Feeding a starter
A sourdough starter is a culture of wild yeast and lactic acid bacteria living in flour and water. Each feeding, or refreshment, keeps part of the old starter and adds fresh flour and water. Bakers describe a feeding with a ratio of starter : flour : water by weight.
When you build a levain for a recipe, you use the same idea to make exactly the amount your dough needs, plus a little extra to keep as your starter.
The calculation
With a target weight T and a ratio s : f : w:
- Starter = T × s ÷ (s + f + w)
- Flour = T × f ÷ (s + f + w)
- Water = T × w ÷ (s + f + w)
The new levain’s hydration counts the flour and water already in the starter. A starter at 100% hydration is half flour, half water.
Worked example
A 200 g levain at 1:2:2, from a starter at 100% hydration:
| Result | Value |
|---|---|
| Starter | 40 g |
| Flour | 80 g |
| Water | 80 g |
| Hydration of the levain | 100% |
| Starter as a share of fresh flour | 50% |
Choosing a ratio
| Ratio | Peak at about 75 °F (24 °C) | Use |
|---|---|---|
| 1:1:1 | 4–6 hours | Daily feeding, quick levain |
| 1:2:2 | 6–8 hours | Daytime levain |
| 1:5:5 | 10–12 hours | Overnight levain |
| 1:10:10 | 12–16 hours | Long overnight, warm kitchen |
Temperature matters as much as the ratio: fermentation roughly doubles in speed for every 15 to 18 °F (8 to 10 °C) warmer. Use a higher ratio in summer and a lower one in winter.
Hydration and flavor
- Liquid starters (100% or more) ferment quickly and produce more lactic acid, giving a mild, creamy tang.
- Stiff starters (50 to 65%) ferment more slowly and favor acetic acid, giving a sharper sourness. They are traditional in some European breads.
Whole-grain flours such as rye or whole wheat feed the culture faster than white flour and make a more active starter.
Signs that the levain is ready
It has roughly doubled, its top is domed or just starting to flatten, it is full of bubbles and it smells pleasantly sour. Use it near its peak for the best rise. If it has collapsed and smells sharply of vinegar or acetone, it is past its peak; feed it again.