Cider science

The chemistry of apple juice

What is apple juice actually made of?

In short

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.

The minority components decide almost everything else. Malic acid supplies nearly all the acidity; phenolic compounds supply bitterness, astringency and colour; pectin governs clarity and makes keeving possible; and nitrogen, present in small and highly variable quantities, governs how the fermentation runs.

None of these is constant. Every one varies with variety, season, orchard, ripeness and how the fruit was handled, which is why a single figure for any of them is only ever a figure for a particular pressing.

Sugar, acid, phenolics and nitrogen as they arrive in the juice and as they leave the ferment.

Described in full

Layout: Four rows, one per component. The left column is what is in the pressed juice; an arrow crosses to the right column, which is what is there after fermentation.
Sugar: Apple juice carries glucose, fructose and sucrose, with fructose the largest. Nearly all of it becomes ethanol and carbon dioxide; roughly half the sugar mass leaves the vessel as gas.
Acid: Malic acid is almost the whole acid content of apple juice. Fermentation itself changes it little. If malolactic fermentation follows, the malic becomes lactic acid, acidity falls and pH rises.
Phenolics: Procyanidins extracted from skin and flesh. Fermentation does not create or destroy them so much as rearrange them: chains polymerise, some bind to yeast and precipitate, and the perceived balance shifts from bitterness towards astringency and then softens.
Nitrogen: Amino acids and ammonium, together the yeast-assimilable nitrogen. It is consumed almost entirely, converted into yeast biomass. Whatever is left at the end is a food supply for anything that follows.
Why nitrogen decides the aroma: Yeast short of nitrogen produces hydrogen sulphide and mercaptans — rotten egg, drains, rubber. Yeast with plenty ferments fast and clean but leaves less of the fruit’s own character. Keeving works precisely by removing nitrogen deliberately.
What the table omits: Pectin, sorbitol, potassium and the trace compounds that decide most of the aroma. These four are the ones a cidermaker measures and can act on.

What is in it, by fraction

The principal components of pressed apple juice and what each governs. Proportions are indicative and vary widely by variety and season.
Component	Roughly how much	What it governs
Water	About 85–90% by mass	The solvent; everything else is dissolved or suspended in it
Sugars — fructose, glucose, sucrose	The great majority of the dissolved solids	Potential alcohol, and sweetness in juice and in any cider fermented short of dryness
Malic acid	The dominant acid, with traces of quinic, citric and others	Acidity, pH, microbial safety, and the perception of freshness
Phenolics — procyanidins, chlorogenic acid, phloridzin, others	From very little in dessert fruit to a substantial fraction in bittersweets	Bitterness, astringency, colour, browning and ageing capacity
Pectin	A small but significant fraction	Viscosity, clarity, press yield, and whether keeving is possible
Nitrogen compounds — amino acids and ammonium	Small, and highly variable between varieties and orchards	Fermentation speed and completion, and whether the yeast produces sulphide off-flavours
Sorbitol	Small in apples; large in pears	Unfermentable sweetness and body
Minerals, chiefly potassium	Trace	Buffering capacity, and therefore how much acid a given pH corresponds to
Starch	Present in under-ripe fruit, absent when fully ripe	Haze in the finished cider where fruit was pressed too early

Sugar, and why the composition matters as well as the amount

Fructose is the largest single sugar in apple juice, with glucose and sucrose making up the rest; sucrose is hydrolysed to glucose and fructose early in fermentation. All three are fermented by Saccharomyces cerevisiae, though not at the same rate: glucose is consumed preferentially and fructose more slowly, which is why the last stages of a ferment are disproportionately fructose and why a stuck ferment leaves fructose behind.

That matters sensorially because fructose is the sweetest of the common sugars on a per-gram basis. A cider that has stopped short therefore tastes sweeter than its total sugar figure alone would suggest, because what is left is enriched in the sweetest component.

Total dissolved solids are measured as specific gravity or in degrees Brix, and both are used to estimate potential alcohol. The estimate is approximate rather than exact, because a hydrometer responds to everything dissolved rather than to sugar alone, and non-fermentable material — sorbitol, acids, phenolics — contributes to the reading.

The phenolic fraction is where cider fruit is different

Apple phenolics fall into several classes. The procyanidins are polymers of catechin and epicatechin units and are the material usually meant by cider tannin; the hydroxycinnamic acids, chiefly chlorogenic acid and p-coumaroylquinic acid, are the principal substrates for enzymatic browning; phloridzin is a dihydrochalcone essentially specific to apple; quercetin glycosides sit in the skin; and anthocyanins appear in red-fleshed varieties.

The distribution across these classes differs by variety far more than the total does, and it is the distribution rather than the total that predicts the palate. In particular, the mean degree of polymerisation of the procyanidins — the average chain length — governs the split between bitterness and astringency, and two juices with identical total phenolics can behave quite differently if their chain-length distributions differ.

Phenolics are concentrated in the skin and in the tissue immediately beneath it, which is why milling fineness and maceration time have such a large effect on how much ends up in the juice. A gentle whole-fruit press extracts markedly less than a finely milled pomace left to stand.

The components that are easy to overlook

Nitrogen is the one that most often decides whether a fermentation works. Yeast requires assimilable nitrogen — free amino acids and ammonium — to build the machinery for consuming sugar, and cider apple juice frequently carries far less than a wine must. Traditional cider varieties from low-input orchards are often at the low end, which is a large part of why traditional ciders ferment slowly and why keeving works at all.

Pectin governs whether the juice will clarify and whether it can be keeved. It is a chain of galacturonic acid units with methyl groups attached; the enzyme pectin methylesterase removes those groups, which allows calcium to cross-link the chains into a gel. Commercially, pectinase preparations are added to do the opposite — break the pectin down so the juice clears and presses more freely.

Potassium, present as a mineral, sets much of the juice’s buffering capacity. Two juices with the same titratable acidity can sit at noticeably different pH values if their potassium content differs, which is one reason pH and titratable acidity have to be measured separately rather than inferred from one another.

Why this page gives ranges rather than numbersPublished analyses of apple juice come from particular cultivars, grown at particular sites, in particular seasons, by particular methods. Averaging them would produce an authoritative-looking figure that describes no real juice. Where a number is quoted on this site it carries the context that makes it meaningful.

Also answered on this page

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

How much sugar is in apple juice?
What acid is in apples?
What are apple polyphenols?
Why is nitrogen important in cider juice?
What is Brix?

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.

What acids are in cider and what does each taste like — Malic acid is the dominant acid in apple juice and in most cider, and it tastes sharp, clean and persistent — the acidity of a green apple. Lactic acid, produced when bacteria convert malic acid, is softer and rounder and reads as creamy rather than sharp.
What is malic acid and why does it matter in cider — Malic acid is the acid of apples and supplies almost all the sharpness a cider has. It also sets pH, which decides how well sulphite works and how exposed the cider is to spoilage, so it does far more work than its taste suggests.
What is the difference between bitterness and astringency in cider — Bitterness is a taste, detected by receptors on the tongue. Astringency is not a taste at all: it is a mechanical sensation caused by tannins binding and precipitating the proline-rich proteins in saliva, which strips away the lubricating film in the mouth.
How do i know when cider fermentation has finished — Stable gravity readings several days apart, not the airlock going quiet. A ferment that has stalled and a ferment that has finished both stop bubbling, and only a hydrometer tells you which one you have.
What are procyanidins in cider — They are the condensed tannins of apples: chains of catechin-type units whose length decides how much of the phenolic load reads as bitterness and how much as astringency. Two ciders with identical total tannin can taste nothing alike.
What does pectin do in cider — Pectin is the structural polysaccharide that holds fruit cells together. In juice it holds haze in suspension, and it is the molecule keeving depends on: strip its methyl groups and it will gel with calcium and float the nutrients out of the juice.

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.
Long Ashton Research Station cider fruit analyses
National Fruit and Cider Institute / University of Bristol · research institute · registered as competent for this subject · covers 1903–2003
The foundational body of cider-fruit science in English. Long Ashton produced the acid-and-tannin classification that divides cider apples into sweet, sharp, bittersweet and bittersharp, and analysed hundreds of cultivars grown at its Somerset site. Its figures are historic measurements of specific fruit at a specific place, not universal constants — a distinction CiderHQ preserves in every measurement record that cites it.
Institut national de recherche pour l’agriculture, l’alimentation et l’environnement (INRAE)
INRAE · research institute · retrieved 2026-08-24
French national agricultural research. Its Angers programme produced much of the published work on apple procyanidin chain length and on the relationship between polymer size, bitterness and astringency.
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.
Peer-reviewed literature on apple phenolics and cider sensory perception
Various journals · peer-reviewed literature · registered as competent for this subject
Registered as a class rather than as one paper, because the mechanisms CiderHQ describes — tannin chain length driving the split between bitterness and astringency, salivary protein precipitation, enzymatic browning — are established across many studies rather than resting on any single one. Individual papers are cited where a specific number is quoted.