are obtained from the same basic fermentation. Steamed wheat bread (man tou) produced in China and throughout Southeast Asia differs from bread only insofar that the baking process is replaced by steaming. Dough fermentation for steamed bread relies on sponge dough fermentation or back-slopped sourdoughs, resulting in fermentation microbiota that are comparable to sourdough used in baking.
Kvass, widely consumed in Russia, and boza, consumed in Turkey and surrounding countries, are two examples of cerealbased beverages. Kvass is produced from malt or sourdough bread, whereas boza is produced from boiled wheat, maize, rice, and/or millet flours. Both beverages are sweetened with sucrose, are slightly alcoholic (0.5–1%), and undergo lactic fermentation. Fermentation microbiota consist of S. cerevisiae and lactic acid bacteria, including dextran-producing Leuconostoc spp. Cereal fermentations in Africa and South Asia employ corn, sorghum, millet, or teff as raw materials to produce porridges, gruels, or cakes. Many of the fermentations documented in the scientific literature are based on spontaneous fermentation, but the use of back-slopping has also been reported. Examples include mawè and ting, porridges produced in West Africa and Botswana, respectively, and idli, a soft cake produced in South India and Sri Lanka. The high ambient temperature in these countries selects for thermophilic fermentation microbiota. It is noteworthy that cereal fermentations in tropical climates frequently harbor amylolytic lactobacilli. This may relate to the low amylase activity of the substrates employed.
Conclusion Sourdough fermentation is the most ancient way of producing bread and has retained its relevance in contemporary bread production. The continued importance of sourdough in bread production relates to the unique quality of sourdough bread that cannot be reproduced with alternative fermentation methods or ingredients. Sourdough can replace several ingredients or processing aids and allow a substantial reduction of the production cost. Traditional procedures for sourdough fermentation retain their relevance in the artisanal production of (specialty) bread. Moreover, traditional processes were adapted and modified to meet the requirements of large-scale and automated bread production.
See also: Bacteriocins: Potential in Food Preservation; Bread: Bread from Wheat Flour; Candida; Ecology of Bacteria and Fungi in Foods: Influence of Redox Potential; Ecology of Bacteria and Fungi in Foods: Effects of pH; Fermentation (Industrial): Basic Considerations; Fermentation (Industrial): Control of Fermentation Conditions; Fermented Foods: Origins and Applications; Fermented Foods: Fermentations of East and Southeast Asia; Beverages from Sorghum and Millet; Lactobacillus: Introduction; Lactobacillus: Lactobacillus brevis; Metabolic Pathways: Release of Energy (Aerobic); Saccharomyces – Introduction; Saccharomyces:Saccharomyces cerevisiae; Starter Cultures; Starter Cultures: Importance of Selected Genera; Torulopsis; Yeasts: Production and Commercial Uses; Yersinia: Introduction.
Further Reading Brandt, M.J., 2007. Sourdough products for convenient use in baking. Food Microbiology 24, 161–164. De Vuyst, L., Vancanneyt, M., 2007. Biodiversity and identification of sourdough lactic acid bacteria. Food Microbiology 24, 120–127.
BREAD j Sourdough Bread Gänzle, M.G., Vermeulen, N., Vogel, R.F., 2007. Carbohydrate, peptide, and lipid metabolism of lactic acid bacteria. Food Microbiology 24, 128–138. Gänzle, M.G., Loponen, J., Gobbetti, M., 2008. Proteolysis in sourdough fermentations: mechanisms and potential for improved bread quality. Trends in Food Science and Technology 19, 513–521. Hammes, W.P., Gänzle, M.G., 1998. Sourdough breads and related products. In: Wood, B.J.B. (Ed.), The Microbiology of Fermented Foods, second ed. Blackie., London, pp. 199–216. Hammes, W.P., Brandt, M.J., Francis, K.L., Rosenheim, J., Seitter, M.F.H., Vogelman, S.A., 2005. Microbial ecology of cereal fermentations. Trends in Food Science and Technology 16, 4–11. Hansen, A., Schieberle, S., 2005. Generation of aroma compounds during sourdough fermentation: applied and fundamental aspects. Trends in Food Science and Technology 16, 85–103. Jenson, I., 1988. Bread and baker’s yeast. In: Wood, B.J.B. (Ed.), The Microbiology of Fermented Foods, second ed. Blackie., London, pp. 172–198.
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Moroni, A.V., Dal Bello, F., Arendt, E.K., 2009. Sourdough in gluten-free breadmaking: an ancient technology to solve a novel issue? Food Microbiology 26, 676–684. Nout, M.J.R., 2009. Rich nutrition from the poorest – cereal fermentations in Africa and Asia. Food Microbiology 26, 685–692. Schnürer, J., Magnusson, J., 2005. Antifungal lactic acid bacteria as biopreservatives. Trends in Food Science and Technology 16, 70–78. Service, R., 1957. Songs of a Sourdough (Reset Edition). Ernest Benn, McGraw-Hill Ryerson., Toronto. Spicher, V., Pomeranz, V., 1985. Bread and Other Baked Products. In: Ullmann’s Encyclopedia of Industrial Chemistry, fifth ed., vol. A4. VCH Verlagsgesellschaft, Weinheim. 331. Vogel, R.F., Knorr, R., Müller, M.R.A., Steudel, U., Gänzle, M.G., Ehrmann, M.A., 1999. Non-dairy lactic fermentations: the cereal world. Antonie Van Leeuwenhoek 76, 403–411.
Introduction Sourdough bread-making encompasses dough fermentation with yeast and lactic acid bacteria. The use of sourdough in baking is an ancient craft that is currently undergoing a revival of interest. The technology and microbiology of the constituent processes are examined, and the diversity of the processes is illustrated. Connections with other traditional fermentations of cereals and legumes are noted.
History 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 BC 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. Sourdough fermentation starts spontaneously if a mixture of flour and water is left in a warm place for a few hours, and satisfactory bread can be made from such a ferment. Sourdough fermentation to obtain porridges or beverages may have been the original process, out of which the production of bread would develop fairly easily. Bread production relied on the use of sourdough as leavening agent for most of human history; the use of baker’s yeast as a leavening agent dates back less than 150 years. Hieroglyphs in early Egypt as well as the analysis of bread from that time demonstrates that bread production certainly used sourdough fermentation. More detailed descriptions of sourdough fermentations were provided in the first century by Pliny the Elder in the Natural History; here, the use of backslopped, acidified dough as well as the use of yeast from winemaking are described. In early Egypt as well as the Roman Empire, bread was produced at a large, essentially industrial scale. In Europe, sourdough fermentation remained the main process for dough leavening until the use of excess brewer’s yeast became common in the fifteenth century. The dedicated production of yeast for use as leavening agent started in the late nineteenth century and all, but replaced the use of sourdough for production of wheat bread. Nevertheless, sourdough breads continued to play a significant part in the market in much of Europe, particularly in countries where rye bread is common, including Scandinavia, Germany, eastern Europe, and the former Soviet Union, as well as in parts of the Middle East. In the United States, sourdough bread was vital to the pioneers traveling west in slow-moving wagon parties, with no means of preserving yeast for baking. Sourdough starters are relatively easy to maintain, and if all else failed, another starter could be prepared from flour and water. It was so important a part of the survival kit of the adventurers seeking gold in
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Alaska and the Yukon in 1898 that they became known as ‘sourdoughs,’ as featured in the poems of Robert Service (1957) and the novels of Jack London. In North America, sourdough bread is usually associated with San Francisco, California, where the tradition and practice of sourdough bread production survived in numerous small-craft bakeries in the century after the California gold rush. It reemerged in the 1980s with San Francisco sourdough bread on sale throughout the United States. In some cases, bakers use sourdough technology without realizing that they are doing so. The use of sponge dough, extended fermentation of a part of the dough after addition of baker’s yeast, is commonly used to improve the quality of wheat bread and soda crackers. If the fermentation time extends to more than 8–12 h, a lactic microbiota invariably develops, resulting in moderate acidification of the dough. The growth of lactic acid bacteria in sponge dough exerts a decisive influence on product quality, but is often not adequately controlled by starter cultures or process parameters. In these cases, minor changes in the recipe or the process (e.g., a different supplier of baker’s yeast) can lead to unexpected and entirely undesirable consequences for product quality. Likewise, the practice of overnight soaking of whole grains used in bread recipes to ensure full uptake of water by the grains is commonly associated with lactic fermentation.
Contemporary Use of Sourdough and Pattern of Consumption The sourdough process is the original type of bread-making, but it is easy for a consumer in the Anglo-Saxon world to assume that sourdough bread has been replaced in all but a few specialist cases by baker’s yeast-leavened bread. The expanding interest in the San Francisco bread is seen as a rather new phenomenon. However, Scandinavia, Germany, the Low Countries, eastern Europe, and the countries of the former Soviet Union all maintained thriving baking industries based on sourdough technology. This continued use of sourdough is, in part, related to the use of rye in bread production because rye flour requires acidification for optimal bread quality. Sourdough use in these countries also reflects the demand of consumers for bread variety and quality. To a British visitor, the variety of breads on offer in these countries can seem most bewildering – or stimulating, if forewarned and interested in this subject. Mediterranean countries also maintained the use of sourdough as leavening agent for specialty products with unmatched quality. The best example of this are the Italian Panettone and Colomba, sweet breads associated with the Christmas and Easter festivities, respectively. Both are produced with sourdough as sole leavening agent, which is labeled as lievito naturale or natural yeast on the ingredient list.
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In addition to the use of sourdough as leavening agent, which retains its place in bread production, current industrial practice predominantly employs sourdough or sourdough products as ingredient to achieve dough acidification, and as baking improver. In these cases, sourdough fermentation is used in combination with baker’s yeast as leavening agent, although the use of sourdough can substantially reduce the amount of yeast in the recipe. The most basic form of these dough fermentations is sponge dough, which typically includes a significant contribution of lactic acid bacteria to biochemical conversions in dough. Other processes are derived from traditional fermentations, and current fermentation equipment allows the automated fermentation of sourdough at a scale that is compatible with industrial bread production. This development was pioneered in the Soviet Union in the early twentieth century as a result of the (forced) industrialization of bread production and was replicated independently in Western countries more than 50 years later. The large-scale sourdough fermentation requires liquid, pumpable sourdoughs, and is typically fermented to higher levels of acidity to reduce the amount of sourdough in the final recipe. Dough consistency and acidity levels alter fermentation microbiota, favoring lactic acid bacteria over yeasts. Moreover, fermentation control does not allow maintaining metabolic activity of sourdough microbiota at a level that achieves dough leavening without baker’s yeast. Other benefits of sourdough fermentation, however, are achieved with these processes combining sourdough fermentation with baker’s yeast. Because sourdough fermentation allows obtaining improved bread quality and product diversification without adding ingredients or additives, up to 50% of (industrial) bread production in European countries currently includes sourdough or sourdough products. Dried or pasteurized sourdough products provide a third avenue to the use of sourdough in baking. Dried sourdough has been produced by specialized suppliers to the baking industry since the 1970s and has surpassed the economic importance of sourdough starter cultures. Drying or pasteurization inactivates fermentation microbiota but also stabilizes the product, providing a long shelf life and allowing distribution without refrigeration. Drying facilitates transportation as water is removed; drying at high temperatures (e.g., drum drying) also generates flavor compounds through the Maillard reaction. Pasteurization of sourdough retains fermentation flavors and partially gelatinizes the starch with concomitant improvements of dough hydration in the final recipe. In sourdough fermentations performed at artisanal or industrial bakeries, the composition of fermentation microbiota results from the raw materials and the choice of fermentation parameters. In contrast, fermentation for production of dried sourdough allows for control of fermentation microbiota by direct inoculation of pure cultures with desired properties. The array of products ranges from dried sourdoughs to achieve the desired level of acidity to products that are fermented and dried to achieve a high level of flavor volatiles or hydrocolloids. The quality and flavor intensity of bread produced with stabilized sourdough does not quite match that of the ‘originals’ produced by traditional sourdough fermentation. When compared to straight dough processes, however, stabilized sourdoughs offer significant opportunities
for improved bread quality, shelf life, and product diversification without the use of additives.
Raw Materials and Methods of Production A process with a rich history, a widespread geographic distribution, and significant variations in terms of integration in contemporary industrial processes will have many variations. The following sections outline the basic process of sourdough fermentation, to which variations can be linked. The raw materials are flour and water. In continental Europe, much of the sourdough bread is made with rye flour, but the North American and Mediterranean markets are predominantly devoted to wheat-flour sourdough, often using white flour. The increasing production of gluten-free bread in North America and Europe has resulted in the commercialization of glutenfree sourdough on the basis of corn, sorghum, or rice flours. The remarkable diversity of processes and raw materials is matched by a corresponding diversity of lactic acid bacteria isolated from sourdough; sourdough is the only known source for more than a dozen Lactobacillus species. Even if the perspective is limited to the traditional sourdoughs, a remarkable diversity of fermentation procedures is recorded. However, all of these processes rely on continuous propagation to maintain yeasts and lactic acid bacteria in a continuous state of growth and high metabolic activity. These traditional processes select for a fermentation microbiota that shows remarkable convergence across different countries or continents and a high stability over time.
Traditional Sourdough Fermentation Preparation and Maintenance of the Starter Sourdough starter can be initiated by mixing flour and water and leaving the mixture in a warm place overnight for spontaneous fermentation. After 12–24 h, visible fermentation has occurred, and the dough will possess a sour, alcoholic odor. The conditions favor yeasts and lactic acid bacteria that dominate the fermentation rapidly, but the outcome of spontaneous fermentations can be quite variable. Bakers control the fermentation by continuous propagation, also referred to as refreshments or back-slopping – a portion of fully fermented sourdough is used to inoculate the next batch. The inoculation of each new batch with sourdough containing actively fermenting organisms results in more rapid fermentation than would otherwise be the case. It also selects for fast-growing organisms. The yeasts and lactic acid bacteria grow synergistically, and this process, ensuring constant reselection, results in the emergence of a very stable consortium of organisms. Following initial spontaneous fermentation, a stable fermentation microbiota consisting of heterofermentative lactic acid bacteria and yeasts is established after about 10 refreshments. Sourdoughs are regularly refreshed for very long periods of time: some are known to be over a century old, with stable fermentation microbiota documented over a period of more than 20 years. However, a price must be paid for these advantages, and it is expressed in the form of labor. Sourdoughs for use as leavening agent are refreshed every 6–12 h and thus require a labor-intensive process that does not
BREAD j Sourdough Bread lend itself to much automation. Modified fermentation protocols that combine sourdough fermentation with leavening by baker’s yeast reduce the demands of the traditional procedures, but do not afford the same stability of fermentation microbiota.
Examples for Traditional Sourdough Processes There are at least as many protocols for traditional sourdough propagation as there are bakers using sourdough as leavening agent. Two representative examples for sourdough propagation, sourdough for production of Panettone and rye bread, are shown in Table 1. Panettone is sweet sourdough bread without pronounced acidity; likewise, plain white wheat bread without pronounced acidic taste can be produced with sourdough as sole leavening agent. Dough propagation for rye bread production aims to achieve a more or less pronounced acidic taste in the final product. Wheat sourdough for production of San Francisco sourdough bread is also propagated to achieve distinct bread acidity. Despite substantial variations in sourdough processes, there are several common principles for the use of sourdough as leavening agent: (1) Processes are based on continuous propagation; (2) Sourdough is refreshed two or three times before the bread dough is prepared. This usually corresponds to 2–4 refreshment steps per day; (3) Dough propagation is done in the temperature range of 20–30 C. Sourdough does not keep well during storage; storage at refrigeration temperature rapidly reduces the metabolic activity of sourdough microbiota. Freezing of sourdough inactivates sourdough yeasts and much of the leavening activity of sourdough. Thus, the use of sourdough as leavening agent requires continuous refreshment of the dough even when the bakery is not producing bread. Experienced bakers adjust the fermentation time, temperature, dough yields, and level of inoculum to ensure sufficient activity of the fermentation microbiota, and the desired balance between lactic acid bacteria and yeasts, corresponding Table 1
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to the balance between dough acidity and leavening. As a general rule, low temperatures and firm dough favor growth of yeasts over the growth of lactic acid bacteria. Referring to the two examples shown in Table 1, the wheat sourdough is propagated at lower dough yield and at a lower temperature when compared to the rye sourdough, resulting in a higher contribution of sourdough yeasts to the overall metabolic activity, higher leavening activity, and lower acidity. Likewise, the second stage of the rye sourdough propagation shown in Table 1 is conducted at a lower temperature and dough yield to promote yeast growth. Yeasts and lactobacilli in traditional sourdoughs grow optimally at 28 and 32 C, respectively with temperature maxima of 35 and 40 C, respectively. Fermentation temperatures of more than 30 C thus favor growth of lactobacilli over the growth of yeasts and result in sourdoughs with higher acidity and reduced leavening activity. It is an apparent paradox that low temperatures also favor formation of acetic acid in sourdough. Acetic acid is produced almost exclusively by heterofermentative lactobacilli. However, acetic acid formation by heterofermentative lactobacilli is dependent on the availability of fructose (see below), and thus on invertase activity of sourdough yeasts to release fructose from fructooligosaccharides present in wheat and rye flours. Salt is generally not included in the sourdough propagation steps, but added to the final bread dough. The addition of 1% NaCl to sourdough does not fundamentally alter the microbial ecology of the dough, but is sufficient to significantly reduce the growth rate of obligate heterofermentative lactobacilli. Higher salt concentrations (2–5%) shift fermentation microbiota in favor of homofermentative lactobacilli; these sourdoughs are suitable for dough acidification but not for leavening.
Processes to Combine Sourdough Fermentation with Baker’s Yeast Fermentation combining sourdough fermentation with leavening by baker’s yeast eliminates the need to maintain
Examples for propagation of sourdough for use as leavening agent Wheat sourdough for panettone production (Italy)
1st stage 2nd stage 3rd stage
% starter 3.5 10 30
% flour 4.3 13 50
% water 2.2 7.0 20
% total 10 30 100
DYa 150 150 150
Sourdough used for production of ca. 350% Panettoneb in one or two stages of dough preparation
Fermentation temp./time 24–26 C, 4 h 24–26 C, 4 h 24–26 C, 4 h Starter culture (madre) for inoculation of next dough is stored at 15–16 C for 12 h
Rye sourdough for rye bread production (Germany)
1st stage 2nd stage 3rd stage
% starter 0.4 3 28
% flour 1.45 16 35
% water 1.45 9 37
% total 3.1 28 100
DY 200 160 180
Sourdough used for production of ca. 250% bread in one stagec
Fermentation temp./time 25–26 C, 6 h 23–27 C, 8 h 28–31 C, 3 h Starter culture (Anstellsauer) used for inoculation without prolonged storage
DY, dough yield (g dough/100 g flour). A typical Panettone recipe consists of 38% wheat flour, 20% water, 9% sugar, 9% butter, 2% egg yolk, skim milk powder, dried or candied fruit, emulsifiers, and flavors. Rye or mixed rye-wheat bread, final dough yield between 160 and 170 depending on the flour(s) used.
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b c
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metabolic activity of sourdough microbiota at a level that generates sufficient CO2 to leaven the dough. Metabolism of sourdough lactic acid bacteria and yeasts, however, remains sufficient to produce dough acidification and to attain other beneficial effects of sourdough on bread quality. Sourdough propagation is derived from traditional procedures but with a reduced number of refreshments – one or two refreshments per day – is commonly used in combination with baker’s yeast. Sponge dough or ‘poolish’ are a second example. In sponge dough fermentations, 10–20% of the flour used in the bread dough is fermented with addition of baker’s yeast for several hours or overnight. If fermentation times exceed 8–12 h, lactic acid bacteria grow to high cell counts and the pH drops to values of less than 4.5 while the leavening activity is entirely attributable to baker’s yeast. Large-scale and in some cases continuous fermentation systems rely on long fermentation times – 12 h to several days – to achieve high levels of acidity and to obtain sourdough that remains stable for several hours or days of refrigerated storage. Dried or pasteurized sourdoughs are produced on the basis of the same principle; the defining difference is the stabilization step to allow shipment from the sourdough producer to the bakery.
Microbiology The diversity of sourdough fermentation processes is matched by the diversity of lactic acid bacteria and yeasts that are found in sourdough. Over 100 species of lactic acid bacteria have been identified, predominantly lactobacilli, but Weissella and Leuconostoc are also frequently found, and species of the genera Lactococcus, Enterococcus, and Pediococcus are occasionally identified. The diversity of lactic acid bacteria can be categorized to some extent by differentiation between different sourdough processes. Traditional sourdough used for leavening is categorized as Type I sourdough. The frequent refreshments needed to sustain a high metabolic activity selects for fast-growing organisms. In Type I sourdoughs, yeasts and lactobacilli are found in a numerical ratio of about 1:100. In most cases, Lactobacillus sanfranciscensis (previously Lactobacillus sanfrancisco or Lactobacillus brevis ssp. lindneri) is the dominating lactic acid bacterium and occurs together with Candida humilis (syn. of Candida milleri) or Kazachstania exiguus (syn. Saccharomyces exiguus, anamorph Candida holmii, prev. Torulopsis holmii). Sourdough yeasts are more acid tolerant than Saccharomyces cerevisiae. The species L. sanfranciscensis was described with isolates from San Francisco Sourdough as type strains. However, strains of the species dominate Type I sourdoughs worldwide and have no specific association with the Bay Area in the United States. The dominance of this species in a majority of Type I sourdoughs is explained by its rapid growth; L. sanfranciscensis grows optimally between 28 and 32 C and a pH of 5.0–6.0, conditions matching Type I sourdough fermentations. Moreover, its metabolism is highly adapted to maltose and sucrose, the most abundant carbohydrate sources in wheat and rye sourdoughs. Coexistence with C. humilis or K. exiguus relies on the lack of competition for nutrients – L. sanfranciscensis preferentially uses maltose or sucrose and peptides, while sourdough yeasts preferentially metabolize glucose and amino acids. Moreover, yeast invertase
hydrolyzes fructo-oligosaccharides, which are not accessible to lactic metabolism, to release fructose, which stimulates growth of obligate heterofermentative lactobacilli (see below). In Type I sourdoughs, L. sanfranciscensis is sometimes replaced by related hetofermentative lactic acid bacteria (e.g., L. brevis, Lactobacillus hammesii, Lactobacillus rossiae, or W. confusa), and often associated with the homofermentative Lactobacillus plantarum or Lactobacillus paralimentarius. Sourdoughs with long fermentation times, often at elevated temperature, that are fermented to achieve high levels of acidity are categorized as Type II sourdoughs. Owing to the more diverse fermentation conditions when compared to Type I sourdoughs, more diverse microbiota are encountered in different processes. Infrequent refreshments and high levels of acidity select for acid tolerant and typically thermophilic lactobacilli. Lactobacillus reuteri, Lactobacillus pontis, Lactobacillus amylovorus, and Lactobacillus fermentum frequently dominate Type II sourdoughs. Comparable to L. sanfranciscensis, L. reuteri, L. fermentum, and L. pontis are heterofermentative lactobacilli that preferentially metabolize maltose and sucrose. In contrast to L. sanfranciscensis, these species generally convert arginine to ornithine and glutamine to g-aminobutyrate. Both conversions contribute to the acid tolerance of these species. Type II sourdough microbiota show remarkable overlap with Lactobacillus species in intestinal microbiota of humans and animals, and the intestinal origin of Type II lactobacilli was shown for sourdough isolates of L. reuteri. Sponge doughs that are started by addition of baker’s yeast are categorized as Type 0 sourdoughs. The microbiota of sponge doughs that are not started by back-slopping of mature sourdough is dependent on lactic acid bacteria from the bakery environment, the raw materials, or those present in baker’s yeast. Baker’s yeast is probably the most significant source of lactic acid bacteria present in sponge doughs, but yeast from different suppliers may be contaminated with different levels and types of lactic acid bacteria. Lactobacillus sakei, L. plantarum, and Pediococcus species were isolated from sponge doughs in France, Germany, and the United States.
Biochemistry of Sourdough Fermentation In contrast to most other food fermentations, obligately heterofermentative lactic acid bacteria are numerically dominant in most sourdoughs. Heterofermentative metabolism converts hexoses via the phosphoketolase pathway to lactate, ethanol or acetate, and CO2. Heterofermentative lactobacilli contribute to the leavening power of sourdough, and experimental sourdough fermentations have demonstrated that sufficient leavening can be achieved by L. sanfranciscensis in pure culture. The competitiveness of heterofermentative lactobacilli in sourdough is attributable to the efficient metabolism of maltose and sucrose. Utilization of these disaccharides is not repressed by glucose and is preferred over glucose metabolism by many sourdough lactobacilli, including L. sanfranciscensis and L. reuteri. Maltose and sucrose metabolism by maltose phosphorylase and sucrose phosphorylase generates glucose-1phosphate without expenditure of ATP and thus increases the energy yield of hexose metabolism. The effective utilization of fructose as electron acceptor to achieve cofactor regeneration is a second important contributor to the competitiveness of
BREAD j Sourdough Bread heterofermentative lactobacilli. Hexose metabolism via the phosphoketolase pathway generates acetyl-phosphate as energy-rich metabolic intermediate, which is reduced to ethanol as end product with concomitant oxidation of two reduced cofactors, NADH, to NADþ. If fructose is present, heterofermentative lactic acid bacteria generally reduce fructose to mannitol with concomitant oxidation of NADH to NADþ. This allows the conversion of acetyl-phosphate to acetate, coupled to synthesis of ATP from ADP and a further increase of metabolic efficiency. The use of fructose as electron acceptor is preferred over the use of fructose as carbon source by heterofermentative lactobacilli. Virtually all strains of L. sanfranciscensis reduce fructose to mannitol, but many strains do not use fructose as carbon source. A majority of sourdough lactobacilli are not capable of oligo- or polysaccharide metabolism and rely on cereal- and yeast-derived amylases and invertase, respectively, to release maltose and fructose from starch and fructo-oligosaccharides, respectively. Likewise, sourdough lactobacilli generally do not exhibit extracellular protease activity and rely on cereal enzymes to provide peptides, which are taken up by oligopeptide or dipeptide transporters.
Technological Effects of Sourdough Fermentation on Bread Quality Sourdough fermentation affects all aspects of bread quality, volume, texture, microbial shelf life and staling, and taste and aroma. Principal effects can be attributed to acid production by lactic acid bacteria and to the increased fermentation time allowing for increased activity of cereal amylases, proteases, phytases, and pentosanases. Dough acidification is particularly important in rye bread, a fact that contributed to the continued use of sourdough in countries where rye bread has a substantial share of the bread market. The importance of dough acidification in rye baking is attributable to two factors. First, rye flour lacks structure-forming gluten proteins; dough hydration and gas retention are largely dependent on pentosans. The partial hydrolysis of pentosans by cereal pentosanases during sourdough fermentation increases their solubility and water binding, and improves the volume and texture of the bread. A solubilization of water-insoluble pentosans also occurs in wheat sourdough, but is of secondary importance due to the presence of polymeric gluten proteins. Second, rye flour has a higher amylase activity and rye starch has a lower gelatinization temperature when compared to wheat. This results in a small temperature range during which active amylase and gelatinized starch coexist. During heating of the crumb in the baking process, the amylase activity of rye flour is sufficient to compromise or destroy the crumb structure unless amylase is inhibited by low pH. The low pH also enhances the activity of cereal proteases and phytases. Minerals in wheat and rye flours are mainly bound in insoluble complexes with phytate. Dough acidification allows for optimal activity of cereal phytase (pH 5.0–5.5) and solubilizes the insoluble phytates (pH < 5.0). Phytate hydrolysis during sourdough fermentation reduces the binding of minerals and increases their bioavailability. It is doubtful if this is significant for a consumer eating a reasonably varied diet. However there is an increasing concern that even the American
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diet is only marginally adequate in terms of trace minerals, and so a reduction in phytate may be significant. There is no evidence that phytases from lactic acid bacteria contribute to phytate degradation during sourdough fermentation. Proteases in wheat and rye flours are optimally active in the pH range of 3.5–4.5. Sourdough fermentation thus allows for high protease activity, and the amino acid concentration increases 5–10-fold during fermentation. Amino acids are important precursors for flavor formation by lactic and yeast metabolism, or in thermal reactions during baking. Excess proteolysis in wheat sourdoughs, however, compromises the gluten network and results in a reduced bread volume. Microbial metabolites with specific effect on bread quality particularly include acetic acid, ornithine, glutamate, and exopolysaccharides. Acetic acid is produced by heterofermentative lactic acid bacteria if fructose is available as electron acceptor. Acetic acid is volatile and thus influences bread odor as well as taste. Moreover, acetic acid has a stronger antimicrobial activity than lactic acid and contributes to the extended microbial shelf life of sourdough bread. However, acetic acid in concentrations that prevents fungal bread spoilage has such a strong impact on bread flavor that the bread is inacceptable to most consumers. Ornithine is the product of arginine conversion by L. reuteri, L. pontis, L. rossiae, and other sourdough lactobacilli. During baking, ornithine reacts to 2-acetyl-1-pyrroline, the character impact compound of wheat crust odor. Experimental strategies to specifically augment the ornithine content of sourdough also increased the pleasant, roasty crust odor. Glutamine is the most abundant amino acid in wheat and rye proteins; individual gliadins contain up to 50% glutamine. Sourdough lactobacilli convert glutamine to glutamate, which imparts umami taste. Glutamate addition to levels matching microbial glutamate accumulation improved the sensory properties of bread. All sourdough lactobacilli are capable of glutamine conversion, although the extent of conversion is strain specific. The conversion of amino acids by yeast metabolism results in formation of flavor volatiles; for example, methylbutanal and phenylethanol are formed from leucine or isoleucine and phenylalanine, respectively. Exopolysaccharide formation by lactobacilli in sourdough is based on glucansucrase or fructansucrase activity. These enzymes are extracellular or cell wall associated and convert sucrose to polymeric fructans (fructansucrases) or glucans (glucansucrases). Polymers produced by lactobacilli in sourdough include the fructans inulin and levan, and the glucans dextran, reuteran, or mutan. The frequency of exopolysaccharides producing sourdough strains is high, and any given sourdough likely contains at least one exopolysaccharideproducing strain. The amount of exopolysaccharides produced during sourdough fermentation is dependent on the strain employed, sucrose concentration, and process conditions. In experimental and industrial sourdough fermentations, exopolysaccharides accumulate to more than 20 g kg1 dough. This quantity is sufficient to improve the volume and texture of sourdough bread and to delay bread staling. For example, the long shelf life of Panettone was attributed to dextran formation by Leuconostoc spp. during sourdough fermentation. Sucrose conversion by glucansucrases and fructansucrases, however, also releases fructose and increases acetic acid formation by heterofermentative lactobacilli. Most Weissella strains are an
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exception among heterofermentative lactic acid bacteria as they do not employ fructose as electron acceptor to support acetate formation and dextran-producing Weissella spp. are highly suitable to improve bread volume and texture. Sourdough fermentation delays the spoilage of bread by rope-forming bacilli and molds. Moderate acidification of bread that is achieved in most sourdough breads is sufficient to prevent ropy spoilage. Experimental sourdough fermentations were also shown to delay the growth of fungal spores on bread. This antifungal effect is strain specific. Active antifungal metabolites remain to be identified; known antifungal metabolites of lactobacilli are not produced to inhibitory concentrations during sourdough fermentation. The antifungal effect of sourdough is likely attributable to a combination of several microbial metabolites and substrate-derived antimicrobial compounds.
Sourdough Starter Cultures Sourdough starter cultures have been available since 1910 and were among the first commercially available starter cultures. Initial culture preparations were based on cereal substrate – essentially refrigerated sourdough with mixed strain composition containing yeasts and lactic acid bacteria and a shelf life of a few weeks. Culture preparations on cereal substrate have retained their relevance to date, owing to their superior activity upon refreshment at the bakery when compared to dried cultures. Recent developments include the commercialization of rice- or sorghum-based gluten-free starter cultures for use in gluten-free baking. Freeze-dried, pure culture preparations for sourdough fermentations have also been available for several decades. However, freeze-dried cultures fail to develop sufficient metabolic activity in straight dough processes; their use requires one or more refreshment in the bakery. Moreover, freeze-dried cultures of lactic acid bacteria cannot replace traditional sourdoughs that contain sourdough yeast as well as lactic acid bacteria. Dried or pasteurized sourdough products do not contain relevant numbers of viable lactic acid bacteria and are used as baking improver rather than as starter culture.
Related Cereal Fermentations Numerous other cereal fermentations exist worldwide that are highly related to sourdough fermentation in terms of fermentation conditions and microbial ecology, but are used to produce beverages or porridges rather than bread. A detailed description of these fermentations is beyond the scope of this article, but a few examples are presented to indicate that very diverse products are obtained from the same basic fermentation. Steamed wheat bread (man tou) produced in China and throughout Southeast Asia differs from bread only insofar that the baking process is replaced by steaming. Dough fermentation for steamed bread relies on sponge dough fermentation or back-slopped sourdoughs, resulting in fermentation microbiota that are comparable to sourdough used in baking.
Kvass, widely consumed in Russia, and boza, consumed in Turkey and surrounding countries, are two examples of cerealbased beverages. Kvass is produced from malt or sourdough bread, whereas boza is produced from boiled wheat, maize, rice, and/or millet flours. Both beverages are sweetened with sucrose, are slightly alcoholic (0.5–1%), and undergo lactic fermentation. Fermentation microbiota consist of S. cerevisiae and lactic acid bacteria, including dextran-producing Leuconostoc spp. Cereal fermentations in Africa and South Asia employ corn, sorghum, millet, or teff as raw materials to produce porridges, gruels, or cakes. Many of the fermentations documented in the scientific literature are based on spontaneous fermentation, but the use of back-slopping has also been reported. Examples include mawè and ting, porridges produced in West Africa and Botswana, respectively, and idli, a soft cake produced in South India and Sri Lanka. The high ambient temperature in these countries selects for thermophilic fermentation microbiota. It is noteworthy that cereal fermentations in tropical climates frequently harbor amylolytic lactobacilli. This may relate to the low amylase activity of the substrates employed.
Conclusion Sourdough fermentation is the most ancient way of producing bread and has retained its relevance in contemporary bread production. The continued importance of sourdough in bread production relates to the unique quality of sourdough bread that cannot be reproduced with alternative fermentation methods or ingredients. Sourdough can replace several ingredients or processing aids and allow a substantial reduction of the production cost. Traditional procedures for sourdough fermentation retain their relevance in the artisanal production of (specialty) bread. Moreover, traditional processes were adapted and modified to meet the requirements of large-scale and automated bread production.
See also: Bacteriocins: Potential in Food Preservation; Bread: Bread from Wheat Flour; Candida; Ecology of Bacteria and Fungi in Foods: Influence of Redox Potential; Ecology of Bacteria and Fungi in Foods: Effects of pH; Fermentation (Industrial): Basic Considerations; Fermentation (Industrial): Control of Fermentation Conditions; Fermented Foods: Origins and Applications; Fermented Foods: Fermentations of East and Southeast Asia; Beverages from Sorghum and Millet; Lactobacillus: Introduction; Lactobacillus: Lactobacillus brevis; Metabolic Pathways: Release of Energy (Aerobic); Saccharomyces – Introduction; Saccharomyces:Saccharomyces cerevisiae; Starter Cultures; Starter Cultures: Importance of Selected Genera; Torulopsis; Yeasts: Production and Commercial Uses; Yersinia: Introduction.
Further Reading Brandt, M.J., 2007. Sourdough products for convenient use in baking. Food Microbiology 24, 161–164. De Vuyst, L., Vancanneyt, M., 2007. Biodiversity and identification of sourdough lactic acid bacteria. Food Microbiology 24, 120–127.
BREAD j Sourdough Bread Gänzle, M.G., Vermeulen, N., Vogel, R.F., 2007. Carbohydrate, peptide, and lipid metabolism of lactic acid bacteria. Food Microbiology 24, 128–138. Gänzle, M.G., Loponen, J., Gobbetti, M., 2008. Proteolysis in sourdough fermentations: mechanisms and potential for improved bread quality. Trends in Food Science and Technology 19, 513–521. Hammes, W.P., Gänzle, M.G., 1998. Sourdough breads and related products. In: Wood, B.J.B. (Ed.), The Microbiology of Fermented Foods, second ed. Blackie., London, pp. 199–216. Hammes, W.P., Brandt, M.J., Francis, K.L., Rosenheim, J., Seitter, M.F.H., Vogelman, S.A., 2005. Microbial ecology of cereal fermentations. Trends in Food Science and Technology 16, 4–11. Hansen, A., Schieberle, S., 2005. Generation of aroma compounds during sourdough fermentation: applied and fundamental aspects. Trends in Food Science and Technology 16, 85–103. Jenson, I., 1988. Bread and baker’s yeast. In: Wood, B.J.B. (Ed.), The Microbiology of Fermented Foods, second ed. Blackie., London, pp. 172–198.
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Moroni, A.V., Dal Bello, F., Arendt, E.K., 2009. Sourdough in gluten-free breadmaking: an ancient technology to solve a novel issue? Food Microbiology 26, 676–684. Nout, M.J.R., 2009. Rich nutrition from the poorest – cereal fermentations in Africa and Asia. Food Microbiology 26, 685–692. Schnürer, J., Magnusson, J., 2005. Antifungal lactic acid bacteria as biopreservatives. Trends in Food Science and Technology 16, 70–78. Service, R., 1957. Songs of a Sourdough (Reset Edition). Ernest Benn, McGraw-Hill Ryerson., Toronto. Spicher, V., Pomeranz, V., 1985. Bread and Other Baked Products. In: Ullmann’s Encyclopedia of Industrial Chemistry, fifth ed., vol. A4. VCH Verlagsgesellschaft, Weinheim. 331. Vogel, R.F., Knorr, R., Müller, M.R.A., Steudel, U., Gänzle, M.G., Ehrmann, M.A., 1999. Non-dairy lactic fermentations: the cereal world. Antonie Van Leeuwenhoek 76, 403–411.