Fermentation uses microbes to convert sugar into three families of product: alcohols and carbon dioxide (yeast, no oxygen), organic acids such as lactic acid (lactic bacteria) and acetic acid (Acetobacter, which needs oxygen), and methane (anaerobic digestion). One kilogram of glucose gives at most about 0.51 kg of ethanol and 0.49 kg of carbon dioxide. The organism and the oxygen supply decide which product you get.
Part of Organic Chemistry
The acids, alcohols and gases that microbes make from sugars and starches, and which ones a rebuilding community can use as fuel, food and chemicals.
The Same Sugar Gives Different Products Under Different Conditions
The carrier’s subtitle names the outputs: acids, alcohols and gases from microbes.
- Alcohol and carbon dioxide. Yeast acting without oxygen on sugar. The reaction is C6H12O6 → 2 C2H5OH + 2 CO2. The deeper treatment is Fermentation Science and Yeast Biology.
- Acetic acid. Acetobacter oxidises alcohol to vinegar and needs oxygen, so it is an aerobic second stage after the yeast stage. See Vinegar Production.
- Lactic acid. Lactic bacteria acidify vegetables and milk; the corpus covers this under food (Lacto-Fermentation).
- Methane and carbon dioxide. Mixed anaerobic microbes digest manure and plant waste into biogas, covered in Anaerobic Digestion Science.
What You Need
- Sugar or starch. Starch must be converted to sugar first (Grain Mash).
- A yeast you can propagate, in a vessel that vents CO2 (Fermentation Vessel Design).
- Temperature control in the range the organism likes; hot or cold stalls the ferment.
- Hygiene, because contaminating bacteria spoil the product.
Worked Example: How Much Ethanol And CO2 From A Kilogram Of Sugar
Use glucose, C6H12O6, with rounded atomic masses C = 12, H = 1, O = 16.
- Mass of one mole of glucose: 6 × 12 + 12 × 1 + 6 × 16 = 72 + 12 + 96 = 180 g.
- Mass of one mole of ethanol, C2H6O: 2 × 12 + 6 × 1 + 16 = 46 g. Two moles: 92 g.
- Mass of one mole of CO2: 12 + 2 × 16 = 44 g. Two moles: 88 g.
- Check: 92 + 88 = 180 g, so mass is conserved.
- Ethanol per kilogram of glucose: 92 ÷ 180 = 0.511 kg. CO2: 88 ÷ 180 = 0.489 kg.
This is a theoretical maximum. Real ferments fall below it, because yeast also grows and makes by-products, and the corpus’s Fermentation page reaches the same 0.51 and 0.49 figures.
Common Mistakes
- Stoppering a fermenter tightly. CO2 builds pressure; use an airlock or open vent.
- Working with CO2 in a closed space. It is heavier than air, collects at floor level and displaces oxygen; the Fermentation page above gives the volume a 100 L wash produces.
- Assuming a ferment is safe to drink because it smells normal. See the Safety block.
Safety
Fermentation makes carbon dioxide, and the products have their own hazards
CO2 pools in pits, cellars and low rooms: ventilate. Methanol is formed from pectin, so fruit and pomace ferments carry far more than grain or sugar washes. The EU limits are 30 g per hectolitre of pure alcohol in neutral spirit against 1,000 to 1,500 in fruit and pomace spirit, a 33- to 50-fold difference (see Methanol Hazards, which is the authoritative page). Discarding the first part of a distillation run is a flavour step and does not remove the methanol. Anaerobic digesters release hydrogen sulfide; never enter a digester pit.
What’s Next
- Learn next: Bioethanol — fermenting for fuel
- Combine with: Esterification — the alcohol and the acid made here are the inputs