How do I make a sourdough starter from scratch?

A starter is a stable culture of wild yeasts and lactic acid bacteria living in flour and water. Everything you need is already on the flour, so making one is not about adding anything — it is about creating conditions those organisms like and then waiting through a stage that looks exactly like failure.

What you'll need

• Wholemeal or rye flour to begin, strong white to continue
• Unchlorinated water — tap water left out for an hour, or filtered
• A clear jar, roughly 500ml, with a loose lid
• Digital scales that read to 1g

Step by step

1. Start with wholemeal or rye flour.The wild yeast and bacteria live on the bran, so wholegrain flour carries far more of both than white. You can switch to strong white flour once it is established, or keep feeding it wholemeal — rye starters are notably vigorous.
2. Use water that has no chlorine in it.Leave tap water in an open jug for an hour, or use filtered water. Chlorine and chloramine are there to suppress exactly the sort of organisms you are trying to grow, and in some areas it makes a real difference.
3. Mix equal weights, by scales.50g flour, 50g water, stirred to a thick paste. Weigh it — a cup of flour and a cup of water are not the same ratio at all, and inconsistent ratios are the commonest reason a starter behaves unpredictably.
4. Keep it somewhere warm and stable.20 to 25°C. In a cold UK kitchen this is the single biggest obstacle — an unheated worktop in February is often 15°C, at which a starter takes three or four times as long. An airing cupboard, the top of the fridge, or an oven with only the light on all work.
5. Feed once a day, discarding first.Each day, throw away all but about 50g, then add 50g flour and 50g water. The discard feels wasteful and is essential — without it the acidity builds up until the yeast cannot work, and the jar becomes unmanageably large.
6. Do not be fooled by day three.Around day two to four many starters froth up dramatically and smell strongly, sometimes of cheese or drains. That is leuconostoc and other bacteria that colonise first, and it dies back as the mixture acidifies. It is not the yeast and it will stop. Keep feeding.
7. Push through the flat spot.Days four to seven are usually quiet, and the starter smells sharp, vinegary or of nail varnish. This is where almost everyone gives up. Nothing is wrong. Keep feeding once a day, keep it warm, and wait.
8. Watch for the real pattern.From about day seven to fourteen it starts to rise and fall predictably: it doubles within four to eight hours of a feed and then sinks back. The smell settles to yoghurty and beery. Two or three consecutive cycles like that mean it is ready.
9. Use it at its peak.A starter has the most lifting power at the top of its rise, when it is domed and full of bubbles, not before and not after it has collapsed. Mark the jar level with an elastic band after each feed so you can see where it is in the cycle.
10. Test it before you commit a loaf's worth of flour to it.Drop a teaspoon of starter at its peak into a glass of water. If it floats, it has enough gas to raise a loaf. It is a rough test and a sinking starter can still work, but a floating one is a good sign.

Tips

• A pinch of rye flour in every feed keeps a white starter more active. Rye has more of everything the culture wants.
• Warmth is the lever that matters most. A starter that looks dead in a cold kitchen is usually just cold.
• Do not add yeast, sugar, honey, grapes or yoghurt to hurry it along. They produce a burst of activity that is not the culture you are trying to establish, and they mask what is actually happening.
• Once it is established, dry some on baking parchment, crumble it and keep it in a jar. It rehydrates and gives you an insurance copy.

Common mistakes

• Throwing it away during the flat spot — Days four to seven look and smell exactly like failure and are a normal stage of a healthy starter's development. Almost every abandoned starter is abandoned in this window, days before it would have come good.
• Sealing the jar tightly — A starter produces carbon dioxide and needs to vent. A tightly sealed jar can build pressure and, in glass, that is a genuine hazard. Rest the lid on, or use a cloth and a band.
• Feeding by volume rather than weight — Flour compacts and water does not, so a cup of each is roughly a 1:2 ratio by weight rather than 1:1. That inconsistency is why some starters behave differently from one day to the next for no apparent reason.

If it doesn't work

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