· Nora Whitfield
What is a sourdough starter? What is actually in the jar
Most explanations stop at "flour and water left to ferment", which is true and tells you nothing. Flour and water left alone goes off. What makes a starter a starter is that two different organisms move in, settle into a stable arrangement, and then defend it. That arrangement is the whole subject.
Two organisms, two jobs
Open any healthy starter and you are looking at a partnership, not a single ingredient.
The wild yeast raises the bread. It eats simple sugars released from the flour and gives off carbon dioxide. That gas is caught in the gluten network of the dough, and the loaf rises because of it. This is the same job a packet of yeast does, done by strains that arrived on their own.
The lactic acid bacteria flavour it and protect it. They produce lactic acid, which reads as the mild tang, and acetic acid, which reads as the sharper sour note. They also drop the pH of the jar low enough that most spoilage organisms cannot establish themselves.
Neither half is optional. Yeast alone gives you a rise with almost no character. Bacteria alone give you an acidic paste that never lifts. A starter is the two of them holding a ratio, and that ratio is roughly a hundred bacteria for every yeast cell in a mature culture — which is why sourdough is often described as a bacterial ferment that happens to also raise bread.
Where the organisms come from
Not from the air, mostly. That is the romantic version and it is largely wrong.
They come from the flour. Wholegrain flour in particular carries the yeast and bacteria on the bran, which is why the standard advice is to begin a starter with rye or wholewheat even if you intend to feed it white flour afterwards. Your hands and your kitchen contribute, but the flour does the heavy lifting.
This has a consequence people find surprising: two starters begun from the same bag of flour in the same kitchen will not necessarily end up identical, and a starter carried to a new house does not change into a local one. What settles in the jar in the first fortnight tends to stay.
How it differs from a packet of yeast
Both raise bread. Everything else is different.
| Commercial yeast | Sourdough starter | |
|---|---|---|
| What it is | One strain, dried | A community of yeasts and bacteria, alive |
| Rise time | 1 to 2 hours | 4 to 12 hours, temperature dependent |
| Flavour | Neutral | Acidic, developed by the long ferment |
| Keeping | Stale bread in two days | The acids slow staling and mould |
| Upkeep | None, buy a sachet | Feeding, forever |
| Cost per loaf | Pennies | Flour for the discard |
Read the last two rows honestly. A starter is more work than a sachet, and the reason to keep one is the third row, not the first. If you want bread on the table in three hours, commercial yeast is the correct tool and no amount of enthusiasm changes that. A starter is for the loaf you plan a day ahead.
The vocabulary, in one table
Every sourdough guide assumes you already know these six words. Here they are.
| Word | What it means |
|---|---|
| Starter | The culture you keep permanently, fed on a routine |
| Levain | A portion taken off and fed specifically for one bake |
| Discard | The part you remove before feeding, so the jar does not double in size every day |
| Hooch | The grey liquid that gathers on top when a starter is hungry. Harmless. Stir it in or pour it off |
| Peak | The highest point of the rise, before it starts to fall. The moment to bake with it |
| Float test | A spoonful dropped in water. If it floats it is gassy enough to bake with |
Two of these cause most of the confusion. Discard sounds wasteful and is not: without it, a jar fed daily at a 1:1:1 ratio triples every feed and you would be carrying kilos of starter by the end of the week. And hooch gets thrown out by beginners who think the starter has died, when it is only asking to be fed.
Why it does not go off
This is the part that makes a starter genuinely unusual as a kitchen ingredient: it is a jar of flour and water left at room temperature that you are supposed to leave at room temperature.
It works because of the acid. A mature starter sits somewhere around pH 3.5 to 4.5 — comparable to orange juice — and most of the organisms that would spoil it cannot grow at that acidity. The bacteria that live there can, and so can the yeast alongside them. The culture builds the conditions that keep everything else out.
That protection is real but it is not unconditional. It depends on the culture being fed often enough to keep producing acid. A starter left for months on a counter, unfed, does eventually lose that defence, which is why long gaps belong in the fridge rather than on the worktop.
Reading it is the actual skill
Knowing what a starter is matters less than being able to tell what yours is doing today, and that comes down to one observation: how far did it rise, and how long did it take?
A starter that doubles in six hours at room temperature is in good shape. One that takes eighteen hours is sluggish, cold, or underfed. One that shoots up in three hours and collapses is being fed too little for how warm the room is. Same jar, same flour, three different conversations — and you can only have any of them if you can see the level.
This is the practical reason a starter lives in a straight-sided jar with a scale on it rather than in whatever container was free. Judged by eye against a curved wall, a rise from 200 to 400 ml and a rise from 400 to 500 ml look similar, and they mean opposite things. Read against printed graduations they are unmistakable. The jar this site is built around puts the scale on the glass for exactly that reason, and what size to use works the capacity out from the same arithmetic.
How long one lives
Indefinitely, in principle. A starter has no natural lifespan: it is a population that renews itself at every feed, so there is nothing in it that ages the way a food ages.
In practice they die of neglect, not of old age — forgotten at the back of a fridge for a year, or left unfed through a hot summer. Bakeries maintain cultures that have run continuously for decades, and there is no technical reason a domestic one cannot do the same. It needs flour, water, and someone who remembers.
A starter that looks dead often is not. Grey liquid, a sharp acetone smell and no rise usually mean hungry rather than gone. Two or three feeds on a warm counter revive most of them. The genuine end point is fuzzy growth in colours — pink, orange, black — and that one is not recoverable.
Some numbers
starters sequenced across four continents in the largest survey of sourdough microbiomes to date, which found the communities in home cultures to be far more varied than the textbook account suggested
— Landis et al., eLife, 2021
the capacity that suits a home starter: room for 200 ml to triple, with a printed scale so the rise can be read rather than estimated
— Technical sheet, 2026
Where to go next
If you do not have one yet, making a starter takes the first fourteen days one day at a time, including the false start on day three that convinces most people they have failed. Once it is alive, feeding covers ratios and timing, and cleaning the jar turns out to be a ten-second job rather than a chore. For everything that lives alongside it, see sourdough tools, or the kits if you would rather start with the whole set.