What Is a Sourdough Starter? Everything You Need to Know

A sourdough starter is a living culture of wild yeast and lactic acid bacteria, kept alive by regular feeding with flour and water. It's what makes sourdough bread rise, gives it its characteristic tang, and makes it different — biologically and flavour-wise — from every other kind of bread.

What's Inside a Sourdough Starter?

A sourdough starter is a complex ecosystem in a jar. Two main groups of microorganisms work together:

• Wild yeast (primarily Saccharomyces cerevisiae and various wild species): These consume sugars in flour and produce carbon dioxide, which makes bread rise. Wild yeast produces a more complex flavour than commercial yeast because it ferments more slowly and produces additional flavour compounds.
• Lactic acid bacteria (primarily Lactobacillus species): These produce organic acids — mainly lactic acid (mild, yoghurt-like) and acetic acid (sharp, vinegary). These acids give sourdough its tang, improve the bread's shelf life, and affect the crumb structure.

The two groups have a symbiotic relationship: the bacteria produce acids that protect the starter from unwanted microorganisms, while the yeast produces CO2 that helps the bacteria thrive. This is why a well-maintained sourdough starter is remarkably stable and resistant to contamination.

How Does a Sourdough Starter Make Bread Rise?

When you add active starter to bread dough, the wild yeast begins consuming the sugars in the flour. As the yeast ferments, it produces CO2 gas. This gas is trapped by the gluten network in the dough — it inflates the dough like tiny balloons, making it airy and light. When the bread goes into a hot oven, the gas expands rapidly (oven spring), the gluten sets, and the structure becomes permanent. The result is the open, irregular crumb characteristic of sourdough bread.

This is why a weak or inactive starter produces dense bread — if the yeast isn't producing enough gas, there's nothing to inflate the dough.

Where Do the Microorganisms Come From?

The wild yeast and bacteria in a sourdough starter come from several sources: the flour itself, the water, the environment (air, the baker's hands), and any previous culture introduced into the mixture. Different flours, environments, and water sources produce starters with different microbial communities — which is part of why sourdough made in Sydney tastes different from sourdough made in San Francisco or Warsaw.

What Does Feeding a Starter Mean?

Feeding (or refreshing) a starter means removing some of it and adding fresh flour and water. This replenishes the food supply for the microorganisms and keeps the culture healthy and active. Without regular feeding, the yeast and bacteria exhaust the available sugars, the environment becomes too acidic, and the culture eventually weakens.

The discard you remove at each feeding — the portion of old starter — is used in discard recipes rather than thrown away. Our What Is Sourdough Discard? article explains how to use it.

Do You Need to Make a Starter From Scratch?

You can, but it takes 7–14 days and results can be inconsistent. Starting with an established culture from a reliable source gives you a headstart — you get a starter that's already balanced, predictable, and proven in baking. Our sourdough starter kits include both wheat and rye starters, available live or dehydrated, with full feeding and baking instructions. See our guide Should You Make a Starter From Scratch or Buy One? to help you decide.