The oldest fermentation in the world
Alcoholic fermentation is a yeast survival mechanism hundreds of millions of years old. Saccharomyces cerevisiae ferments sugar to gain energy under anaerobic conditions, and the ethanol it produces is — from the yeast's perspective — merely metabolic waste.
The global equation — and why it is incomplete
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (ΔG = −235 kJ/mol)
Correct in atomic balance, but between glucose and ethanol there are ten enzymatic reactions in two blocks: glycolysis (glucose → pyruvate, net +2 ATP and +2 NADH) and fermentation proper (pyruvate → ethanol, regenerating NAD⁺).
Block 2: the two decisive steps of fermentation
Step 1 — Pyruvate decarboxylation: Pyruvate decarboxylase (PDC), unique to yeasts and plants, removes CO₂ from pyruvate to produce acetaldehyde. This requires thiamine pyrophosphate (TPP) as cofactor.
CH₃CO·COOH → CH₃CHO + CO₂
Step 2 — Acetaldehyde reduction to ethanol: Alcohol dehydrogenase (ADH) reduces acetaldehyde to ethanol using NADH from glycolysis. NAD⁺ is regenerated and glycolysis can continue.
CH₃CHO + NADH + H⁺ → CH₃CH₂OH + NAD⁺
The yeast's real biological purpose in step 2 is not to produce alcohol — it is to regenerate NAD⁺ to maintain glycolysis running under anaerobiosis.
Beyond ethanol: fermentation byproducts
Real-world fermentation (wine, beer, cacao) produces dozens of compounds besides ethanol: glycerol (body and smoothness), succinic acid (umami, clean acidity), ethyl acetate (fruity notes at low concentration), fusel alcohols (leucine-derived isoamyl alcohol = banana aroma in ales).
Temperature and process control
Fermentation temperature determines which metabolic pathways predominate: white wines at 10–15 °C retain volatile aromatic esters; red wines at 25–28 °C extract more anthocyanins; lager beers at 5–10 °C produce clean profiles; cacao at 45–52 °C selects heat-tolerant Hanseniaspora yeasts with specific aroma contributions.