Sourdough
The Bread That Ferments Itself
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The book walks through the essential steps from creating your first starter to understanding the different types of sourdough cultures. Chapters cover everything from basic mixing and shaping techniques to the science behind lactic acid bacteria and gluten development. You'll learn how fermentation affects bread characteristics, how to recognize proper vs. over-fermented dough, and what regional baking traditions exist around sourdough.
Whether you're starting your first loaf or perfecting your technique, this practical guide shows you exactly how to keep a healthy starter alive through daily care and seasonal changes in your baking routine.
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Sourdough stands as one of humanity’s oldest and most widespread methods for making bread rise. For most of history, it was the only way people baked bread until the Middle Ages, when it began to be replaced by barm. Barm itself gave way to industrial baker's yeast in the late 1800s and early 1900s.
Sourdough bread is one of humanity’s oldest fermented foods, with evidence of its use dating back to around 3700 BC, when archaeologists found a specimen in Switzerland. But the practice likely began even earlier, tied to the development of agriculture in places like the Fertile Crescent and Egypt. For most of human history, people made bread using sourdough as a leavening agent. Pliny the Elder wrote about different techniques for creating sourdough starters. It wasn’t until the European Middle Ages that sourdough gave way to barm from beer brewing, and only after 1871 did commercial yeast begin to replace it. The Encyclopedia of Food Microbiology notes that baker’s yeast as a leavening agent is less than 150 years old, dating from around 2014.
French bakers arrived in Northern California during the Gold Rush, bringing sourdough techniques that took root in San Francisco. The nickname “Sourdough Sam” lives on as the mascot of the San Francisco 49ers. Though the term is linked to 1849 prospectors, those men more often used commercial yeast or baking soda for bread. The distinctive sourdough style of San Francisco is a white bread with a strong tang. A specific type of bacteria found in these starters is named Fructilactobacillus sanfranciscensis, and the yeast found alongside it is called Kasachstania humilis.
The sourdough tradition traveled to Alaska and the Yukon Territory during the Klondike Gold Rush of 1898. Miners and settlers there relied on sourdough starters because conventional leavenings like yeast and baking soda often failed in the harsh conditions. Prospectors kept their starters close—around their necks or on belts—and protected them fiercely from freezing. But freezing didn’t kill the starters; too much heat did. Over time, experienced residents came to be known as “sourdoughs,” a term still used today for old-timers in Alaska and the Klondike. The cultural significance of this nickname was captured in the poetry of Robert Service, especially in his collection titled "Songs of a Sourdough."
In English-speaking countries, where wheat-based breads are common, sourdough is no longer the usual way to make bread rise. It was slowly replaced first by barm from beer making, and then, after Louis Pasteur confirmed germ theory, by cultured yeasts. Although sourdough fell out of use in commercial bakeries during the 20th century, it has seen a comeback among artisan bakers and, more recently, in industrial ones. In places without legal definitions of what sourdough bread should be, some products labeled as such may actually use baker’s yeast or chemical leaveners instead of a live starter culture. The Real Bread Campaign has called these breads “sourfaux,” criticizing not just their preparation but also the use of agents like baking soda and baking powder, which they say may be connected to celiac disease.
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A sourdough starter is a living mixture of flour and water that contains wild yeast and lactobacilli, creating a pre-ferment known by many names like chief, chef, head, mother, or sponge. This starter gives bread its rise and its distinctive flavor. The way it's made depends on the ratio of water to flour, which is called hydration. Some starters are thick doughs, others are more liquid batter. Each kind brings its own character to the final loaf.
To begin a sourdough culture, water and flour are mixed together and left in a warm place for about a week or two. When wheat flour touches water, the enzyme amylase breaks down the starch into sugars like glucose and maltose. These sugars feed the natural yeast already present in the flour. Given enough time, warmth, and regular feeding with fresh dough, the mixture begins to develop a stable culture. This culture causes bread dough to rise. The lactic acid bacteria in the culture ferment starches that the yeast can't handle, producing maltose and other byproducts. The yeast then consumes these, generating carbon dioxide gas, which makes the dough rise and gives sourdough its distinctive flavor.
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As the starter culture ferments, sometimes over the course of days, it grows in volume through regular additions of flour and water—what's called refreshment or feeding. As long as this culture receives flour and water on a consistent basis, it stays alive and active. The process keeps the sourdough alive and ready for baking.
When you feed your sourdough starter, the amount of old starter you mix with fresh flour and water matters a lot. This mix is called the refreshment ratio, and it helps keep your starter healthy and strong. In San Francisco sourdough, they use a 40% refreshment ratio, which means about 67% of the new dough's weight comes from the old starter. A higher ratio makes the dough more acidic, with a lower pH level. When the pH drops below 4.0, it stops lactobacilli from growing and favors yeasts that can handle acidity. This balance keeps the sourdough stable and ready to use.
A sourdough starter made from a salted wheat-rye dough needs about fifty-four hours to mature at twenty-seven degrees Celsius. During that time, it reaches a stable pH level between 4.4 and 4.6. The dough contains four percent salt, which slows down the activity of L. sanfranciscensis, but not C. milleri, which can handle up to eight percent salt.
A starter that's drier and cooler produces more acetic acid than lactic acid because it has less bacterial activity and more yeast growth. On the other hand, a wetter and warmer starter favors bacterial activity and reduces yeast growth, leading to more lactic acid. The yeasts in the starter mainly create carbon dioxide and ethanol. For rye and mixed-rye breads, having more lactic acid is preferred, while wheat fermentations benefit from higher levels of acetic acid. A dry, cool starter results in a sourer loaf compared to a wet, warm one. Some starters, like the Flemish Desem, are kept firm and can be buried in flour to prevent drying out, though they require more care than wetter ones.
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A stable culture where F. sanfranciscensis thrives needs a temperature range of 25 to 30 degrees Celsius, about 77 to 86 Fahrenheit. To keep it healthy, you have to feed it every twenty-four hours for roughly two weeks, which adds up to three hundred thirty-six hours. If you go longer than three days between refreshments, the dough starts to get too acidic, and that can shift the balance of microbes in the culture.
When you refresh your sourdough starter less often, you can speed up the fermentation process, which is called "acceleration." This change affects the balance between yeasts and lactobacilli in the culture. If you're not cutting down the time between feedings from once a day to just a few hours, then you’ll need to use less starter in your final dough to get the dough to rise properly during proofing.
Faster methods of making sourdough have been developed, needing fewer feedings, sometimes using commercial starters. These starters come in two kinds. One type uses traditional, stable doughs that are often dried, where the mix of microbes is unclear. The other type starts with microbes taken from lab dishes, grown in large tanks, and turned into baker’s products with exact microbial counts, designed for specific bread types.
Keeping sourdough alive and active needs regular feeding, especially when it's being used as the only leavening agent in bread-making. In bakeries where sourdough is the sole source of rise, refreshments happen several times a day. That kind of routine isn’t typical for home bakers, who might use their starter once a week or even less often. The difference lies in how frequently the sourdough is fed and handled, which affects its strength and ability to leaven dough. For amateur bakers, maintaining high leavening activity means more work, but it’s necessary if you want the best results from your sourdough.
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Bakers have found different ways to build a strong, stable group of microorganisms in their sourdough starter. Using flour that hasn’t been bleached or treated with bromine helps, since it holds more natural life. Wholemeal flour, which includes the bran, brings the most variety of organisms and extra minerals. Some bakers start with a mix of white flour and rye or whole wheat, or use organic grapes—unwashed and whole—to introduce wild yeasts. The skins of grapes and grape must are known to carry lactic acid bacteria, just like many other plants. In Greece, people soak basil leaves in room-temperature water for an hour to help start a traditional sourdough. Another method uses water from boiled potatoes, which is said to give the bacteria more starch to feed on and boost their activity.
Most city water is treated with chlorine or chloramines to stop harmful bacteria, but these chemicals don’t hurt people or animals. Some bakers say using water without those treatments works better for feeding sourdough cultures. Bottled water is fine for this purpose. You can remove chlorine from tap water by boiling it or letting it sit uncovered for at least a day. Chloramines require boiling or activated carbon filters to be removed.
The taste of sourdough bread changes depending on where it’s made, shaped by the techniques used, the moisture content of the starter and dough, how often it's fed, the length of time it ferments, and the conditions around it like temperature, humidity, and altitude. All these factors influence the unique mix of microorganisms that give each loaf its character.
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The starter needs to be fed anywhere from four to twelve hours before it's used in the dough. Feeding means mixing flour and water into the starter, which creates an active leaven. The leaven will grow in size, and it’s ready to use when it’s bubbly and floats in water.
When making sourdough bread, you mix a portion of your leaven with flour and water to form the final dough. The amount of starter used usually weighs between 13 and 25 percent of the total flour in the recipe. Some bakers choose to use a smaller amount—between 5 and 10 percent—of cold, unfed starter, often called sourdough discard. This method takes longer to ferment but can produce a more complex flavor in the finished loaf.
The dough gets shaped into loaves and left for a final rise, called proofing, before going into the oven. Because sourdough takes a long time to rise, many bakers choose to slow things down by chilling the dough in the refrigerator before baking. This method is called retardation, and it helps create a more complex flavor in the bread.
Working with sourdough takes more time than regular dough because the yeast in it isn’t as strong. But in the presence of lactic acid bacteria, some of these yeasts have been seen to produce twice as much gas as baker’s yeast. The acidic environment of sourdough, along with enzymes from the bacteria breaking down proteins, leads to weaker gluten formation and can result in a denser final loaf.
Because sourdough starters take longer to rise than those using baker’s yeast, traditional starters usually won’t work in a bread machine. Some manufacturers do offer recipes designed for their machines, where you can make the starter directly in the pan using a special setting, and then add things like apple cider vinegar later. You can also use sourdough that's already been proofed for hours—whether from a starter or mother dough—and transfer it into a bread machine to bake, skipping the kneading part. That works well if you want just one loaf. But the intricate, blistered crust and slashes typical of oven-baked sourdough can’t be duplicated in a machine, since that usually needs a baking stone and steam from misting the dough.
In sourdough baking, when the dough has risen and is ready to go into the oven, bakers make one or more deep cuts into the surface. These cuts are called primary slashes, and they let the bread expand evenly as it puffs up in the hot oven. Some bakers add extra shallow slashes for looks — these decorative cuts don’t affect how the bread rises but make it more pleasing to see. The shape and placement of these slashes can be simple or intricate, depending on the baker’s style and preference. It's all part of shaping the loaf before it goes into the oven.
Once baked, sourdough bread stays fresh longer than most other kinds, and it resists going bad or growing mold without needing any artificial preservatives that other breads require.
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Bakers don’t start fresh each time they bake; instead, they keep going with the same starter, an unbroken line that traces back to the original first batch. This living culture is known by many names—mother dough, mother sponge, chef, or seed sour. Once the bread dough is mixed, there are two main ways to carry on the sourdough cycle.
When someone starts a sourdough culture, they take a small piece of it to use later, because the acidic environment and natural antibacterial agents keep it safe from harmful bacteria and unwanted yeasts. This means the same starter can be used for years. To keep it alive, they mix flour, water, and sometimes other ingredients like sugar, milk, apples, or potatoes into the reserved portion. Each time they bake, they use some of this fed starter, but always save a bit more to continue the process.
The French-style levain technique offers another method for maintaining sourdough. Instead of saving a portion of the starter, bakers keep a piece of raw, uncooked dough. Once that dough has risen, it’s mixed into the ingredients for a new loaf. Before the new dough is shaped and baked, a small amount is removed and set aside. This portion will later be used to start the next batch of bread, keeping the fermentation process going.
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Sourdough is a living mixture of lactic acid bacteria and yeast suspended in flour and water, where the yeast produces carbon dioxide gas to help the dough rise, and the lactic acid bacteria generate lactic acid, giving the bread its distinctive sour taste. While the yeast consumes sugars, the lactic acid bacteria break down other compounds, and in turn, the yeast metabolizes by-products from the bacterial fermentation. Throughout this process, the acidic environment activates enzymes like phytases, proteases, and pentosanases, which are crucial for the biochemical transformations that occur during sourdough fermentation.
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Every sourdough starter is home to a unique mix of lactic acid bacteria, naturally introduced from the air, water, and flour used to build it. These gram-positive microbes feed on carbohydrates, turning them into organic acids like lactic, acetic, formic, and propionic acid. The buildup of these acids makes the environment tough for harmful bacteria and mold, helping the starter stay healthy and active.
In sourdough, lactic acid bacteria are the key players, and among them, Leuconostoc, Pediococcus, and Weissella are commonly present. But the most abundant species come from a large and varied genus called Lactobacillus. These bacteria are what give sourdough its distinctive tang and help it rise. They work slowly, breaking down starches into lactic acid, which creates the sour flavor and helps develop the bread's structure. Without these microorganisms, there would be no sourdough at all.
Lactic acid bacteria are a group that includes aerotolerant anaerobes, which are microbes that can survive and even multiply in the presence of oxygen, and microaerophiles, organisms that thrive when oxygen levels are lower than what's found in the atmosphere. These bacteria play a key role in fermentation processes, like those that make sourdough rise and develop its distinctive tangy flavor. They're not picky about oxygen — some can handle it, others prefer to work without it, but all contribute to the complex chemistry of fermented foods.
In sourdough, the main lactic acid bacteria are heterofermentative, meaning they create more than one product during fermentation. These bacteria convert sugars called hexoses through a specific process known as the phosphoketolase pathway. This breakdown results in lactate, carbon dioxide, and either acetate or ethanol. Heterofermentative lactic acid bacteria often work alongside another group of bacteria called homofermentative lactobacilli, especially species from the genera Lactobacillus and Companilactobacillus.
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Type I sourdoughs are traditional bread starters relying entirely on natural fermentation, like those used in San Francisco Sourdough Bread, Panettone, and rye bread. These doughs tend to be firm, with a pH between 3.8 and 4.5, fermenting best at temperatures ranging from 20 to 30 degrees Celsius. A bacterium called Fructilactobacillus sanfranciscensis was named after its discovery in San Francisco starters but isn't unique to the city. It often works alongside other lactic-acid bacteria such as Limosilactobacillus fermentum and Levilactobacillus brevis. The yeast Saccharomyces exiguus, along with Kasachstania humilis and Candida holmii, typically lives symbiotically in these cultures. S. exiguus is related to C. milleri and C. holmii, which were once considered the same species but are now known to be different based on DNA testing. C. milleri can survive in low pH environments with high acetate levels, helping maintain stability in sourdough flora. Notably, F. sanfranciscensis needs maltose to grow, while C. milleri does not.
For F. sanfrancisensis to make acetic acid, it needs maltose and fructose. In wheat dough, starch and some polyfructosanes are broken down into those sugars, along with a bit of glucose. These polyfructosanes are long chains of fructose molecules linked to a single glucose, and they make up about 1–2% of the dough. Yeasts can pull fructose from these compounds. When L. sanfrancisensis uses up all the available fructose, it stops making acetic acid and starts producing ethanol instead. If the dough gets too warm, yeast activity slows and less fructose is produced. Fructose depletion becomes a bigger issue in doughs with lower enzyme activity.
In a Belgian lab study, wheat and spelt doughs were refreshed daily and fermented at 30°C, showing how sourdough ecosystems develop over time. In the first two days, bacteria like Enterococcus and Lactococcus dominated. By days 2 through 5, strains from genera Lactobacillus, Pediococcus, and Weissella took over. Yeast growth was slower, peaking around days 4 to 5. By days 5 to 7, well-adapted lactobacilli such as L. fermentum and Lactiplantibacillus plantarum appeared. At their peak, yeast populations were about 1 to 10 percent of lactobacilli levels. A stable sourdough is marked by heterofermentative lactobacilli outcompeting the homofermentative kind. F. sanfranciscensis was rarely found in spontaneous sourdoughs, even after repeated refreshments, though it was quickly introduced when plant materials were used to start wheat fermentations.
When scientists studied wheat sourdough, they found that S. cerevisiae disappeared after just two refreshment cycles. This yeast doesn’t handle acetic acid well compared to other sourdough yeasts. Because of this, a stable sourdough culture can't accidentally become contaminated by S. cerevisiae if it's kept going regularly.
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Type II sourdoughs include baker’s yeast, or Saccharomyces cerevisiae, to help the dough rise. The main lactic acid bacteria in these sourdoughs are L. pontis and Limosilactobacillus panis, along with other Lactobacillus species. These sourdoughs have a pH below 3.5 and are fermented between 30 and 50 degrees Celsius for several days without being fed. This process slows down the microorganisms' activity. Some industrial bakers adopted this method because it simplifies the complex build process used in Type I sourdoughs.
In Type II sourdoughs, yeast growth is slowed or stopped because of higher fermentation temperatures. These doughs are more liquid than others and, once they've fermented, can be chilled and stored for up to a week. They remain pumpable, which makes them useful in continuous bread production systems.
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Type III sourdoughs start as Type II sourdoughs, which then go through a drying process—usually spray or drum drying. These dried sourdoughs are mainly used in industrial settings as flavoring agents. They're dominated by lactic acid bacteria that can survive the drying, especially Pediococcus pentosaceus, L. plantarum, and L. brevis. The conditions of the drying process, like time and heat, can be adjusted to influence caramelization and affect the final baked product.
Manufacturers of non-sourdough breads compensate for the absence of natural yeast and bacterial culture by adding a man-made mixture called bread improver or flour improver into their dough.
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Many breads use sourdough techniques including Danish rugbrød, dense rye bread from smørrebrød sandwiches. Mexican birote salado from Guadalajara started as a short baguette using sourdough instead of yeast, creating a crunchy exterior and soft interior. Amish friendship bread uses a starter with sugar and milk, plus baking powder and soda. An Amish sourdough is fed every few days with sugar and potato flakes. German pumpernickel is made with traditional sourdough, though modern versions often use commercial yeast and acids to affect the rye flour. Flemish desem bread is a whole-wheat sourdough. Whole-wheat flatbreads are eaten in Azerbaijan. In Ethiopia, injera is made from fermented teff flour, and a similar bread is found in Somalia, Djibouti, and Yemen as lahoh. In India, idlis and dosa come from sourdough fermentation of rice and black gram.
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Sourdough bread tends to have a lower glycemic index than other breads, which means it doesn’t raise blood sugar as quickly. That happens because of the fermentation process. During sourdough fermentation, enzymes from the cereal grains break down phytates. This makes it easier for the body to absorb certain vitamins and minerals, especially those found in the bran of the grain.
Sourdough fermentation can help reduce certain wheat components that might cause issues for people with non-celiac wheat sensitivity or irritable bowel syndrome. The process also involves lactic acid bacteria, which may improve the quality of gluten-free breads, enhancing their texture, aroma, and shelf life.
A review from 2023 looked at the health effects of sourdough and found that while many publications, social media posts, and bakers talk about its benefits, there isn’t solid proof yet that it has real health impacts on clinical outcomes.
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Sourdough baking has a loyal following today, with people sharing their starters and advice online. Hobbyists often post their creations on social media. Each sourdough culture is home to a community of living organisms, and bakers feel responsible for keeping them alive. The local air carries different yeasts, which means each starter develops its own unique character based on where it's kept.
Ed Wood discovered ancient yeast in an Egyptian bakery near the pyramids of Giza, linking today’s sourdough to millennia past. Among those who preserve living starters are bakers like Carl Griffith, whose 1847 culture has been handed down through generations. “I like the throwback of traditional bread, the things our great grandmothers ate,” writes professional baker Stacie Kearney. Some bakers speak of starters that span decades, but Griffith’s stands out as especially remarkable.
Sourdough baking took off during the pandemic, as more people started baking at home. The surge in demand led to shortages of baker's yeast in stores. But sourdough doesn't need store-bought yeast—it can be kept alive and grown right in your kitchen. That made it a go-to for bakers looking to continue their craft while staying safe indoors.
Sourdough baking needs just flour, salt, water, and a little time—but it asks for practice. For some, the purity of the process is part of its charm. As one baker put it, “If you take flour, water, (wild) yeast and salt, and play around with time and temperature, what comes out of the oven is something utterly transformed.” Some bakers follow strict schedules, others give their starters names. A few treat it like science, fine-tuning flavor and acidity with careful notes and even consulting microbiologists. Starters can be shared or bought, but many prefer to grow their own. There are debates over methods, like using commercial yeast to kickstart a culture while trapping wild yeasts, or adding grapes or milk.
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In northern Europe, where rye bread is a staple, people rely on sourdough instead of yeast because rye doesn’t have enough gluten to work with baker’s yeast. The structure of rye bread depends on starch and pentosans, carbohydrates in the flour. But rye amylase stays active at higher temperatures than wheat amylase, which can break down the starches during baking and weaken the bread. Sourdough helps here by lowering the pH, which stops the amylases from breaking down the starches when heat alone can’t do it. That lets the carbohydrates gel and set properly in the final loaf.
In southern Europe, sourdough is still used traditionally for certain breads like panettone. But in the 20th century, it became less common, replaced by baker's yeast, which works faster. Sometimes, that yeast was combined with longer fermentation times to let bacteria add flavor. Then, during the 2010s, sourdough fermentation came back into style as a main method in bread making, often mixed with baker’s yeast as a leavening agent.
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