Yeast
A single-celled fungus that ferments sugars to carbon dioxide and ethanol, providing both lift and a large part of bread’s flavour.
Also called baker’s yeast (TRADE); fresh yeast (UK) — The compressed form; behaves the same but is dosed differently by weight from dried.
Baker’s yeast is a living organism, and every practical fact about handling it follows from that.
It metabolises simple sugars and produces carbon dioxide, which inflates the dough, and ethanol and other compounds, which contribute flavour. It cannot use starch directly, so it depends on flour’s own enzymes to supply fermentable sugars — which is why a lean dough with no added sugar ferments perfectly well.
Its activity roughly increases with temperature up to a point and then falls sharply as the cells are killed, which happens well below baking temperature. This is why liquid that is too hot kills yeast outright, and why dough temperature is the most effective control over fermentation rate available to a home baker.
Salt restrains it, sugar in high concentrations stresses it osmotically, and fat coating the dough slows gas movement — all reasons an enriched dough ferments more slowly than a lean one.
What changes if it is left out
Without a biological leavener the product must be leavened chemically, mechanically or by steam, and it becomes a different product. A chemical leavener produces gas but none of the fermentation flavour, which is why a soda bread and a yeasted loaf taste unrelated.
Unsourced because it restates the fermentation-flavour relationship documented on the fermentation science page rather than adding a claim. The comparison between soda bread and a yeasted loaf is an observation about two products rather than a measured result.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Storage and deterioration
Dried yeast keeps for months sealed and cool and loses activity once opened. Fresh yeast is perishable and keeps for a short time refrigerated. Reduced activity shows as slow fermentation rather than as no fermentation, which makes it an easy fault to misattribute.
Unsourced. Shelf lives are set by manufacturers and printed on the packet, and quoting a duration here as a property of yeast in general would be the kind of false precision this model exists to avoid. The point about partial loss of activity being easy to misattribute is BakeHQ’s own observation.
RecordedEstablished practice, not a measured result
Related
Used in
A measured dose of a single organism, which is what makes this loaf predictable in a way sourdough is not.
The only leavening.
A small dose over a long fermentation.
Leavened, for flavour and handling.
Slowed by everything else in the list.
Fast at this scale.
Slowed by the enrichment.
Fermenting while the butter needs the dough cold.
What it does
Aeration
Carbon dioxide from fermentation inflates cells the dough already contains rather than creating new ones.
Flavour contribution
Fermentation by-products account for a large part of why a slowly fermented loaf tastes of more than flour and salt.
The science behind it
Every practical rule about handling yeast follows from it being a living organism with a temperature-dependent metabolism.
Cultures built from this
Apply this with a tool
Converting between yeast forms, with the conventional ratio labelled as a convention rather than a measurement.
Compared in
Instant yeast vs active dry yeast
Both sides are the same organism dried differently, so the comparison points at one ingredient rather than two. The difference readers are asking about is handling, not species.
One side of the comparison, so the comparison is reachable from the record a reader is more likely to land on first.
One side of the comparison.
Covered in
What flavourings actually do, beyond tasting of themselves
The salt-and-yeast contact question, stated with the concentration and the contact rather than as either absolute.
Commonly confused with
The same organism wet or dry, with a weight conversion between them that is widely quoted as though it were exact.