Sourdough starter vs poolish
One is a wild population kept alive indefinitely; the other is built with commercial yeast for a single bake and used up.
The short answer
They are not variants of one another. A starter is a maintained ecosystem that both raises and acidifies bread; a poolish is a short-lived yeasted pre-ferment that contributes flavour and dough character but almost no acidity. Substituting one for the other changes the timing, the flavour and the handling all at once.
Where they differ
| Dimension | First | Second |
|---|---|---|
| What is living in it | A wild mixed population — lactic acid bacteria dominant, yeasts at roughly a tenth to a hundredth of their number. | Commercial baker’s yeast, plus whatever bacteria the flour contributes over the fermentation time. |
| How long it lasts | Indefinitely, if refreshed. Some are decades old. | Hours. It is built for one bake and used entirely. |
| What it contributes | Leavening AND acidification — the acid changes flavour, gluten behaviour, keeping quality and mould resistance. | Flavour and extensibility. Very little acid, and no meaningful change to keeping quality. |
| How predictable it is | Variable. Its power depends on when it was last fed and at what temperature, which is why sourdough method is written around observation. | Predictable. The yeast quantity is known, so the timing can be specified. |
| Effort | Ongoing. It has to be fed whether or not you are baking. | None between bakes, because there is nothing to keep. |
Differences that do not matter
These are real distinctions that are unlikely to change your decision. They are listed so you can stop thinking about them rather than left out so the comparison looks decisive.
- Hydration. A choice, commonly 100 per cent for arithmetic convenience. 100 per cent by definition. A stiff one is called something else.
Why you cannot simply swap them
The quantity of active organisms in a starter is unknown and changes with its schedule, while a poolish carries a known quantity of yeast. So a formula converted from one to the other cannot keep its timings.
The acid matters at least as much. A sourdough’s acidity strengthens the dough early and degrades it late, changes how the crumb keeps, and is a large part of the flavour. A poolish supplies none of that, so a loaf built on one tastes and behaves differently even where the rise is equivalent.
Related
Compares
One side of the comparison, and the entity most of the observed demand is actually asking about.
The other side. Poolish exists as a record largely because this comparison needed a real entity to point at rather than a paragraph.