What a Sourdough Starter Is — Amarabosch — baking sourdough bread at home

A starter is a live culture of wild yeast and lactobacillus bacteria kept alive by regular feeding — the leavening agent and the source of sourness in one.

Sourdough is a leavening agent containing naturally occurring yeast and lactobacillus bacteria. The term sourdough describes the bread baked with it, while the leavening culture itself is called a starter.

The two organisms do different jobs. Wild yeast produce carbon dioxide, which raises the dough. Lactobacillus bacteria produce lactic acid, which gives the bread its distinctive sour taste and slows spoilage.

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Keeping both alive requires feeding: replacing spent flour and water with fresh ones on a schedule. A common baseline is 1 part starter to 1 part flour to 1 part water by weight.

At around 24 °C a 1:1:1 feed typically reaches peak activity in 4–8 hours. In a cooler kitchen the same feed can take 12 hours or more, so the schedule is a guide, not a rule.

A starter kept in the fridge needs two or three feeds at room temperature before it is strong enough to raise a loaf. Using a cold, unfed starter is the most common cause of a dense first bake.

Wild yeast → carbon dioxide → lift.
Lactobacillus → lactic acid → sour taste and keeping quality.
Feeding 1:1:1 by weight → predictable activity.
Peak activity → dome, doubled volume, pleasantly sour smell.