A sourdough starter is flour and water colonised by a small community of microbes: yeasts, which make the carbon dioxide that raises the bread, and lactic acid bacteria, which make the acids that give it its sour taste – often joined by acetic acid bacteria. Each time you feed it, you hand that community fresh flour and dilute the acid, and the microbes best suited to your routine come out ahead. (It isn’t a SCOBY: that word describes the rubbery cellulose mat that kombucha grows. A starter is just the batter.)
What Lives in a Starter: the 500-Starter Study
The largest survey so far sequenced 500 home starters sent in by bakers from North America, Europe and Australasia (Landis et al., 2021):
- Yeast: baker’s yeast’s own species, Saccharomyces cerevisiae, made up more than half of the fungal DNA in 77% of starters. Species of Kazachstania, such as K. humilis, also turned up.
- Lactic acid bacteria: Fructilactobacillus sanfranciscensis was the dominant bacterium in most starters where it occurred – especially older starters and starters bought from a business – and it tended to pair with Kazachstania humilis rather than with S. cerevisiae.
- Geography didn’t matter much. Contrary to the idea that a starter reflects its city, location didn’t predict the microbes. How the starter was made and kept mattered more.
- Acetic acid bacteria, mostly overlooked before, were common – and starters with more of them tended to rise more slowly and smell more of vinegar.
In a follow-up experiment with lab-built starters, adding any of the acetic acid bacteria strains tested increased acidification by 18.5% on average and changed the aroma (Rappaport et al., 2024).
Why the Bacteria Have New Names
In 2020 the old genus Lactobacillus – 261 species – was split into 25 genera based on whole-genome data (Zheng et al., 2020). That’s why Lactobacillus sanfranciscensis is now Fructilactobacillus sanfranciscensis and L. plantarum is Lactiplantibacillus plantarum; older articles use the old names. The new groups reflect lifestyles: a starter that is fed and used for leavening over and over selects for fast-growing specialists, which is why F. sanfranciscensis, a species with a small genome adapted to this niche, dominates long-kept wheat and rye starters (Gänzle & Zheng, 2019).
Steering Your Starter
These are bakers’ rules of thumb rather than precise laws – starters differ – but they follow from how the microbes grow:
- Feed timing matters most. Use the starter when it has risen and domed, before it collapses. A starter used long after its peak is acidic and has fewer active yeasts.
- Larger feeds (e.g. 1 part starter to 5 parts flour and 5 water) give a milder starter that peaks later, because the acid is diluted more; small feeds (1:1:1) peak sooner and taste sharper.
- Warmth speeds everything up. At around 24–28°C a starter peaks within hours; in the fridge it barely moves, which is how most home bakers keep one between bakes.
- Vinegary smell or a sluggish rise fits a starter rich in acetic acid bacteria. More frequent feeding and stirring usually brings it back to a cleaner, yogurt-like sourness.
- Liquid on top (“hooch”) means it’s hungry; stir it in or pour it off, and feed. Discard a starter with fuzzy mold or pink or orange streaks.
To start one from scratch, follow our sourdough starter guide.
What Sourdough Does to Bread – and What It Doesn’t
- Phytate and minerals: in a rat study with reconstituted whole wheat, sourdough bread had 71% less phytate than the flour (yeast bread: 52% less), and the rats absorbed more magnesium, iron and zinc from it (Lopez et al., 2003). The effect matters most for whole-grain bread; it hasn’t been shown to the same extent in people.
- FODMAPs: enzymes of sourdough lactobacilli can lower FODMAP levels in bread (Gänzle & Zheng, 2019), which is why some people with IBS find long-fermented sourdough easier – but it depends on the recipe and fermentation time.
- Gluten: ordinary sourdough is not safe for celiac disease. In a trial, only wheat baked goods that had been fully hydrolysed in a special process with selected lactobacilli and fungal enzymes (8 ppm residual gluten) were tolerated; regular wheat baked goods caused problems (Greco et al., 2011). Home fermentation doesn’t come close.
- Live cultures: baking kills the yeasts and bacteria, so sourdough bread isn’t a probiotic food.
Sourdough has relatives around the world: Ethiopian injera is a fermented teff batter, and Russian kvass is brewed from bread. For how fermentation works in general, see the science of fermentation.