How to Make Sourdough Bread: From Starter to Loaf

What Is Sourdough, and Where Does It Come From?

Sourdough bread is leavened without commercial yeast, relying instead on a culture built purely from the natural fermentation of flour and water. It's one of humanity's oldest known leavening methods — one of the earliest surviving sourdough breads, excavated in Switzerland, dates to 3700 BC, and the technique likely traces back even further to the spread of agriculture through the Fertile Crescent and Egypt.

Sourdough dominated breadmaking until the Middle Ages, when it gave way first to a method called barm, then to commercial baker's yeast in the late 19th century. But the technique never disappeared. French bakers carried sourdough to Northern California during the Gold Rush, laying the foundation for San Francisco's still-famous sourdough tradition.

The 1898 Klondike Gold Rush carried it further, into Alaska and the Yukon, where prospectors guarded their starter cultures closely. The fact that "sourdoughs" is still used as a nickname for old-timers in those regions says something about how deeply the technique took hold there.

Building the Starter

At its simplest, a sourdough starter is built by mixing flour and water and leaving it in a warm spot for a week or two. Over that time, wild yeast and lactobacilli naturally present in the flour and the environment multiply on their own, forming a stable culture — no commercial yeast required.

Keeping the environment at 25–30°C during that initial one-to-two-week stabilization period favors the bacterial species that end up dominating the culture, and plays a real role in how balanced the resulting mix of yeast and bacteria turns out to be.

The Feeding Routine

Keeping a starter alive means feeding it regularly — adding fresh flour and water at set intervals. During the initial two-week stabilization period, that typically means feeding every 24 hours; letting more than three days pass between feedings can shift the culture's microbial balance and change which species dominate.

The feeding ratio shapes the starter's character too. San Francisco-style sourdough uses roughly 40% fermented starter by total weight, with fresh flour and water making up the rest. Hydration — the water-to-flour ratio — isn't fixed either: some bakers keep a loose, liquid starter, others a stiff, dough-like one, and that choice directly shapes the acid profile.

Temperature and Hydration: The Two Levers on Sourness

A starter's temperature and hydration are the two main variables behind how sour the final bread turns out. Cooler, drier starters tend to produce more acetic acid, giving the bread a sharper, vinegary sourness. Warmer, wetter starters favor lactic acid production instead, leading to a softer, yogurt-like tang.

Adjusting these two variables deliberately makes it possible to get very different character out of the same basic method. Whether a baker keeps their starter stiff and cool, or loose and warm, shapes the final loaf's flavor directly.

The Microbiology: Wild Yeast and Lactic Acid Bacteria

Sourdough fermentation runs on a symbiotic relationship between wild yeast and lactic acid bacteria naturally present in flour and the surrounding environment. San Francisco cultures are typically dominated by Fructilactobacillus sanfranciscensis; these gram-positive bacteria convert carbohydrates into lactic, acetic and formic acid, which both create the characteristic sourness and make the environment inhospitable to spoilage organisms, extending the bread's shelf life.

On the yeast side, common species include Kazachstania exigua and K. humilis. These produce the carbon dioxide that leavens the dough while consuming byproducts of bacterial fermentation. In a stable culture, yeast populations typically make up only 1–10% of the bacterial population — sourdough is, numerically, a bacteria-dominated ecosystem.

The two organisms divide labor: bacteria ferment starches the yeast can't metabolize on its own. Enzymes break wheat starch down into glucose and maltose; bacteria ferment those sugars into compounds that the yeast then metabolizes, generating additional CO₂ for leavening in the process.

Mixing Ratio, Proofing and Retardation

When mixed into dough, the starter typically makes up 13–25% of the total flour weight, though some bakers go as low as 5–10%. A lower ratio extends fermentation time, but that extra time often deepens flavor.

Because wild yeast is less vigorous than commercial baker's yeast, sourdough dough takes noticeably longer to proof than standard yeasted dough. Many bakers manage this through retardation — refrigerating the shaped loaf before baking. It extends fermentation and deepens flavor complexity while making it easier to fit baking into a production schedule.

That long, unpredictable proofing time is exactly why sourdough doesn't translate well to standard bread machines without significant modification. It remains, by nature, a hands-on process that resists full automation.

Folding Instead of Kneading

Because sourdough is typically worked at higher hydration than commercial yeasted dough, bakers usually reach for the stretch-and-fold technique instead of classic kneading. The dough is gently stretched and folded over itself at intervals, developing the gluten network with far less disruption than kneading while preserving much of the gas already trapped inside.

That gentler handling is also what produces sourdough's characteristic open, irregular crumb: the less the dough is compressed, the more the gas pockets formed during fermentation survive intact, showing up as the loaf's signature holes once baked.