Fermentation
The metabolism of sugars by yeast and bacteria into carbon dioxide, alcohol, acids and a large share of bread’s flavour.
Established science
Measured and reproducible food science, described in textbooks or peer-reviewed work. The mechanism and the numbers are both supported.
Fermentation in bread is the metabolic activity of yeast and, in a sourdough, of lactic acid bacteria alongside it.
Yeast converts simple sugars principally to carbon dioxide and ethanol. The carbon dioxide inflates the gas cells the dough already contains — it does not create new ones — and the ethanol and other by-products contribute flavour, most of which is driven off or transformed during baking.
In a sourdough, lactic acid bacteria are present in far greater numbers than yeasts and produce lactic and acetic acid, lowering the pH. That acidity affects gluten behaviour, enzyme activity, flavour and keeping quality.
Rate depends most strongly on temperature, then on the quantity of leaven, then on the availability of fermentable sugar. Salt restrains it. This is why fermentation is properly judged by the dough’s state rather than by a clock.
What is actually established
Sourdough microbiota
A mature sourdough is a community of lactic acid bacteria and yeasts, with bacteria outnumbering yeasts by roughly 10:1 to 100:1.
SPRINT 7: re-cited from an unread handbook to a paper BakeHQ had already read for Sprint 4 and which states the figure directly — the ratio “is mostly 10:1 to 100:1”, and the Discussion confirms this held for most of the sourdoughs the study itself analysed. The claim gained a number in the process: “far outnumbering” was vaguer than the evidence available.
RecordedA. Comasio, M. Verce, S. Van Kerrebroeck and L. De Vuyst, Frontiers in Microbiology, Marco Gobbetti and Michael Gänzle (eds.), Springer
Source of fermentable sugar in a lean dough
Flour’s own amylases break damaged starch down into maltose, which is the most abundant sugar available to yeast in a lean dough — so added sugar is not required for fermentation.
SPRINT 7: the read source states this almost word for word for a dough of flour, water, yeast and salt, and names the sugar. BakeHQ’s claim previously said only “sugars”; it now says maltose, and says which starch the enzymes reach — the damaged granules, not the intact ones.
RecordedV. J. Higgins, M. Braidwood, P. Bell and P. Bissinger, Applied and Environmental Microbiology, Jan A. Delcour and R. Carl Hoseney, AACC International
What you can do with this
Dough temperature is the most effective control a baker has over fermentation rate, and it is far more powerful and more precise than adjusting the amount of leaven. Controlling the temperature of the water is the easiest way to control the temperature of the dough.
What is not settled
Which specific organisms are present in a given sourdough culture, and how stable that population is, varies considerably and is not predictable from how the culture was started. Popular claims that a culture reflects the flora of a particular place or of the baker’s hands are not well supported and should be treated as a traditional account rather than a finding.
Related
Ingredients this explains
Every practical rule about handling yeast follows from it being a living organism with a temperature-dependent metabolism.
Added amylases do at a standardised rate what the flour’s own enzymes already do, which is the claim the fermentation record carries a verified passage for.
Techniques this explains
The stage’s whole behaviour — its temperature dependence, its window, its flavour development — is fermentation kinetics.
Causes this explains
Underproofed
Fermentation rate is strongly temperature-dependent, which is exactly why a stated time fails in a different kitchen.
Overproofed
Gas production continues while enzymes and acidity degrade the network, so the two curves cross.
It has gone too long between feeds
A culture consumes what its enzymes release and excretes acid as it goes. Left unfed it runs out of the first and is suppressed by the second.
The dough was too cold to ferment at the expected rate
Rate against temperature, which is the whole of this cause.
The dough was warmer than intended
The same relationship read from the other end, plus the softening of the network that makes a warm dough handle wet.
The dough was enriched, and enrichment slows fermentation
Why sugar and fat slow a dough that is otherwise identical.
There was more yeast than the dough needed
Gas, ethanol and acids are produced together; more organisms produce all of them faster.
History and culture
The traditional account is a story about the discovery of a process the science page describes mechanistically.
Why regional breads differ, and what actually causes it
Climate decides fermentation rate, which is one of the four constraints the record identifies behind regional difference.
Variables that move this
Dough temperature
Fermentation rate is strongly temperature-dependent, which is why dough temperature is a more precise control than the quantity of leaven.
Refreshment ratio
Carrying less old culture forward dilutes both the accumulated acid and the population, which is the most effective control a baker has over how sour a starter becomes.
Cultures this explains
Everything a starter does — the gas, the acid, the flavour, the gluten changes — is fermentation by two groups of organisms with different products.
A pre-ferment is a way of buying fermentation time for part of the flour, so the science it depends on is the same one with a different schedule.
Compared in
Rapid process vs long fermentation
The other side. Time is the thing the rapid process removes, and this is the record describing what time was doing.
Both states are fermentation gone too far or not far enough, which is why temperature matters more than the clock.
Covered in
Oven spring — what it is and what causes it
Covered by the guide, which is the resource a reader arrives at with this question.
What freezes well, and what freezing actually does
Why a yeasted dough is the hard case — freezing kills a proportion of the yeast, and the final proof takes longer than the recipe says.
What flavourings actually do, beyond tasting of themselves
Where the rate effect of salt belongs, and why an unsalted dough is over-proved by the time the recipe says to bake it.