A sourdough starter is flour and water left to ferment until wild yeast and lactic acid bacteria colonize it and can leaven bread. That's it. No commercial yeast, no starter from a friend, no expensive kit. You need a jar, a spoon, whole grain flour, bread flour, and water. In seven days you'll have an active starter; give it two more weeks of regular feeding before you rely on it for a loaf.
A starter can be built in a 20°C (68°F) kitchen in winter or a 29°C (84°F) kitchen in July. The timing shifts, the smell shifts, but the process below works in both. Let's walk through it.
What you need before day 1
- A clear glass jar, 500ml or larger, with a loose lid (a canning jar with the ring but no seal works well)
- A digital kitchen scale that reads to 1 gram
- Whole grain flour, rye or whole wheat, for the first few days. Whole grain flour carries more wild yeast and bacteria on the bran than white flour does.
- Unbleached bread flour or all-purpose flour for the maintenance days
- Filtered or bottled water if your tap water is heavily chlorinated. If you can smell chlorine at the tap, filter it or leave a pitcher uncovered overnight to let the chlorine off-gas. That doesn't work for chloramine, which many US utilities use (your water report says which): use a carbon filter or a Campden tablet, or bottled water.
- A rubber band or marker to track the level in the jar
Volume measurements will get you close, but grams remove the guesswork. A starter is a ratio, and ratios need weight.
The 7-day plan at a glance
| Day | Action | Flour | Water | What to expect |
|---|---|---|---|---|
| 1 | Mix | 50g whole rye or whole wheat | 50g water | Nothing visible |
| 2 | Wait / observe | Maybe small bubbles | ||
| 3 | First feed: keep 50g | 50g flour (25g whole grain + 25g bread flour) | 50g water | Bubbles, sour-sharp smell |
| 4 | Feed once: keep 50g | 50g flour (25g + 25g) | 50g water | Possibly a false peak, then quiet |
| 5 | Feed twice (12h apart) | 25g starter + 50g flour + 50g water | Steady rising activity | |
| 6 | Feed twice | Same 1:2:2 ratio | Doubles within 8–12 hours | |
| 7 | Feed and check | Same | Doubles within about 6 hours, domed top |
Day 1: mix
Combine 50g whole rye (best) or whole wheat flour with 50g water in your jar. Stir until no dry flour remains; the consistency should be like thick pancake batter. Scrape down the sides, mark the level with a rubber band, and cover loosely. Leave it somewhere warm: 24–27°C (75–80°F) is ideal. On top of the fridge, in a turned-off oven with the light on, or near (not on) a radiator all work.
Do nothing else for 24 hours.
Day 2: mostly wait
You might see a few bubbles or none at all. Either is fine. Don't feed it yet. Feeding too early dilutes the acidity that keeps bad bacteria out and slows the process paradoxically. Give it another 24 hours.
If you see nothing at all by end of day 2, don't panic. Cold kitchens push everything back a day.
Day 3: first feed
By now you should see bubbles through the side of the jar and smell something sharp, often unpleasant, sometimes like old socks or acetone. This is normal. The early bacteria (mostly Leuconostoc) are dominating before the yeast catches up.
Discard about half the mixture (roughly 50g), then add 25g whole grain flour, 25g bread flour, and 50g water. Stir well. Mark the new level.
The discard question comes up constantly: yes, you have to. Discarding keeps the jar small enough that each feed is a meaningful amount of fresh food relative to the culture. Without it, acid and spent flour build up and you'll be feeding an ever-growing jar of weak, hungry culture. You can save discard for pancakes once the starter smells pleasant, but for now, bin it.
Day 4: quiet day (this is where people give up)
Day 4 is the trap. Many starters rise dramatically on day 3, then collapse and go still on day 4. First-time builders think they've killed it. They haven't. The early bacterial bloom has died off, the pH has dropped, and the actual sourdough microbes (lactic acid bacteria such as Fructilactobacillus sanfranciscensis and yeasts such as Kazachstania exigua) are just starting to establish.
Feed the same as day 3: keep 50g, discard the rest, then add 25g whole grain flour, 25g bread flour and 50g water.
A starter that sits completely flat on day 4 looks dead, and that is exactly when many get thrown out. Left alone through the quiet phase, the same jar is typically climbing by day 6.
Day 5: switch to twice-daily feeds
If you see any activity (bubbles, a slight rise, a milder smell), start feeding twice a day, roughly 12 hours apart.
New ratio: 25g starter + 50g flour + 50g water (a 1:2:2 feed). Use 25g whole grain and 25g bread flour, or shift to all bread flour if you want a milder final flavor. Whole grain feeds faster; white gives a cleaner taste.
The point of twice-daily feeding is to keep the culture fed before it goes hungry and produces the acetic acid that stunts yeast growth. You're training it to be reliably active.
Day 6: it should be rising predictably
By day 6 the starter should be roughly doubling between feeds, showing a domed top with a fine bubble structure across the surface, and smelling tangy and yeasty: sour but not chemical. If it's not doubling yet, keep feeding twice a day on the same 1:2:2 ratio. Give it another day or two.
If your kitchen is below 21°C (70°F), everything runs slower. Find a warmer spot: a proofer, an oven with the light on (check the temperature; some oven lights push the interior past 29°C (85°F), and tape a note to the controls so nobody preheats with the jar inside), or a heating pad set low.
Day 7: check the rise
Feed 1:2:2 as usual and mark the level. A working starter doubles within about 6 hours at 24°C (75°F), with a domed top and bubbles throughout. If it takes longer, keep feeding twice a day and check again tomorrow.
The float test (a spoonful in water) is an optional extra check for a wheat-based starter. It isn't reliable for rye, and a starter can sink and still bake fine, so the rise against the mark is what counts.
Then keep feeding it daily for two more weeks before the first bake. A new starter gets stronger as its yeast and bacteria build up. When you're ready to bake, the schedule calculator works out how much levain to build from it.
What "ready" actually means
A day-7 starter is active but young. The flavor will be mild for the first few bakes and gets more complex over several more weeks of regular feeding.
Signs it's ready to bake with:
- Doubles within 4–6 hours of a 1:2:2 feed at 24°C (75°F); in a cooler room allow longer (the model gives about 5½ hours at 22°C (72°F) and 6½ hours at 20°C (68°F))
- Has a strong, even bubble structure, not just a few big holes
- Smells tangy, yeasty, slightly fruity or beer-like, not sharp like vinegar or nail polish remover
- Behaves consistently over three or four feed cycles
Storage once it's established
Feed daily at room temp if you bake often (2+ times a week). Otherwise, move it to the fridge and feed once a week. To feed from cold: discard down to 25g, add 50g flour and 50g water, leave out for 2–4 hours until active, then refrigerate again.
The night before a bake, take it out and give it two feeds at room temp to wake it up fully.
Troubleshooting
Pink, orange, or fuzzy growth
Throw it out. Pink or orange streaks and any fuzzy mold mean contamination. Wash the jar in hot soapy water and start over. Note: a gray liquid on top ("hooch") is not mold. That's alcohol from a hungry but healthy starter. Pour it off or stir it in and feed.
Smells like nail polish remover
Acetone smell means it's starving. Feed more often or use a warmer spot. This is common on day 3–4 and fixes itself.
Won't rise after day 7
The usual cause is temperature. Measure your kitchen with a probe thermometer. If it's below 22°C (72°F), your starter needs help. The other cause is chlorinated or chloraminated water; switch to filtered.
Rises then collapses fast
It's peaking and falling between checks. Either use a looser feed (1:5:5) or catch it earlier. A starter that peaks in 3 hours and collapses is strong, not weak; you just need to time your bake to its schedule.
Common questions
Can I use all-purpose flour instead of bread flour?
Yes. All-purpose works fine for maintenance. Bread flour just gives slightly more vigorous rises because of higher protein. The whole grain flour on days 1–3 matters much more than the white flour choice later.
Do I have to use rye, or is whole wheat fine?
Whole wheat works. Rye is faster because it has more accessible sugars and typically carries a heavier microbial load, so it tends to shave a day off the process. Either gets you there.
Why does my starter smell like vomit or vinegar?
Overly sharp, harsh smells usually mean it's too acidic, most often from being underfed. Feed it more often or with a looser ratio (1:5:5 instead of 1:2:2), and keep it at 24–26°C (75–79°F): a cool jar leans toward vinegar-like acetic acid.
Can I skip discarding to save flour?
Not in the first week. After it's established, you can build smaller starters (as little as 5g mother + 25g + 25g) to minimize waste. But during the build, discarding is what keeps each feed large relative to the culture, diluting the acid and spent flour, and drives the culture forward.
Is the "pineapple juice trick" necessary?
No. Some old guides recommend using pineapple juice on day 1–2 to lower the pH and prevent bacterial off-smells. It works, but it's optional. A standard flour-and-water build gets to the same place by day 4 or 5.
Can I speed this up?
Warmer temperatures (26–28°C (78–82°F)) can knock a day or two off, but there's a floor. The microbial succession takes time, and no trick reliably produces a bakeable starter in 3 days. Plan for a week and be pleasantly surprised if it's ready on day 6.
Don't bin it on day 4
Don't throw a new starter out on day 4: one that looks dead midweek can still work by day 6. If the jar smells odd and looks dead midweek, keep to the feeding plan and check again in 24 hours.