Sourdough starter
A maintained population of wild yeasts and lactic acid bacteria in flour and water, kept alive by regular refreshment and used to raise and acidify bread.
Also called starter (INTERNATIONAL); mother (INTERNATIONAL); levain (INTERNATIONAL) — Used loosely as a synonym, but in French practice a levain is the portion built from the starter for a particular bake. BakeHQ keeps them separate.; sourdough culture (INTERNATIONAL); chef (HISTORICAL)
A sourdough starter is a stable ecosystem rather than an ingredient. Flour carries yeasts and bacteria; adding water and keeping the mixture fed selects for the few organisms that tolerate the acidity they themselves create, until the population becomes self-reinforcing and resists most newcomers. That competitive exclusion is the reason a well-kept starter is robust, and it is also why the significant risks concentrate in the first days, before the culture is established, and after long neglect, when it has weakened.
The culture does two jobs that a packet of yeast does only one of. The yeasts produce carbon dioxide, which raises the loaf. The bacteria produce lactic and acetic acid, which lower the pH, change the flavour, strengthen and then eventually weaken the gluten, and slow staling and mould growth in the finished bread. A baker who thinks of a starter purely as a leavening agent is missing the half of it that determines flavour, keeping quality and dough handling.
When to throw it away
Fuzzy or coloured growth — green, black, pink, orange, or anything raised and hairy — is mould, and a starter showing it should be discarded rather than scraped and fed. Mould in a soft, wet medium is not confined to the part you can see, and some moulds produce toxins that persist after the visible growth is removed. Pink or orange discolouration in particular has long been treated in baking guidance as a discard signal. This is the one judgement on this page where the cautious answer is the correct one: a starter costs a week to replace, and the cost of being wrong is not worth the saving. A dark liquid layer, a strong acetone or nail-varnish smell, and a sharply sour taste are all NORMAL signs of a hungry culture and are not spoilage.
What is living in it
Microbiology
A mixed population dominated by lactic acid bacteria, with yeasts present at a much lower cell count — the ratio is mostly between 10 to 1 and 100 to 1. More than 90 lactic acid bacteria species and more than 40 yeast species have been isolated from sourdoughs worldwide, so no single species list describes a starter. Which species come to dominate is influenced by dough yield, pH and temperature; it is NOT determined by the flour used or by where in the world the culture is kept.
The last sentence contradicts a claim repeated constantly in baking writing — that flour choice or geography gives a starter its character. The cited paper found neither structures the mature community. Flour and temperature still change the RATE and the acid balance, which is what bakers actually observe and then misattribute.
RecordedA. Comasio, M. Verce, S. Van Kerrebroeck and L. De Vuyst, Frontiers in Microbiology
Hydration
Typical hydration
50–125% (typically 100%)
A convention rather than a measured property. 100% — equal weights of flour and water — is the common domestic choice because it makes the arithmetic of a formula trivial. Stiffer cultures at 50–60% favour acetic acid production and ferment more slowly; slacker ones ferment faster and sour more sharply toward lactic. No hydration is correct in itself.
RecordedEstablished practice, not a measured result
How it is kept
Refreshment ratio
The ratio of old culture to fresh flour and water sets how long the culture takes to peak and how sour it becomes. A small proportion of old culture — 1 part to 5 or 10 of fresh — dilutes the accumulated acid, takes longer to reach peak and produces a milder result. A large proportion peaks quickly and stays acidic. This is the primary control a baker has, and it is more effective than changing the feeding frequency.
Professional consensus with a clear mechanism — dilution of acid and of cell count — rather than a controlled comparison. Stated as a direction of effect, with no ratio recommended as correct.
RecordedEstablished practice, not a measured result
Temperature
Temperature changes both the speed of fermentation and which organisms are favoured — it is one of the few variables shown to affect which species are present at all. Warmer maintenance shortens the time to peak and tends to favour lactic acid production; cooler, slower maintenance gives more acetic character. A starter kept somewhere consistently warm will need feeding more often than the same starter in a cool kitchen, which is why a schedule that worked in winter fails in summer.
The source supports temperature as a determinant of species present. The lactic-versus-acetic direction is professional consensus rather than a figure taken from this paper.
RecordedA. Comasio, M. Verce, S. Van Kerrebroeck and L. De Vuyst, Frontiers in Microbiology
Flour
Wholemeal and rye flours ferment faster than white, because they carry more of the minerals and enzyme activity that fermentation draws on. A starter switched from wholemeal to white will usually slow down for several refreshments before settling, which is routinely mistaken for the culture dying. Note what this does NOT mean: changing flour changes the rate, but the evidence does not support the common claim that it changes which organisms live there.
The rate claim is professional consensus with an accepted mechanism and no passage-level source. The cited paper is attached for the second half of the statement, which is a CORRECTION of a widespread claim rather than support for one.
RecordedA. Comasio, M. Verce, S. Van Kerrebroeck and L. De Vuyst, Frontiers in Microbiology
How to tell it is ready
These are observations, not a schedule. Time to peak depends on temperature, refreshment ratio and flour, and those interact — so the clock tells you roughly when to look, and the list below tells you what you are looking for.
- Sign 1
- Roughly doubled in volume from its post-feed level — judged against a mark on the jar, not against a remembered impression.
- Sign 2
- Domed rather than flat on top, with the surface just beginning to lose its curve at peak.
- Sign 3
- A network of bubbles visible through the side of the jar, not only on the surface.
- Sign 4
- Smells sharp and yeasty rather than solvent-like or cheesy.
- Sign 5
- A spoonful holds together and is aerated throughout when stirred, rather than collapsing to liquid.
Dormancy and revival
Keeping
Refrigeration slows the culture rather than stopping it, so a starter can be kept for one to several weeks between feeds in a domestic fridge. The organisms remain viable well beyond the point at which the culture looks unpromising: a dark liquid layer, a sharp acetone smell and no visible activity are signs of a hungry starter, not a dead one, and two or three refreshments at room temperature will usually restore normal behaviour. A culture that shows no gas production after several consecutive refreshments in warm conditions has genuinely failed.
The revival behaviour is consistent professional and domestic experience. The claim is directional — that dormancy is usually recoverable — and deliberately carries no maximum period, because the honest answer depends on the culture and the conditions.
RecordedEstablished practice, not a measured result
Why a starter is not a yeast substitute
Swapping a starter for packet yeast changes four things at once: the leavening rate, the acidity, the flour and water already in the culture, and the timescale. A formula converted by simply replacing one with the other will usually under- or over-ferment, because the quantity of active organisms in a starter is unknown and depends on when it was last fed. This is why sourdough method is written around observation of the dough rather than around stated times.
Discard, and what it is for
Discard is the portion removed before refreshment. It exists because feeding without removing anything means the culture doubles in mass at every feed, and because the discarded portion carries the accumulated acid that would otherwise keep building. It is not a waste product in the sense of being spoiled — it is simply an over-acidified, under-fed portion of the same culture. Its baking behaviour is different from active starter: it has little leavening power left but retains its acidity and its enzymes, which is why it is used where sourness and tenderising are wanted and lift comes from elsewhere.
What the clock cannot tell you
Almost every difficulty with a starter comes from following a schedule rather than an observation. The time to peak depends on temperature, refreshment ratio, flour and the culture's own condition, and these interact — so a starter that peaked in six hours last week may take four this week and twelve next. The indicators above are the actual answer to "is it ready"; the clock is a rough guide to when to look.
Related
Products raised with this
The defining relationship of the product: sourdough is bread raised by a maintained culture rather than by added yeast, which is why its timing is read from the dough rather than from a recipe.
A gluten-free starter ferments perfectly well — the organisms need starch, not gluten. What it cannot do is supply the gas-retaining phase, so a gluten-free sourdough loaf still needs a hydrocolloid. Recorded because bakers arriving from wheat sourdough routinely expect the culture to do the gum’s job.
Built and refreshed from
Flour supplies the starch the culture’s enzymes convert to sugar, and the minerals the organisms need. Changing the flour changes how fast the culture works — but not, on the evidence, which organisms live in it.
Water sets the hydration, which controls both how fast the culture ferments and whether it favours lactic or acetic character. Its temperature is also the one way a baker can damage a culture irreversibly.
The science behind it
Everything a starter does — the gas, the acid, the flavour, the gluten changes — is fermentation by two groups of organisms with different products.
Compared in
One side of the comparison, and the entity most of the observed demand is actually asking about.
Glossary terms defined here
Hooch
Hooch has no meaning outside a maintained culture, and the distinction from mould is the single most useful thing on the starter page.
Discard
Discard is defined by the refreshment cycle, so it belongs to the culture rather than standing alone.
A next step from
A poolish gives most of the flavour benefit of a pre-ferment with none of the maintenance, so it is the honest starting point for a baker who wants better bread but not a dependant.
History and culture
Sourdough, before and after commercial yeast
The maintained culture the whole history turns on.