Science
The chemistry of alcoholic fermentation
What yeast actually do to sugar, why the schoolbook equation is a simplification, and where the flavour compounds come from.
The short answer
Yeast metabolise sugar through glycolysis and then convert the product to ethanol and carbon dioxide, releasing energy they use to grow. The familiar one-line equation describes the overall balance, not the pathway, and it accounts for none of the hundreds of secondary compounds that give fermented wine its character.
Overview
Fermentation is often reduced to a single reaction, and that reduction is where a great deal of confusion starts.
The overall balance is real: roughly, one molecule of glucose yields two of ethanol and two of carbon dioxide. As a rule of thumb, sugar at about 17 g/L yields around 1% alcohol by volume, though the conversion varies with yeast strain and conditions.
What the equation omits is everything responsible for wine tasting of anything at all.
How it works
The mechanism, at the scope it has actually been established.
The pathway is glycolysis: glucose is broken down through a sequence of enzyme-catalysed steps to pyruvate, generating ATP for the cell. In the absence of oxygen, pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol, regenerating the cofactor the cell needs to keep glycolysis running.
That last step is the point of the whole exercise from the yeast’s perspective. Ethanol is a waste product; the cell is after the energy.
Fermentation is strongly exothermic. Uncontrolled, a red fermentation can rise well above 30°C, and temperature control is one of the defining technologies of modern winemaking.
Alongside ethanol, yeast produce glycerol (contributing texture), higher alcohols, esters, aldehydes, organic acids and volatile sulfur compounds. Several hundred compounds have been identified, and the balance among them varies with strain, temperature, nutrition and oxygen.
Esters in particular are strongly temperature-dependent, which is the mechanism behind cool fermentation being commonly used for aromatic whites: lower temperatures are associated with higher retention of the fruity esters produced.
Carbon dioxide production is substantial and is a genuine hazard in an enclosed cellar. It also forms the protective blanket that limits oxidation during fermentation.
Why it matters in the glass
It explains why fermentation temperature is a stylistic control and not just a safety measure, and why the same juice fermented two ways gives two different wines.
It also explains stuck fermentations, which are principally a nutrition, temperature or alcohol-tolerance problem rather than bad luck.
Compounds named on this page
Where a compound comes from is chemistry; what it smells like is perception. These are kept apart deliberately.
- Ethanol — Ethyl alcohol
- The principal product of fermentation, and the compound that makes wine wine. Caution: Ethanol does several distinct things at once, and "high alcohol" is routinely used as a single criticism when the actual complaint is usually imbalance.
- Glycerol
- A fermentation by-product contributing viscosity and a slight sweetness. Caution: Frequently credited with producing the legs on a glass. Alcohol dominates that effect; glycerol modifies it.
- Esters
- Fermentation-derived compounds responsible for much of the fruitiness in young wine. Associated with banana, pear drop, green apple, strawberry, tropical fruit. Caution: No single ester accounts for a descriptor. Attributing "banana" to isoamyl acetate is a shorthand, not an identification.
- Acetaldehyde
- The principal oxidation product — a fault in fresh wine, definitional in Fino Sherry. Associated with bruised apple, sherry, nuts. Caution: Bound by free sulfur dioxide, which both removes the aroma and explains why a wine low in free SO₂ becomes vulnerable.
The contribution of individual yeast strains to finished wine character is real and measurable in controlled trials, but how much of it survives ageing, blending and bottle development in a commercial wine is much less clear.
Go shallower or deeper
The beginner page and the chemistry page are both correct, and neither substitutes for the other.
Sources
Sources consulted
- The Australian Wine Research Institute — AWRI
- UC Davis Department of Viticulture and Enology — University of California, Davis
- International Code of Oenological Practices — OIV
More in wine science
Science
Acidity: the acids in wine and what each does
Tartaric, malic, lactic, citric and acetic acid — where they come from, how they differ, and why pH and total acidity are not the same measurement.
Science
Cool climate and warm climate
The single most useful distinction in wine — what it actually changes in the grape, and where the shorthand breaks down.
Science
Minerality: what the evidence actually shows
A word almost every taster uses, no agreed chemical definition, and no demonstrated route from vineyard rock to matching flavour.