Fermenting a fuel-ethanol wash means dissolving 15-20% sugar in water, cooling it to 25-35°C, adding yeast and holding 25-30°C under an airlock for 5-14 days until bubbling stops, which gives a wash of roughly 9-12% alcohol. Each kilogram of sugar becomes at most 0.51 kg of ethanol and 0.49 kg of carbon dioxide, so a 100-litre wash releases about 4.2 cubic metres of CO₂, which suffocates anyone who stands in a closed room or pit where it collects.
Part of Ethanol Fuel Production
Fermentation is the stage where yeast converts dissolved sugar into ethanol and carbon dioxide, producing the dilute alcohol wash that distillation concentrates.
The Reaction Sets How Much Fuel And How Much Gas
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. In mass: 180.16 g of glucose gives 92.14 g of ethanol and 88.02 g of CO₂, so about 0.51 kg ethanol (0.511) and 0.49 kg CO₂ (0.489) per kg of sugar. Yeast keeps some sugar for its own growth, so plan on about 95% of the ideal.
Strength follows from the sugar you start with. A wash with 150-200 g of sugar per litre can make at most 150 × 0.511 × 0.95 = 73 g and 200 × 0.511 × 0.95 = 97 g of ethanol per litre, which is 73 ÷ 0.789 = 92 mL and 97 ÷ 0.789 = 123 mL of ethanol per litre, so about 9% to 12% by volume. The parent’s 8-15% range reaches 15% only with far more sugar than the 15-20% it recommends, and with a yeast that tolerates it (Yeast Selection & Propagation).
Method
Step 1: Make the wash. Sugar crops are pressed (Sugar Crops); starch crops are cooked and converted first (Mashing Starch Crops). Dilute to 15-20% sugar.
Step 2: Cool and pitch. At 25-35°C add yeast; above about 40°C it dies. Fermentation shows as bubbling in 6-24 hours.
Step 3: Hold temperature and keep air out. Keep 25-30°C under an airlock: a one-way route for CO₂ so air and bacteria cannot enter (Anaerobic Conditions). Vessel details are on Fermentation Vessel Design.
Step 4: Watch it finish. Expect 5-14 days depending on yeast and temperature (3-5 with turbo yeast, 5-7 with bread yeast). It is done when bubbling stops and a hydrometer reading stops falling over two days (Monitoring & Troubleshooting). Then draw the wash off the settled yeast and distil (Distillation).
Worked Example: The Gas In A Cellar
100 L of wash at 10% holds 10 L of ethanol = 7.89 kg = 7,890 ÷ 46.07 = 171 mol. Each mole of ethanol comes with one mole of CO₂, and a mole of gas fills about 24.5 litres at 25°C, so 171 × 24.5 = 4,190 L, about 4.2 m³ of CO₂, over the few days of the main fermentation.
A closed room 3 m × 3 m × 2.5 m is 22.5 m³. If all of the gas stayed in it, 4.2 ÷ 22.5 = 19% of the air would be CO₂. Anything above a few percent causes headache and rapid breathing, and well above that collapse; the US NIOSH figure for “immediately dangerous to life” is 4% (40,000 ppm, recalled). CO₂ is 44 ÷ 29 = 1.52 times as heavy as air and settles into pits and cellar floors (Gas Hazards (Methane, CO), written for coal workings, sets out where each gas collects).
Carbon dioxide from a fermenter can kill in a closed space
Ferment outdoors or in a room with a permanent opening at floor level and another high up. Never ferment in a sealed room, cellar or pit, and never climb down into one that has held a ferment without airing it first. Fit a vent or airlock, never a sealed lid: a tightly closed container of fermenting wash will burst. Never enter a fermentation pit to rescue someone; ventilate and haul them out with a line.
Common Mistakes
- Sealing the vessel completely. Gas pressure bursts the container; use an airlock.
- Starting too sweet. Above the 15-20% range the yeast stalls and leaves sugar behind.
- Pitching hot. Wash above 40°C kills the yeast and the batch never starts.
What’s Next
- Learn next: Yeast Selection & Propagation, because the yeast sets the strength
- Combine with: Yeast Biology, the corpus’s alcohol-oriented guide to the organism