Fix your next loaf.

One culture doing two jobs. The starter is the jar you keep alive and feed; it never all gets used, and it outlives every loaf. The levain is a small batch built from part of that jar, and all of it goes into the dough. Build it a few hours before you mix, or the night before at 1:4:4 (1 part starter to 4 each of flour and water, by weight), which peaks in about 12 hours at 78°F / 26°C; a cooler kitchen takes longer. Plenty of recipes say "starter" for both, which is where the confusion starts.

Building a levain rather than tipping in the jar means it goes into the dough at its peak, you can give it different flour than the jar gets, and a failed bake never costs you the culture. In this app the Starter tab is the jar; "Starter (levain)" in a formula is what goes in the dough, and the first step of a bake gives you the exact weights for a build a few hours ahead. Building it overnight? Use the same total, split 1:4:4: 40 g each of starter, flour and water becomes 13 g starter, 53 g flour and 54 g water. Tap Done on that step once it has peaked.

Sources King Arthur Baking: Baking trials: Testing different sourdough feeding ratios

Baker's percentages are shares of the total flour, not of the dough. 20% starter with 500 g of flour means 100 g of levain.

You do not need to work it out. Pick your pot and set the percentages in the calculator, or when starting a bake, and every number you have to weigh is listed underneath in grams.

Most often an under-fermented bulk, or a levain that was not lively when it went in. The dough never built enough gas to open up. A loaf that over-proofed and collapsed can cut dense too, but then it is flat as well.

Bulk until the dough has risen about 50% and feels alive — domed, jiggly, bubbles at the edges — rather than until the clock says so. If a levain of equal weights took more than about 8 hours to peak somewhere warm, feed the starter twice a day for a few days before trying again.

Sources King Arthur Baking: Baking trials: Testing different sourdough feeding ratios·King Arthur Baking: Debunking the biggest sourdough myths

Two different faults look identical: cut too soon, or baked too little. Over-fermentation gives a third version, where the crumb is gummy and the loaf is also flat.

Bake until the crust is deep brown; a thermometer in the centre should read at least 190°F / 88°C. Then leave it on a rack at least two hours before cutting. If it is flat as well as gummy, shorten the bulk next time.

Sources King Arthur Baking: How to tell if bread is done baking·King Arthur Baking: Using a thermometer with yeast bread

Three different faults, each with its own fix. Over-fermented: by shaping, the dough was slack, very bubbly and smelled sharp. Weak shaping: it held together but spread as soon as it left the basket. Too much water: it was sticky and hard to handle from the first fold.

Change the one that fits, not all three. Over-fermented: cut the bulk by 30 minutes. Weak shaping: pre-shape and shape with more tension. Too much water: drop the hydration by five points. All-purpose (plain) flour makes a slacker dough at 80% than bread flour, which soaks up more water.

Sources King Arthur Baking: How to make your sourdough bread more (or less) sour: part 3

Over-proofed dough has no lift left. A cool oven or no steam will do it too.

Bake in a properly preheated Dutch oven: heat it in the oven for at least 30 minutes. A cold-proofed loaf can go straight in from the fridge or sit out about 45 minutes first; both work. Enamelled or ceramic pot? Always let a cold-proofed loaf sit out at least 45 minutes, since a fridge-cold loaf can crack a hot pot, and check the maker's care notes on heating it empty. Score one decisive cut about 1 cm deep: a curved blade at about 30° to the surface, a straight blade at 90°.

Sources King Arthur Baking: How to score bread dough·King Arthur Baking: Artisan sourdough bread tips, part 3·Lodge Cast Iron: Cleaning and care: enameled cast iron·King Arthur Baking: Pain au levain

Two loaves get called this. One big hole under the crust, with an even crumb below it, is gas trapped by loose shaping or a skin that formed during the bench rest. Big holes with dense, gummy crumb around them usually mean the bulk was too short.

For the first, pre-shape with real tension, keep the bench rest to 30 minutes or less, and cover the dough if the kitchen is dry. For the second, give the bulk longer, until the dough has risen about 50%.

The oven was not hot enough, or the lid stayed on too long.

Give it longer uncovered and take the colour further than feels right. A deep brown crust is where most of the flavour lives.

Sourness comes from acid. A warm dough, a levain used past its peak and whole wheat or rye in the flour all tend to build more of it, and a night in the fridge often makes the loaf more sour too.

Use the levain as soon as it has domed and is just starting to flatten, not later. Or feed the starter more often, so it never sits long past its peak. Or keep the dough a little cooler, about 73°F / 23°C, and give the bulk longer; in the app, enter that dough temperature when you start a bake and the times stretch to match. Or skip the fridge: proof at room temperature and bake the same day, as "Country loaf, same day" does. Or use bread flour in place of any whole wheat or rye. Flavour is hard to steer exactly, so change one thing per bake.

Sources King Arthur Baking: How to make your sourdough bread more (or less) sour: part 2·King Arthur Baking: How to make your sourdough bread more (or less) sour: part 3·King Arthur Baking: Artisan sourdough bread tips, part 3·PeerJ, 2021: A review of sourdough starters: ecology, practices, and sensory quality

A cool dough, a levain used right at its peak, white flour only and a same-day bake all tend to keep a loaf mild, and the fridge does not always make it more sour.

Let the levain go a little past its peak, until the dome starts to sink. Or keep the dough warmer through the bulk, 79–83°F / 26–28°C, and judge the bulk by its rise, not the clock; in the app, enter that dough temperature when you start a bake and the times shorten to match. Or, if it is all white flour, make 10–15% of it whole wheat or rye. Or, once the bulk has risen about 50%, shape the loaf and proof it in the fridge overnight, as "Country loaf, overnight" does. Flavour is hard to steer exactly, so change one thing per bake; push it too far and the loaf may not rise well.

Sources King Arthur Baking: How to make your sourdough bread more (or less) sour: part 2·King Arthur Baking: How to make your sourdough bread more (or less) sour: part 3·King Arthur Baking: Artisan sourdough bread tips, part 3

Yes. Starter is sold fresh, alive in a tub, or dried in a packet that you wake up with flour and water. Either way, you skip growing one from nothing.

Feed a fresh one within a day of it arriving, then twice a day at room temperature until it doubles after each feed. That is when it is ready to bake with. In the app, tap "I already have one" on the Starter tab.

Sources King Arthur Baking: Fresh sourdough starter·King Arthur Baking: French sourdough starter

Bread flour or all-purpose works on its own, so a bag of either is all the flour you need. Whole wheat or whole rye may get a new starter going a little faster, because wild yeast is more likely to be found in whole grain flour, but you do not need them. Rye on its own works too, and so does spelt, a kind of wheat; barley has made sourdough in lab tests as well.

You can switch between these once it is going, for example from whole grain to white. A starter fed a different flour for a long time may come out thicker or thinner, or a different colour, and act differently. Some gluten-free flours, such as teff, can make a starter too, if it only ever gets gluten-free flour, but results vary: in lab tests, buckwheat sourdoughs sometimes ended up with no yeast at all. Switching a wheat or rye starter to gluten-free flour is not recommended.

Sources King Arthur Baking: Sourdough starter recipe·King Arthur Baking: Best flour for sourdough starter·Microbiology Spectrum, 2026: The role of flour type and feeding schedule on the sourdough microbiome·MicrobiologyOpen, 2020: Bacterial community dynamics in spontaneous sourdoughs made from wheat, spelt, and rye wholemeal flour·Letters in Applied Microbiology, 2020: Microbial consortia involved in fermented spelt sourdoughs: dynamics and characterization of yeasts and lactic acid bacteria·International Journal of Food Microbiology, 2016: Community dynamics and metabolite target analysis of spontaneous, backslopped barley sourdough fermentations under laboratory and bakery conditions·Food Microbiology, 2011: Biodiversity of lactic acid bacteria and yeasts in spontaneously-fermented buckwheat and teff sourdoughs·International Journal of Food Microbiology, 2010: Development of buckwheat and teff sourdoughs with the use of commercial starters

Less than 50 g is fine. A smaller starter, such as 20 g kept and fed 20 g of flour and 20 g of water, works too, as long as flour and water are equal by weight. A scale matters most with a small starter, so weigh every feed, and use a jar small enough to see it rise, with room for it to double.

Check you will have enough to bake. One loaf usually builds its levain from about 35 to 55 g of starter (more in a bigger pot), and you keep some back to carry on. A 20 g starter is only 60 g after a feed, so at the feed before you build the levain, throw none away, and use a bigger jar or bowl: 60 g of starter plus 60 g each of flour and water makes 180 g. Growing one in the app? Change Keep under "Feed it" and the day card follows.

Sources King Arthur Baking: A smaller sourdough starter·King Arthur Baking: Maintaining a smaller sourdough starter·Public Science Lab, NC State University: Wild Sourdough

Usually too cold (below about 70°F / 21°C, day or night), or fed too rarely.

Feed 1:1:1 twice a day and keep it at the warm end of the ideal range, 75–78°F / 24–26°C. An oven with only the light on works if it is not running hotter than that; tape a note to the door so nobody preheats it. If you have whole rye, swap 10–20% of the flour for it for a feed or two; a little whole wheat can help too. Tap water is fine unless you can smell chlorine in it. Give it a week before deciding it has failed.

Sources King Arthur Baking: Sourdough starter recipe·King Arthur Baking: Best flour for sourdough starter·King Arthur Baking: Sourdough starter troubleshooting·Cultures for Health: Sourdough starter troubleshooting FAQ

Hooch. It is alcohol, and it means the starter has eaten everything and is hungry. Grey, brown or black, the colour does not mean it has spoiled.

Stir it back in for a little extra flavour, or pour it off. A layer deeper than about half an inch (1.3 cm), especially a very dark one, is worth pouring off: that much liquid can thin the starter enough to change the bake. Then feed it. Keeps coming back? Feed at a bigger ratio such as 1:5:5, feed more often, or keep it cooler.

Sources King Arthur Baking: Sourdough starter troubleshooting·King Arthur Baking: Black liquid on top of sourdough starter is totally fine·King Arthur Baking: Baking trials: Testing different sourdough feeding ratios·Cultures for Health: Sourdough starter troubleshooting FAQ

This is contamination, not fermentation. Healthy starter never grows fuzz and is never pink or orange.

Throw the whole jar out without sniffing it. Do not scrape the top off and carry on: in a wet food, mould reaches below the surface and bacteria can grow alongside it. Wash everything and start again.

Sources USDA: Molds on food: are they dangerous?·King Arthur Baking: Sourdough starter troubleshooting

Volume alone lies, because it depends on the vessel. Read the surface and the feel.

Look for a 40–60% rise, a domed top, bubbles at the edge, and a wobble when you shake the bowl. The dough should feel full of air rather than heavy.

The poke test, which reads the gas structure rather than the clock.

Flour a fingertip and press 1 cm into the dough. Springs back fast: not ready. Springs back slowly and not all the way: bake it. Stays dented and sighs: over-proofed, bake it now and expect it flatter. Use it on dough at room temperature. A loaf proofed in the fridge goes by the schedule: bake it straight from the fridge, or let it sit out about 45 minutes first. In an enamelled or ceramic pot, always give it at least 45 minutes out: a fridge-cold loaf can crack a hot pot.

Sources King Arthur Baking: Artisan sourdough bread tips, part 3·Lodge Cast Iron: Cleaning and care: enameled cast iron