Alcoholic Fermentation: From Sugar to Ethanol, the Complete Chemistry

The world's oldest fermentation

About 9,000 years ago, in what is now China, someone left rice, honey and fruit in a pottery vessel. Wild yeasts did the rest. The result is the oldest fermented drink ever identified chemically (in jars from the Neolithic site of Jiahu).

But alcoholic fermentation did not start with humans. It is a survival mechanism of yeasts that is hundreds of millions of years old. Saccharomyces cerevisiae ferments sugar to obtain energy without oxygen, and the ethanol it produces is, from the yeast's point of view, simply metabolic waste. From the human point of view, it is the active ingredient of every alcoholic drink, and fermentation is also what makes bread rise.

The overall equation, and why it is incomplete

Alcoholic fermentation is usually summed up in one equation:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (ΔG ≈ −235 kJ/mol)

The atom balance is right, but the equation is deceptively simple. Between glucose and ethanol lie twelve linked enzymatic reactions in two blocks: glycolysis (which breaks glucose down to pyruvate) and fermentation proper (which turns pyruvate into ethanol). Understanding these steps explains why fermentation always produces more than just ethanol and CO₂.

Block 1: glycolysis, from glucose to pyruvate (10 steps)

Glycolysis happens in the yeast's cytoplasm and turns one molecule of glucose into two of pyruvate, with a net gain of 2 ATP and 2 NADH:

The investment phase (steps 1–5): glucose is phosphorylated twice (using 2 ATP) and split into two three-carbon fragments, glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). DHAP is isomerised to G3P, giving two G3P per glucose.

The payoff phase (steps 6–10): each G3P is oxidised to pyruvate in five reactions that generate a total of 4 ATP and 2 NADH. Net balance: +2 ATP and +2 NADH per glucose.

Pyruvate is the central metabolic hub: with oxygen, it would enter the mitochondrion for the Krebs cycle (aerobic metabolism). Without oxygen, the yeast takes the fermentation exit.

Block 2: alcoholic fermentation in two decisive steps

With pyruvate made and no oxygen for the respiratory chain, the yeast has a problem: the 2 NADH from glycolysis must be reoxidised to NAD⁺ for glycolysis to keep going. Alcoholic fermentation solves this in exactly two steps:

Step 1: decarboxylation of pyruvate. Pyruvate decarboxylase (PDC), found in yeasts and plants but absent in mammals, removes CO₂ from pyruvate to give acetaldehyde. It needs thiamine pyrophosphate (TPP) as a cofactor.

CH₃CO·COOH → CH₃CHO + CO₂

Acetaldehyde is responsible for much of alcohol's toxicity, more than ethanol itself. In human ethanol metabolism, acetaldehyde is the intermediate between ethanol and acetate, and its build-up (in people with variants of aldehyde dehydrogenase ALDH2) causes the facial flushing and nausea of the "alcohol flush reaction".

Step 2: reduction of acetaldehyde to ethanol. Alcohol dehydrogenase (ADH) reduces acetaldehyde to ethanol using NADH from glycolysis as the electron donor. NAD⁺ is regenerated and glycolysis can continue.

CH₃CHO + NADH + H⁺ → CH₃CH₂OH + NAD⁺

This second step gives fermentation its name and produces the ethanol. But its real biological purpose for the yeast is not to make alcohol; it is to regenerate NAD⁺ so glycolysis can keep running without oxygen.

Ethanol: structure and physical properties

Ethanol (C₂H₅OH, ethyl alcohol) is, after methanol, the simplest alcohol. Its physical properties come straight from its structure:

Hydrogen bonds. Ethanol's –OH can act as both donor and acceptor of hydrogen bonds. That explains its relatively high boiling point (78.4 °C) for its molecular weight (46 g/mol), far above that of propane (−42 °C), which has a similar mass but no hydrogen bonds.

Complete miscibility with water. The (hydrophobic) ethyl group is small enough not to prevent the –OH from interacting with water. As its concentration rises, ethanol progressively disrupts the structure of water, lowering its surface tension and viscosity; that is why wine forms "tears" on the glass (the Marangoni effect).

Dissolving organic compounds. Ethanol is an intermediate solvent that dissolves both polar compounds (sugars, acids) and non-polar ones (terpenes, lipids). This property is key to macerating herbs, extracting aromas when making liqueurs, and drawing phenolic compounds out of grape skins when making red wine.

Beyond ethanol: the by-products of fermentation

The "real" fermentation of wine, beer or cacao produces dozens of compounds besides ethanol. The most important:

Glycerol. Produced especially early in fermentation, when the cell needs to regenerate NAD⁺ to keep its redox balance. Glycerol (3–7 g/L in wine) contributes to body and a smooth mouthfeel.

Succinic acid. Already discussed in the article on cacao fermentation; yeasts also make it as a by-product of a truncated citrate cycle under anaerobic conditions.

Ethyl acetate. Made by yeast enzymes from acetyl-CoA and ethanol (and, more slowly, by chemical esterification of acetic acid with ethanol). At low concentrations (<150 mg/L) it adds fruity notes; in excess it causes the "glue" or "nail-polish remover" fault in wines and beers.

Higher (fusel) alcohols. Leucine, isoleucine and valine are broken down by the Ehrlich pathway into isoamyl alcohol, active amyl alcohol (2-methylbutanol) and isobutanol, the "fusel alcohols" that give warmth and body to spirits. In excess, they contribute to the burn of cheap spirits.

Aroma esters. Isoamyl acetate (banana), 2-phenylethyl acetate (rose), ethyl hexanoate (apple): the aromatic fraction of fermentation that defines the fruity character of young wines and ale-style beers.

Yeast, temperature and process control

Saccharomyces cerevisiae works best between 18 and 30 °C, with maximum speed around 30–35 °C. Yet the fermentation temperatures chosen in industry are often lower:

• White and sparkling wines: 10–15 °C → slow fermentation, better retention of volatile aroma esters
• Red wines: 25–28 °C → more extraction of anthocyanins, more body
• Lager beers: 5–10 °C (Saccharomyces pastorianus) → a clean profile without fruity esters
• Ale beers: 18–24 °C (S. cerevisiae) → a fruity, spicy profile
• Cacao: the yeast phase (Hanseniaspora, Pichia, S. cerevisiae) happens in the first 24–48 hours, below 40 °C; afterwards, acetic acid bacteria oxidise the ethanol and the mass heats up to 45–50 °C

Temperature does not just change speed; it shifts the balance between metabolic pathways, deciding which by-products form and in what proportions. The same sugar, the same yeast, different temperatures: completely different aroma profiles.

The next article looks at the most ubiquitous acid in the food industry: citric acid, found in almost everything processed we buy, with a fascinating industrial history and chemistry more interesting than its label suggests.