A sourdough starter is flour and water that you feed regularly until a stable community of wild yeasts and bacteria takes hold. The yeasts make the gas that raises bread; the bacteria make the lactic and acetic acids that give sourdough its tang. Making one takes nothing but flour, water, a jar and about one to two weeks of daily feeding. This page is the practical method; for what’s happening inside the jar in more depth, see our guide to the microbiology of sourdough starters.
What Ends Up Living in a Starter
The largest study so far analysed 500 home starters from four continents. Baker’s yeast, Saccharomyces cerevisiae, made up more than half of the yeast in 77% of them; lactic acid bacteria and acetic acid bacteria together made up more than 97% of the bacteria. Where a starter came from made little difference to what was in it – how it was made and kept mattered more – and starters rich in acetic acid bacteria smelled more of vinegar and rose more slowly (Landis et al., 2021).
The microbes come mainly from the flour. When bakers in different countries made starters with the same recipe and the same ingredients, the starters still came out different, and some of their microbes matched those on the bakers’ hands (Reese et al., 2020). The community also keeps changing for a long time: in rye starters fed daily for eight weeks in a laboratory, the dominant bacteria were still shifting after six weeks (Bessmeltseva et al., 2014). That’s why a new starter can be lively one day and sluggish the next, and why it settles down with steady feeding.
Is Sourdough Bread Healthier?
Less clearly than often claimed. A 2026 systematic review of 27 human studies concluded that the evidence that sourdough bread is better for blood sugar and insulin than yeast-raised bread in healthy people is “neither sufficient nor convincing” (Starowicz et al., 2026), and an earlier review of 25 trials found no clear consensus on health benefits of sourdough as such, though some specific strains and processes helped (Ribet et al., 2023).
Sourdough is not safe for people with coeliac disease. In a laboratory digestion study, sourdough fermentation changed how gluten broke down but didn’t reduce the total amount of the immunogenic gluten peptides (Ogilvie et al., 2021); in a clinical trial, only wheat products hydrolysed by a special industrial process were tolerated, not regular baked goods (Greco et al., 2011).
Keeping Your Starter
These are practical rules of thumb rather than research findings:
- Baking often: keep it at room temperature and feed it once or twice a day – for example, keep 50 g of starter and add 50 g flour and 50 g water.
- Baking weekly or less: feed it, leave it out for an hour or two, then refrigerate. Feed it about once a week, and give it one or two feeds at room temperature before you bake.
- When to use it: at or near its peak – risen, domed and full of bubbles – usually 4–8 hours after a feed in a warm kitchen. Watching the rise against a rubber band on the jar is more reliable than the float test.
- Grey liquid on top (“hooch”) is alcohol and water from a hungry starter: pour it off or stir it in, and feed more often.
- Backup: spread a little active starter thinly on baking paper, let it dry completely, and keep the flakes in a sealed jar.
- Discard (what you remove before feeding) works in pancakes, crackers and flatbreads, where it adds flavour rather than lift.
Troubleshooting
- Lots of bubbles on day 2–3, then nothing: common – early activity often comes from microbes that don’t last. Keep feeding on schedule and keep it warm (about 24–27°C); it usually picks up within a week.
- Slow rise or a sharp vinegar smell: feed more often or with a bigger feed (1 part starter to 2–5 parts each of flour and water), and keep it a little warmer.
- Nail-polish smell: usually a hungry starter; feed it.
- Fuzzy mould, or pink, orange or red streaks: throw the whole starter away and start again in a clean jar.
Don’t taste raw starter or raw dough: raw flour isn’t a ready-to-eat food and can carry harmful bacteria such as E. coli (FDA).
Other flour ferments to try: injera, made from a fermented teff batter, and kvass, a drink brewed from bread. For films and moulds on other ferments, see our troubleshooting guide.