Sourdough is not an ingredient you weigh out like salt. It is a small ecosystem of flour, water, and two groups of living organisms that share their food, limit one another, and respond to warmth, feed, and time. The official German guidelines for bread (the Leitsätze), which describe the bread types in Germany, define it exactly this way: a dough with lactic acid bacteria and yeasts that keeps forming acid. What matters is not where it comes from or how old it is, but that its work continues as long as you maintain it. Consistency comes from consistent care, not from age.
This introduction is the map of the book. You get to know the inhabitants and learn what they do in the dough. You understand what the acid does and why rye does not work without it. You get an overview of the chain from starter to first slice and learn the levers you turn. The details are in the specialist chapters of the book; here you get only as much as you need to place them.
What you see, smell, and measure
In the jar. Right after feeding, nothing happens that you could see. Later bubbles appear, the surface rises, and the volume doubles or triples. The smell runs from milky and yogurt-like through sour to vinegary or solvent-like. Then the dome sinks, and the mass turns liquid. A starter counts as ripe when it has risen clearly, shows bubbles on top, at the edge, and inside, and smells slightly sour. Rye starter is stickier, tears when you stir it, and holds its performance longer than wheat or spelt starter.
In the dough. During bulk fermentation the final dough becomes airier and more extensible, domes, and shows bubbles along the wall of the container. When it is over-acidified, it turns slack and sticky.
In the bread. You taste acid and aroma. Sourdough bread stays fresh longer and molds later than yeast bread with the same recipe. When rye bread lacks acid, it shows in a gummy, wet crumb.
What you can measure. The pH value describes the strength of the acid on a logarithmic scale: one step lower means ten times as acidic. It falls from about 6 in freshly mixed flour-and-water dough to 3.8 to 4.3 in ripe sourdough. Titratable acidity comes from the bakery: you titrate a sample with lye, and the milliliters used per 10 grams of sourdough show the amount of acid, not its strength. At home you work with pH strips, with the volume increase in a straight-sided container, with a probe thermometer, and with the time to peak at a known temperature.
Tip: Your nose is the measuring instrument
Yogurty means young, sour means ripe, solvent means overripe or hungry. When you feed, put a rubber band at the height of the mass, and you learn smell and height together.
Who works in the dough
In a ripe sourdough there are mostly ten to a hundred bacteria for every yeast cell, as an order of magnitude and not as a fixed rule; in individual sourdoughs the ratio was a thousand to one and above, and it varies with the culture, the measuring method, and the stage of ripeness. In absolute numbers, measurements on sourdoughs of different origins found lactic acid bacteria at roughly half a billion to five billion per gram and yeasts at a few tens of thousands to well over a hundred million. More than ninety species of lactic acid bacteria and over forty species of yeast have been described worldwide, but in a stably maintained sourdough a few determine the picture.
Among the bacteria, Fructilactobacillus sanfranciscensis (formerly Lactobacillus sanfranciscensis) leads, joined by Levilactobacillus brevis (formerly Lactobacillus brevis). The old genus Lactobacillus was split into 25 genera in 2020, which is why many species carry new names while books and recipes often still use the old ones. If you know both, you can read both worlds. From here on the text refers to it as the sourdough bacterium. Among the yeasts, Saccharomyces cerevisiae comes first, the species of baker's yeast: in 77 % of 500 starters examined it made up more than half of the yeast sequences. Alongside it there is Kazachstania humilis (formerly Candida milleri), the so-called sourdough yeast. The typical starter (median) contains only a single yeast species. In a little under 30 % of starters, acetic acid bacteria also live, which slow the rise and bring a vinegar note.
Where they come from. In one study, 18 bakers on two continents started starters from the same recipe with the same ingredients. Each starter stayed most similar to its flour, and the bakers' hands gave species to the starter: about a quarter of the microbial variants found were also on the hands but not in the flour. Although everything was the same, the starters turned out different, and these differences went along with acidity: starters with a similar pH had a similar composition, and more lactic acid bacteria went along with a more sour impression. A second investigation of 500 starters, mostly from the USA (429), plus Canada, Europe, and Australia and New Zealand, found no link to where people lived; the 33 factors recorded together explained less than a tenth of the differences, and an effect of age, for example, is documented. The numbers come from the two original studies (see the list of sources) and apply to these samples. The conclusion holds: the culture comes from flour and hands, not from the region, and anyone can get a "San Francisco sourdough" anywhere.
How stable they are. A sourdough often stays stable for years, but not automatically: the studies on this come to contradictory results. Anyone who changes the flour changes the culture more than someone who moves house.
Tip: Old and new names
"Lactobacillus sanfranciscensis" and "Fructilactobacillus sanfranciscensis" are the same bacterium. When a book, a video, or a recipe names one of the two, you know it is about the same organism.
Tip: A pet with two residents
The yeast likes 26 to 28 °C and fresh food; the bacteria are fine with warmer and more acidic conditions. So maintain your starter at 24 to 26 °C and feed it before the mass collapses.
What they do
Two crafts. The yeasts ferment sugar to carbon dioxide and alcohol and deliver the larger part of the rise. The bacteria are mostly heterofermentative: unlike homofermentative species, which form only lactic acid, they turn sugar into lactic acid plus carbon dioxide and either alcohol or acetic acid. Their main product is the acid; the gas is a by-product.
One menu for two. The sourdough bacterium has an enzyme that splits maltose, the most common sugar in the dough, and prefers to use it. According to textbook knowledge, it releases glucose in the process, which the sourdough yeast lives on, because it cannot use maltose. The enzyme is documented; the exchange is the common explanation. So the two hardly compete. The yeast supplies amino acids and vitamins, the bacteria lower the pH and keep foreign microbes away; acid, alcohol, and special inhibitors from the bacteria also protect against mold.
Lactic acid versus acetic acid. Lactic acid tastes mild and yogurty, acetic acid sharp. The ratio of the two is called the fermentation quotient, meaning the ratio of their molar amounts. According to one encyclopedia, 2 to 4 counts as good, for rye bread rather 2 to 3; a textbook gives 2.0 to 2.7. The sources contradict one another, and a reliable measurement is missing; so take the numbers as an order of magnitude: lactic acid clearly dominates. The direction is documented: kept warm and soft, the dough forms more lactic acid and so a milder taste; kept cool and firm, more acetic acid and a stronger taste. Fructose, which is released from sucrose and fructans in whole grain, rye, and sugar, and oxygen steer the bacteria toward acetic acid.
What the acid does. There are four effects. The first is taste: more than 300 aroma compounds have been found, and starters that differ in acid and aroma bake breads that taste different. The second is shelf life. With 20 % sourdough, packaged white bread kept twice as long in one study; some strains form substances against mold, and staling sets in later. The third is minerals. Flour contains about 1 % phytate, which holds minerals tight. The enzyme phytase from the grain releases them, and it works at pH 3.5 to 5. The acidification wakes this phytase, not the bacteria themselves. In the lab and in animal studies, minerals become more available this way; in humans the result varies. The fourth effect belongs to rye: below pH 4.5, the starch-degrading amylases work at a reduced rate. In rye, their optimum is reached at about 50 °C, shortly before the starch swells and gelatinizes in the oven. Without acid, the amylase breaks down exactly this starch, and the crumb turns sticky. Newer rye varieties contain less amylase, but souring remains standard.
The gluten reacts too. Acid and substances from the bacteria make the gluten protein more soluble and more extensible; less than 5 % of the proteins are broken down in the process. The dough turns slack only with oversouring.
The types. Type I is the traditional sourdough: maintained at room temperature, refreshed at regular intervals, usually with the sourdough bacterium and the sourdough yeast. Your kitchen works with Type I, and the Italian lievito madre, a stiff wheat sourdough, belongs to it as well. How warm Type I is maintained is disputed: German teaching speaks of 20 to 30 °C, while Italian practice goes clearly above that, at 30 to 37 °C. According to textbook knowledge, Type II and Type III are industrial. Type II is liquid, is maintained above 30 °C for days, becomes very sour, and carries acid-tolerant species; the rise usually comes from baker's yeast. Type III is dried and serves as an aroma carrier. The German methods, that is the Detmold single-stage and three-stage methods, the Monheim salt sourdough, and the Berlin short sourdough, are Type I recipes in which temperature, time, and inoculation rate are fixed. They are covered in the chapter The Levain.
Wheat, mixed-flour bread, rye: two rule sets
In wheat bread, the gluten protein carries the structure. During kneading and folding it forms a network that holds gas, which is why tension, folds, and shaping come into play. The acid delivers flavor and shelf life, but not structure. It is usual to let 5 to 25 % of the total flour, in bakeries up to 30, ripen in the sourdough.
In rye bread there is hardly any gluten. Instead, according to one study, rye contains 3.6 to 5 % pentosans in light flour, more in whole grain; wheat 1.4 to 2.8. These mucilages bind water and form the crumb together with the starch. Here the acid is structure: it slows the amylase and firms up the swelling of the mucilages. This is why with rye you sour a large part of the flour. The figures refer to the rye flour and depend on the bread type and on the method: rye bread 35 to 45 %, single-stage method 25 to 35, rye-wheat mixed bread 40 to 50; calculated on the total flour, 22 to 35 throughout. How much it really has to be depends on the method and on the enzyme strength of the flour. Kneading in the wheat sense, folding, preshaping, and building tension are dropped. What is usual is a short rest, working the loaf (wirken), that is shaping without building tension and with plenty of flour, and a final proof until cracks appear. This sequence is common practice. Rye starter is considered more robust and more tolerant than wheat starter and is also suitable for wheat breads.
In mixed-flour bread, the share of rye decides which rule set prevails. According to the German Leitsätze, a German classification and not a general definition, the names are: Mischbrot means 50 to 50, wheat-rye mixed bread (Weizenmischbrot) and rye-wheat mixed bread (Roggenmischbrot) carry the name of the grain above 50 % plus a second grain above 10 %, and rye bread and wheat bread begin at 90 %. Those are names, not baking technique.
Tip: With rye, don't measure the sourdough too tightly
Guide values for 1000 g of flour in the whole dough. Sours with as much water as flour, used ripe. One starter is enough for both sours.
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| Bread (rye/wheat) | Sour | Rye flour in it | Wheat flour in it |
|---|---|---|---|
| Rye bread 100/0 | rye sour, 800 g | 400 g (40 % of the rye) | – |
| Rye-wheat mixed bread 70/30 | rye sour, 560 g | 280 g (40 %) | – |
| Mixed-flour bread 50/50 | rye sour 500 g, plus wheat sour 200 g | 250 g (50 %) | 100 g (20 % of the wheat) |
| Wheat-rye mixed bread 30/70 | rye sour 420 g, plus wheat levain 280 g | 210 g (70 %) | 140 g (20 %) |
| Wheat-rye mixed bread 20/80 | wheat levain only, 200 g | – | 100 g (12 %) |
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Below 20 percent rye the wheat levain is enough; above that, you usually sour the rye separately, as the table shows. With a flour that is strong in enzymes, rather use more. The percentages in the table refer to the respective flour: for rye, to the rye flour; for wheat, to the wheat flour of the dough.
Tip: From a third rye on, handle mixed-flour bread like rye
You can then leave out the folding (derivation, not a measurement): the structure increasingly comes from acid and water binding, not from gluten.
The chain
In every sourdough bread the same steps follow one another: refresh the starter, build the sourdough, mix the final dough, bulk fermentation, shaping, final proof, baking, cooling. Three baker-authors describe this for wheat bread almost identically. What differs is the levain share (from 13 to over 40 %) and with it the length of bulk fermentation, plus the question of whether the final proof runs warm or cold.
| Step | Guideline | |
|---|---|---|
| Refresh the starter | ratio 1:1:1 to 1:5:5, at 22 to 27 °C for four to twelve hours | The Starter |
| Build the sourdough | inoculation 13 to 50 %, five to twelve hours | The Levain |
| Autolyse | optional, 20 to 60 minutes | Autolyse and Fermentolyse |
| Mixing, kneading | by hand 5 to 10 minutes, machine 4 | Mixing and Kneading |
| Bulk fermentation | two to four hours at 23 to 25 °C, with 2 to 3 rounds of folds | Bulk Fermentation, Dough Development: Stretch and Fold |
| Preshape, rest | 20 to 25 minutes | Shaping and Bench Rest |
| Final proof | either warm (2 to 2.5 h) or in the fridge (mostly 4 to 7 °C, measure; at 5 to 7 °C eight to fourteen hours, longer if colder; planning range eight to twenty-four) | Final Proof and Cold Retard |
| Baking | 230 °C; with steam 20 minutes, without 25 to 30; internal temperature 96 to 97 °C | Baking |
| Cooling | one to two hours, for rye up to 24 | Cooling and the First Slice |
The short bulk fermentation times go with high levain shares; with less levain, it takes clearly longer.
With rye, the same chain runs in a second, thinner track. The sourdough stage is long: the Detmold single-stage method works with 10 % starter and a very wet dough (German bakers write TA 180 to 200 for the hydration (see Core Variables); classically 180, today mostly softer, up to 200), 15 hours at 26 °C. The final dough, by contrast, is short. Kneading and folding are dropped, the dough rest is brief, working the loaf replaces preshaping and shaping, the final proof lasts until the first cracks appear, and the finished bread needs up to a day for cooling. You find the complete sequence with times in the chapter Rye and Mixed-Flour Breads.
When bulk fermentation ends, the baker-authors answer contradictorily: 20 to 50 % volume increase at 23 to 24 °C if 16 hours of cold retard follow; 75 to 100 % at 21 °C. That looks like a dispute and is still the same principle. A warmer dough keeps fermenting strongly during shaping and in the fridge and has to leave bulk fermentation earlier, a cooler one later. Inoculation rate, length of the cold retard, and the way of measuring the volume add to this. As a rule of thumb: about 30 % at 27 °C, 75 to 100 % at 21 °C.
Tip: Cold does not replace warmth
Twelve hours at room temperature are not the same as twenty-four hours in the fridge. At 3 °C, a whole day amounts, by the reference value of a single baker-author, to half an hour of warm fermentation at most (the exact size of the effect is not established), and the dough needs hours to get cold inside and keeps fermenting during that time.
The levers at a glance
Seven levers determine the process. Each has its own section in the book; here is the short version.
Temperature. It is the strongest lever, because the two residents react differently to warmth: yeasts are fastest at 26 to 28 °C, bacteria at 32 to 34 °C. Cool gives more rise than acid, warm the reverse. For a long bulk fermentation, the target for wheat is 24 to 26 °C in the dough. How you hit it through the water is in the chapter Temperature.
This is where the reading sample ends. The remaining levers and all other chapters are in the Pro edition.
The full chapters, with values, tables, and illustrations, are available in the DoughCalculator app after the in-app purchase of the Pro edition.