Science
Lactobacillus and Wild Yeast: The Two Engines of Sourdough
Your starter is a partnership between wild yeast and lactic-acid bacteria. Here's what each one does and why both matter.
A sourdough starter is a symbiosis between two kinds of microbes: wild yeast (which produces carbon dioxide to make bread rise) and lactic-acid bacteria, mainly Lactobacillus (which produce the acids that give sourdough its tang and keeping quality). Understanding this partnership explains nearly everything about how starters behave.
The two players
| Microbe | Produces | Effect on bread |
|---|---|---|
| Wild yeast | CO2 + alcohol | Rise (leavening) |
| Lactic-acid bacteria | Lactic + acetic acid | Tang + preservation |
What the yeast does
Wild yeasts (often Kazachstania and Saccharomyces species) ferment sugars and release carbon dioxide. That gas, trapped by gluten, is what makes the dough rise and creates the open crumb. The yeast is the leavening engine.
What the bacteria do
Lactobacilli ferment sugars into lactic acid (mild, yogurt-like tang) and, under certain conditions, acetic acid (sharper, vinegary tang). These acids:
- Give sourdough its characteristic flavor.
- Lower the pH, which preserves the bread and slows mold.
- Improve digestibility and mineral absorption.
The partnership
The two work together: bacteria create an acidic environment that the wild yeast tolerates but many competing microbes can't, keeping the culture stable. The yeast, in turn, helps break down starches into sugars the bacteria can use. This balance is why a healthy starter resists contamination.
How you steer the balance
| Want | Conditions favoring it |
|---|---|
| More lactic (mild) | Warmer, wetter, faster ferment |
| More acetic (sharp) | Cooler, stiffer, longer ferment |
| More rise | Active yeast at peak |
Temperature and hydration shift which acids dominate — that's how you control flavor.
Frequently asked questions
Is the yeast in sourdough the same as commercial yeast?
No — sourdough uses wild yeast strains, often different species than the single strain in commercial packets.
Why does my starter smell like vinegar sometimes?
That's acetic acid, favored by cooler, longer ferments and a hungry starter. Feed it and warm it for a milder profile.
Can the bacteria make bread rise?
Some produce a little gas, but the yeast does almost all the leavening. The bacteria handle flavor and preservation.
Reading your starter is really about reading this microbial balance. SourdoughAI tracks its behavior over time so you can dial in flavor and rise.