Why sourdough rises

A mature starter is a stable little ecosystem of two kinds of microbe.

Wild yeasts — often Saccharomyces cerevisiae, the same species as baker’s yeast, or Kazachstania humilis — eat sugars and turn them into carbon dioxide and ethanol. The carbon dioxide is the gas that raises the bread. The ethanol mostly bakes off, though you can smell it in a hungry starter.

Lactic acid bacteria — Fructilactobacillus sanfranciscensis (the one named after the San Francisco bakeries where it was first described), Lactiplantibacillus plantarum and their relatives — eat the same sugars and make lactic and acetic acid. They outnumber the yeasts by something like a hundred to one. They are where the sour in sourdough comes from, and the acid they make is what keeps the jar safe: within a few hours of a feed the pH falls low enough that most spoilage organisms simply cannot compete.

The two get along better than you might expect. Flour contains enzymes, amylases, that slowly break starch into maltose, a sugar made of two glucose units. F. sanfranciscensis loves maltose; K. humilis cannot use it at all and lives on the glucose the bacteria leave behind. They are not competing for the same meal, so neither crowds the other out. That partnership is why a healthy starter can be kept for decades.

Feeding is just dilution. You take a little of the old culture — a crowded, acidic, exhausted population — and give it a lot of fresh food. The microbes wake up, multiply, and the jar rises as gas is trapped in the paste. When the food runs low the rise stalls and the top begins to fall: that is the peak, the moment the culture is most active and the best time to use it.

This view opens on an overnight feed: 10 g of starter with 50 g of flour and 50 g of water, a 1:5:5 feed, at 21 °C. The tool estimates it will peak in about ten and a half hours.

Try this:

What to notice: the relationship is logarithmic. Going from 1:1:1 to 1:2:2 adds less time than you might guess, because what matters is how many doublings the culture needs, and that is the logarithm of how much you diluted it. Bakers use big ratios on purpose: a 1:5:5 feed in the evening is ripe at breakfast, and a more diluted, slower culture tastes milder.

Microbes are chemistry in a bag, and chemistry speeds up with heat. For yeast and lactic acid bacteria in the range a kitchen actually covers — roughly 4 °C to 30 °C — a useful rule of thumb is that activity doubles for every 10 °C warmer. Scientists call this a Q₁₀ of about two. It is not exact, and it breaks down above about 32 °C (the bacteria start to outrun the yeast) and near the fridge (the yeast nearly stop), but over the range that matters it is surprisingly good.

The chart plots hours to peak against the jar’s temperature for four common feeding ratios. The dashed line marks the temperature you typed into Starter sits at.

Try this:

What to notice: the shape is the important part, not the numbers. Your starter, your flour and your kitchen will shift the curves up or down, but the curve stays a curve. A warm summer kitchen and a cold winter one can be ten degrees apart, and that is the difference between a bake that runs on time and one that runs half a day late. Experienced bakers watch the thermometer far more than the clock.

Before the dough can rise it has to be mixed, and bakers write their recipes in a slightly odd way: every ingredient is a percentage of the flour. A dough with 1,000 g of flour and 750 g of water is "75 % hydration". It makes recipes easy to scale, and it makes the one number that most shapes a loaf — how wet it is — visible at a glance.

Water matters to the rise in three ways. It is the medium the microbes and enzymes work in, so a wetter dough ferments a little faster. It lets the gluten proteins move and link up. And it makes the dough more extensible, so gas cells can grow bigger before they tear: wetter doughs tend to give a more open crumb, at the cost of being harder to handle.

There is a catch with sourdough, though. The levain — the portion of starter built up for this particular bake — is itself flour and water. If you only count the water you poured in, you are underestimating how wet the dough really is.

This is a country loaf in the Tartine style: 90 % bread flour, 10 % whole wheat, 75 % water, 2 % salt and 20 % levain at 100 % hydration, scaled to two 900 g loaves.

Try this:

What to notice: "prefermented flour" is a quiet but important number. It is how much of the dough has already been through fermentation — and so how much acid, flavour and active culture you are starting with. More levain means a faster bulk and a tangier loaf; less means a longer, gentler one.

Yeast makes gas, but gas alone does not make bread; it would bubble straight out. What holds it is gluten. Wheat flour carries two families of storage proteins, glutenins and gliadins. Dry, they do nothing. Wet them and work them, and the glutenins link end to end into long elastic chains while the gliadins slip between them like a lubricant. The result is a network that is both springy (glutenin) and stretchy (gliadin) — a sheet that can inflate.

One surprising detail: the yeast do not make the bubbles. Mixing folds in tiny pockets of air, and every gas cell in the finished crumb grew from one of those. The carbon dioxide dissolves in the dough’s water and diffuses into the pockets that already exist. That is why mixing and folding matter so much: they decide how many cells there are and how strong their walls are. Stretch-and-folds and coil folds during the first half of bulk build strength without knocking out gas, and organise the network so the cells grow evenly.

Acid plays both sides. A little acid tightens the gluten and makes dough easier to handle. A lot of acid, and the flour’s own protease enzymes given many hours, start to break the network down again — which is why an over-fermented sourdough turns slack, sticky and impossible to shape. (Rye is a different story: it has very little useful gluten, and the acid in a rye sour is what lets the crumb set at all, by slowing the enzymes that would otherwise turn the starch to paste.)

Bulk fermentation — the long first rise, after mixing and before shaping — is where most sourdough goes wrong. Stop too early and the loaf is tight and dense with a few big tunnels under the crust. Go too far and the gluten weakens, the dough spreads, and it bakes flat and sour. The clock is a poor guide because, as we have seen, it depends so heavily on temperature and levain.

The trick many bakers use is the aliquot jar: right after mixing, drop a small piece of the dough into a straight-sided jar, push it flat and mark the level. The jar sits beside the main dough, ferments at the same rate, and its straight sides turn a vague "looks puffy" into a percentage you can read.

The tab opens on a guide to starter types — liquid levain, stiff levain, rye sour, lievito madre — which is worth a read in itself: note how a stiff, cooler levain favours acetic acid and a sharper loaf, while a liquid, warmer one favours milder lactic acid. Scroll down to Fermentation model. The preset is 15 % levain in a 26 °C dough: about four and a half hours of bulk, and an aliquot-jar range to aim for before shaping.

Try this:

What to notice: the target is a range, not a line, and it moves with the conditions. The tool says so plainly: these are rules of thumb to calibrate against your own crumb. Bake, cut the loaf open, and adjust next time.

Every baker has pulled out a pale, flat disc with no ear. The useful thing about knowing the mechanism is that a bad loaf becomes evidence: the crumb tells you which part of the chain failed.

This preset ticks three symptoms — spread flat, no ear, a tight even crumb — and ranks the likely causes beneath.

Try this:

What to notice: the causes map neatly onto the sections of this page — the microbes, the temperature, the gluten, and (next) the oven. Change one thing at a time between bakes, or you will never know which one fixed it.

After bulk the dough is divided, pre-shaped, rested and shaped. Shaping is not just cosmetic: it drags the outside of the dough into a taut skin that holds the loaf up while it proofs and steers its expansion in the oven. Then comes the final proof — an hour or two on the bench, or a night in the fridge, which slows fermentation to a crawl, deepens the acid flavour and makes a cold, firm loaf that is easy to score.

This view shows shaping a bâtard in four steps, the poke test for judging the proof, and the scoring patterns, opened on the single ear.

Try this:

When the loaf hits a hot oven, the rise has one last act — oven spring. In the first ten minutes or so the dough warms from the outside in. Gas expands with heat; the dissolved carbon dioxide comes out of solution; ethanol and then water turn to vapour inside the cells; and the yeast have a final frantic burst of activity before they die at around 60 °C. A well-made loaf can grow by a third in that time. Then the starch gelatinises and the proteins set, freezing the crumb in place, and the crust dries and browns through the Maillard reaction and caramelisation.

All of that depends on the crust staying soft long enough. That is what steam is for.

The country loaf is set up for a Dutch oven: preheat for 50 minutes at 250 °C, bake 20 minutes with the lid on, then 25 uncovered at 230 °C until the middle reads 96–99 °C.

Try this:

What to notice: the lid-on phase is exactly the window of oven spring. The Dutch oven works because the loaf makes its own steam and the lid traps it. Take the lid off too early and the crust sets while the loaf is still trying to grow: a pale, tight, earless loaf, just like the one in the troubleshooter.

And then — the hardest step — leave it to cool. The crumb is still setting as the steam leaves it. Cut a lean loaf before an hour is up and it will be gummy; cut a high-rye loaf before a day is up and it will be worse.

Follow one loaf through and the whole story fits in a paragraph. A feed dilutes a culture of yeasts and lactic acid bacteria and gives them maltose to share; they double until the food runs out, and the jar peaks. That ripe culture goes into a dough whose water you chose to land it at 25 °C, because temperature sets the pace of everything that follows. Mixing and folding make the air cells and build the gluten network around them; the yeast fill the cells with carbon dioxide while the bacteria make the acids that flavour the loaf and, given too long, weaken it. The aliquot jar tells you when to stop. Shaping makes a skin, scoring makes a weak point, and in a steamy oven the gas expands one last time before heat sets it for good.

None of it needs a packet of yeast. It needs a thermometer, a straight-sided jar and patience.