Vol. XIV · Fermentation Plate 2 of 11yeast, fermentation, carbon dioxide, alcohol, ethanol, bread rise, proofing, wine, beer, 米酒, temperature, stoichiometryMT

Yeast Fermentation.

Every 180 grams of sugar a yeast ferments becomes 92 grams of alcohol and nearly 50 litres of carbon dioxide.

Without air, yeast gets its energy by splitting sugar: C6H12O6 → 2 C2H5OH + 2 CO2. The arithmetic is exact: 180 g of glucose gives 92 g of ethanol and 88 g of carbon dioxide, about 49 litres of gas at room temperature. In a bread dough a few grams of sugar — released from starch by the flour's own enzymes — make the litres of gas that the gluten network traps (VI.10, XXIV); the alcohol evaporates in the oven, leaving flavour. In beer, wine and 米酒 the alcohol is the point, and the gas escapes or is kept for sparkle.

Yeast works fastest in the low thirties °C and slows sharply below 15 °C; above about 40 °C it struggles, and it dies near 55–60 °C. Bakers use that: a warm proof for speed, a cold, slow overnight proof in the fridge for more flavour, since the slow time lets enzymes and bacteria add complexity while the yeast barely moves. Salt and lots of sugar slow yeast; too much time lets the dough over-proof and collapse. The workshop raises a dough.

Below: the volume of a lean dough against hours at the temperature and yeast you choose (dough bench, teaching model calibrated to common bakery timings), and the gas and alcohol from the sugar fermented (exact stoichiometry).

What you will learn
Workshop
Dough bench: gas production ∝ yeast^0.8 × yeast activity at the temperature (optimum near 34 °C), slowed above 2 % salt and 10 % sugar; the dough holds gas up to about 3.2 times its volume, then over-proofs. Calibrated so 1 % instant yeast at 26 °C doubles a lean dough in about 1.6 hours. Stoichiometry exact.

Gas for bread, alcohol for drink

Ten grams of sugar fermented give about 2.7 litres of carbon dioxide — enough, trapped by gluten, to double a kilogram of dough several times over, which is why a little yeast goes a long way. In this model a lean dough with 1 % instant yeast doubles in about 1.6 hours at 26 °C and 1.3 at 32 °C; halving the yeast stretches the time at 26 °C to 2.8 hours; in the fridge at 4 °C it grows to only about 1.1 times its volume in twelve hours.

Use the dial deliberately. A warm, quick proof gives a bread that tastes of little but wheat and yeast; a cool, slow one — overnight in the fridge, or with less yeast at room temperature — gives a deeper, more complex flavour and a more open, blistered crust. Proof by the dough, not the clock: it is ready when it has roughly doubled and a floured finger pressed in leaves a dent that springs back slowly. For drinks, the same chemistry with a sweet liquid and no air makes alcohol; Chinese 米酒 and Japanese sake use koji first to turn rice starch into sugar for the yeast (XIV.7).

At the stove

Mix two small identical doughs. Leave one on the counter and put the other in the fridge overnight. Bake both: the slow one usually has the richer taste and the darker crust.

Commercial baker's yeast was first produced in the 1860s and 1870s by distillers and in Vienna; before that, bakers used barm (the foam from brewing) or kept a sourdough (XXIV.7).

Terms

Yeast
A single-celled fungus; Saccharomyces cerevisiae for bread, beer and wine.
Proofing
Letting a shaped dough rise before baking.
Over-proofing
Rising too long, so the gas network weakens and collapses.
Cold retard
Slowing fermentation in the fridge for flavour and convenience.
Yeast splits sugar into alcohol and carbon dioxide (180 g → 92 g + 88 g). Temperature and yeast amount set the pace; slow and cool gives more flavour.

Sources

  1. Exact stoichiometry: glucose → 2 ethanol + 2 CO2 (180 g → 92 g + 88 g); ethanol + O2 → acetic acid + water (46 g → 60 g); ideal-gas molar volume 24.5 L at 25 °C.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.