Building Your Starter from Scratch

What a starter is

A sourdough starter is a mixture of flour and water that hosts a stable community of wild yeasts and lactic acid bacteria. These microbes consume flour sugars and produce two things: CO2 gas (which makes bread rise) and organic acids (which give sourdough its flavor and tang).

You're not adding anything to make this happen. The yeasts and bacteria are already present on the flour and in your kitchen. You're just creating the right conditions for them to thrive.

Why all-purpose flour

You don't need rye or whole wheat to build a starter. All-purpose flour works fine — it just takes a few extra days to get going. King Arthur Unbleached All-Purpose is the recommendation: it has a consistent protein content (11.7%), is available at most grocery stores, and is the same flour you'll use for baking. One bag, start to finish.

The schedule

What to watch for

Days 1–3: You may see a burst of bubbles early. This is normal — it's not the stable fermentation you want yet. It will slow down before it picks back up.

Days 4–6: Bubbles become more consistent. The starter starts to smell pleasantly sour and yeasty rather than funky.

Day 7+: The starter should double in size between feedings, dome on top at peak, and smell like yogurt or mild vinegar. When it does this reliably for 2–3 days in a row, it's ready to bake with.

If nothing is happening: your kitchen is probably too cold. Find a warmer spot — on top of the fridge, near the oven, or inside the oven with just the light on.

The float test (and why to ignore it)

You may have heard of the float test: drop a spoonful of starter in water — if it floats, it's ready. This test is unreliable. A starter can be perfectly active and sink. A starter can float and still be too weak to raise bread.

The real test: does it double in size within 4–8 hours of feeding, consistently, for 2–3 days in a row? That's the only signal that matters.