How to Make a Sourdough Starter From Flour and Water
Ten days from a paste of flour and water to a wild-yeast culture that doubles on schedule. What to feed it, how often, what you should see each day, and what to do when it goes wrong.
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A sourdough starter is made by mixing equal weights of flour and water, leaving it at room temperature, and feeding it daily — discarding half and adding fresh flour and water — until it reliably doubles within six to eight hours of a feed. That takes about ten days. Once established it lives in the fridge and needs one feed a week, plus one twelve hours before each bake.
What you are actually growing
Flour carries wild yeast and lactic acid bacteria on it already. Add water and warmth and they wake up; feed them and the ones suited to a wet, acidic, flour-rich environment out-compete everything else. By day ten you have a stable community: yeast for the rise, lactobacillus for the acid — and the acid is what does most of the work this series is about.
Two things matter more than any other. Use white bread flour rather than wholemeal, because bran carries more of the wrong organisms and more of the phytate the fermentation is trying to remove. And use filtered or boiled-and-cooled water, because chlorine is there to kill exactly the organisms you are trying to grow.
The ten-day schedule
Ratios are starter : flour : water, by weight. Keep the jar somewhere around 22°C — the top of the fridge, a cupboard near the boiler. Colder is slower, not worse.
| Day | Feed | How often | What you should see |
|---|---|---|---|
| 1 | 50 g flour + 50 g water | once | Nothing. Maybe a bubble by evening. |
| 2–3 | Discard half; 1:1:1 | every 24 h | Sour smell, some bubbles, a rise and then a fall. |
| 4–5 | Discard; 1:1:1 | every 12 h | Doubles in about 8 h. Smells like yogurt and beer. |
| 6–7 | Discard; 1:2:2 | every 12 h | Doubles reliably in about 6 h. Domed top, then a slight collapse. |
| 8–10 | 1:5:5 the night before baking | once | Passes the float test. Ready for Part 3. |
| Maintain | 1:5:5 weekly, in the fridge | weekly | Feed 12 h before each bake and leave it out until it peaks. |
The float test, and why it works
Drop a teaspoon of starter into a glass of water. If it floats it is full of gas, which means the yeast is active enough to raise a loaf. If it sinks, feed it and try again in four hours.
It is a crude test and it fails in one direction: an over-ripe starter that has already peaked and collapsed will sink even though it was ready two hours ago. So test on the way up — when the volume is still rising and the top is domed — not after it has flattened.
When the starter goes wrong
| What you see | What it means | What to do |
|---|---|---|
| Gray liquid on top, smells of nail varnish | Starved — too long between feeds | Pour the liquid off, feed 1:2:2 twice a day for two days |
| Rises then collapses within 3 hours | Kitchen is warm; the culture is fast and hungry | Feed at a higher ratio (1:5:5) or move it somewhere cooler |
| No activity at all after 5 days | Chlorinated water or bleached flour | Switch to filtered water; add a spoon of rye flour to the next three feeds |
| Pink or orange streaks, or fuzzy mould | Contamination | Discard the lot and start again — do not taste it |
| Smells strongly of acetone but looks fine | Hungry, not dead | One normal feed fixes it; the smell is the culture running out of food |
Before Part 3
You need a starter that doubles within six hours of a 1:2:2 feed at room temperature and passes the float test on the way up. Do not move on with one that is merely bubbling — a weak starter gives a dense loaf and no amount of technique downstream fixes it.
The night before you bake, feed it 1:5:5 and leave it out. By morning it will be at its peak, and that is when Part 3 begins.
