Cider science

What fermentation actually does

What happens chemically when cider ferments?

In short

Yeast takes each six-carbon sugar molecule, splits it through the glycolytic pathway into two three-carbon fragments, and converts those to two molecules of ethanol and two of carbon dioxide, capturing a small amount of energy in the process.

That reaction accounts for the mass balance — roughly equal weights of ethanol and carbon dioxide from the sugar consumed — but not for the flavour. Flavour comes from the rest of the yeast’s metabolism: glycerol from redox balancing, higher alcohols from amino acid processing, esters from the reaction of those alcohols with acids, and sulphur compounds where nitrogen runs short.

Alongside the yeast, bacteria may convert malic acid to lactic acid, and, if oxygen is available, acetic acid bacteria will convert ethanol to acetic acid.

Specific gravity falling over eight weeks, with the organisms that dominate each part of the ferment shown above it.

Described in full

The gravity curve: Specific gravity falls from about 1.050 at pitching to about 1.000 at completion. The fall is not even: it is slow for the first few days, steepest between roughly week one and week three, and then flattens into a long tail.
The lag phase: Very little gravity is lost in the first days. Yeast is multiplying rather than producing much alcohol, and a wild ferment can sit apparently inert for a week before anything visible happens.
Apiculate yeasts, weeks nought to one and a half: Hanseniaspora and Kloeckera species carried in on the fruit dominate at the start. They make esters and acetic acid, tolerate very little alcohol, and die back once ethanol reaches a few percent.
*Saccharomyces*, weeks half to five: Takes over as the apiculates fail and does the bulk of the work. This band overlaps the steepest part of the gravity curve, which is the visible consequence of the takeover.
Malolactic, weeks four to eight and beyond: Optional, drawn with a dashed edge because it may not happen at all. Lactic acid bacteria convert malic acid to lactic acid once the sugar is largely gone, softening acidity and sometimes leaving a buttery note.
Why the bands overlap: Succession is a handover, not a relay. Both yeast populations are present together for days, and the aroma of the finished cider is partly decided in that overlap.
The scale is indicative: Temperature changes everything. A cold cellar ferment can run for six months rather than eight weeks, and the same succession plays out slowly rather than differently.

The central reaction

Glycolysis converts one molecule of glucose or fructose into two molecules of pyruvate, generating a small net gain of adenosine triphosphate and reducing two molecules of the cofactor NAD+ to NADH. That reduction is the problem the rest of the pathway exists to solve: the cell has a limited pool of NAD+ and glycolysis stops when it is exhausted.

In the presence of oxygen, respiration regenerates NAD+ and the pyruvate is oxidised fully. Without oxygen, yeast instead decarboxylates pyruvate to acetaldehyde and then reduces the acetaldehyde to ethanol, regenerating NAD+ in the process. Ethanol is therefore not the point of fermentation from the yeast’s perspective; it is the waste product of a redox-balancing manoeuvre.

The mass relationship follows directly. Each mole of hexose sugar gives two moles of ethanol and two of carbon dioxide, which works out at roughly 51% ethanol and 49% carbon dioxide by mass of the sugar consumed, less what is diverted into yeast biomass and by-products. This is the arithmetic behind every potential-alcohol calculation, and the small diversions are why the practical conversion factor differs slightly between authorities.

The by-products that carry the flavour

Fermentation by-products and where they come from.
Product	Formed by	Sensory consequence
Ethanol	The main pathway	Warmth, faint sweetness, body; acts as a solvent for aroma compounds
Carbon dioxide	The main pathway	Carbonation where retained; strips volatile aroma where it escapes
Glycerol	Redox balancing, particularly early in fermentation and under stress	Viscosity and a slight sweetness; contributes to body
Higher alcohols	Amino acid metabolism, via the Ehrlich pathway	Solvent and warming notes; harsh in quantity, and precursors to esters
Acetate esters	Reaction of higher alcohols with acetyl-CoA	Fruit and sweet-shop aromas — isoamyl acetate for banana and pear drop
Ethyl esters	Reaction of ethanol with medium-chain fatty acids	Apple, tropical and waxy fruit aromas
Acetaldehyde	Intermediate in the main pathway; also from oxidation later	Bruised apple and sherry; binds sulphur dioxide
Succinic and other acids	Side branches of central metabolism	A small contribution to total acidity and a salty-bitter note
Hydrogen sulphide and mercaptans	Sulphur metabolism under nitrogen stress	Rotten egg, drains, rubber — a fault at any detectable level

What the yeast is responding to

The balance of those by-products is not fixed. Temperature is the largest single lever: warm fermentations produce more higher alcohols and shift the ester profile towards the acetate esters, while cool fermentations retain more of the fruit-derived volatiles and produce a narrower, fresher profile. This is why fermentation temperature is described as a stylistic decision rather than a technical detail.

Nitrogen supply is the second. The Ehrlich pathway that produces higher alcohols runs on amino acids, so nitrogen availability shifts the alcohol and ester profile directly. More importantly, severe nitrogen deficiency drives the yeast to scavenge sulphur-containing amino acids, releasing hydrogen sulphide.

Oxygen matters at the beginning and is harmful later. Yeast needs a small amount of oxygen early to synthesise the sterols and unsaturated fatty acids its membranes require; deprived of it, the population is weak and the ferment is more likely to stall. Once fermentation is under way, oxygen ingress causes oxidation rather than helping.

The other fermentations

Malolactic fermentation is a bacterial process, not a yeast one. Lactic acid bacteria — Oenococcus oeni and various Lactobacillus species — decarboxylate malic acid, a dicarboxylic acid, to lactic acid, a monocarboxylic one, releasing carbon dioxide. Because one acid group is lost, total acidity falls and pH rises, and because lactic acid tastes softer than malic, the change is easily perceived. Diacetyl produced alongside it contributes a buttery note.

Acetification is the process nobody wants. Acetic acid bacteria oxidise ethanol first to acetaldehyde and then to acetic acid, and they require oxygen to do it. This is why headspace, ullage and unsealed vessels are the standing preoccupation of every cidermaker, and why a cask left half full turns to vinegar.

A number of other organisms may act on the cider. Brettanomyces can ferment residual sugars slowly and produce volatile phenols; Zygosaccharomyces can refement a sweetened cider in bottle; certain lactic bacteria can degrade glycerol to acrolein or produce the polysaccharide behind ropiness. All of these are ordinary microbiology rather than mysteries, and all are described on their own pages.

Also answered on this page

Questions this page covers, so you can tell at a glance whether it is the one you want.

What is the chemical equation for fermentation?
Why does yeast make alcohol at all?
What is glycerol and where does it come from?
Why does fermentation temperature matter?
What is malolactic fermentation?

What people ask next

Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.

Does cider taste of the apples it was made from — Partly, and less than most people assume. The fruit contributes the acid, nearly all the tannin, the sugar and a set of aroma compounds, but a large part of the fruit’s own volatile aroma is lost during fermentation, stripped out by escaping carbon dioxide or metabolised by yeast.
How many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
What is apple juice actually made of — By mass, apple juice is around 85–90% water. Most of the rest is sugar — fructose predominantly, with glucose and sucrose — and it is that sugar which sets how much alcohol the juice can produce.
What temperature should cider ferment at — Most cider is fermented cool, commonly between about 12 and 18 °C. Cooler ferments keep more fruit aroma and run slower; above the low twenties the cider tends towards hot, solvent-like higher alcohols.
Which yeast ferments cider — Saccharomyces cerevisiae finishes almost every cider fermentation, whether it was pitched or arrived from the press house. In a spontaneous ferment it is not the first organism present, only the one that survives the alcohol it makes.
What is malolactic fermentation — Malolactic fermentation is a bacterial conversion of sharp malic acid into softer lactic acid, releasing carbon dioxide. It lowers total acidity and raises pH, and in cider it is often the source of a farmyard or buttery note as well.

Sources

What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.

The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Peer-reviewed literature on cider fermentation microbiology
Various journals · peer-reviewed literature · registered as competent for this subject
Covers the microbial succession of spontaneous cider fermentation, the role of non-Saccharomyces yeasts in the early stages, malolactic conversion by Oenococcus and Lactobacillus species, and the organisms behind the principal spoilage faults.
Hochschule Geisenheim University — beverage technology
Hochschule Geisenheim · university · retrieved 2026-08-24
German beverage-technology research covering apple wine and fruit juice processing, including the enzymology of clarification.
Cornell Cider Research and Extension programme
Cornell University, School of Integrative Plant Science · university · passage verified 2026-08-24
Runs cultivar trials in New York State and publishes juice chemistry for European cider varieties grown in a North American climate — the single most useful counterweight to treating English figures as universal.