Cider science

The acids in cider

What acids are in cider and what does each taste like?

In short

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.

Acetic acid is vinegar, and above a low threshold it is a fault. Citric acid is present in pears and essentially absent from apples, which is one reason a total-acid figure means something different for perry.

Several minor acids — succinic, quinic, and the phenolic acids — contribute at low levels, and a handful of them taste bitter or salty rather than sour.

Two juices at the same pH carrying different titratable acidity, drawn on two separate scales.

Described in full

Two axes, not one: The upper scale is pH, running from 4.2 on the left down to 3.2 on the right. The lower scale is titratable acidity in grams per litre as malic acid, running from two to ten. They measure different things and are drawn separately for that reason.
What pH measures: The concentration of free hydrogen ions — the strength of the acid actually dissociated at that moment. It governs microbial risk, the effectiveness of sulphite, and how stable the cider is. It is a logarithmic scale, so a change of 0.3 is a doubling.
What titratable acidity measures: The total amount of acid present, found by neutralising it with alkali. It tracks perceived sourness far more closely than pH does, and it is what a taster responds to.
The V on the figure: Two juices are marked at the same pH on the upper scale. Lines run down to two clearly different positions on the acidity scale below. Same pH, different acidity, and they would not taste alike.
Why they come apart: Potassium and other cations in the juice buffer it. A juice rich in potassium holds its pH up while carrying a lot of acid, so a high-acid fruit can arrive at a pH that offers less protection than its sourness suggests.
The practical consequence: A cidermaker needs both numbers. Blending to taste uses titratable acidity; deciding on a sulphite addition, or whether a juice is safe to ferment slowly, uses pH. Neither substitutes for the other.
Typical territory: Bittersweet juice sits at the low-acidity, high-pH end and is the most vulnerable to spoilage. Sharp fruit is added to blends as much to bring pH down as to make the drink taste fresher.

The acids and what they do

The acids of cider and perry, their origin and their sensory character.
Acid	Where it comes from	Taste	Significance
Malic	The fruit; the dominant acid of apple juice by a wide margin	Sharp, clean, green-apple; a long, cutting sourness	Sets acidity and pH; the substrate for malolactic fermentation
Lactic	Bacterial conversion of malic acid during malolactic fermentation	Soft, rounded, mildly sour; often read as creamy	Lowers total acidity and raises pH; brings diacetyl with it
Acetic	Acetic acid bacteria oxidising ethanol, given oxygen	Vinegar; sharp and volatile, with a pungent aroma	The principal component of volatile acidity; a fault above a low threshold
Citric	The fruit — present in pears, and in apples only in traces	Bright, immediate, less persistent than malic	Part of why perry acidity reads differently; also convertible by some lactic bacteria
Succinic	Yeast metabolism during fermentation	Sour with a distinctly salty and bitter edge	A small but perceptible contribution to the finish of most ciders
Quinic and chlorogenic	The fruit, as part of the phenolic fraction	Little sourness; chlorogenic acid is a browning substrate rather than a taste	Contribute to titratable acidity without contributing much perceived sourness

Malolactic conversion, and what it changes

Malic acid carries two carboxylic acid groups; lactic acid carries one. When lactic acid bacteria decarboxylate malate to lactate, one acid group is lost as carbon dioxide, so a given quantity of malic acid becomes a smaller quantity of acid overall. Titratable acidity falls, and pH rises.

The sensory change is larger than the numbers suggest, because lactic acid is perceived as softer as well as being present in smaller effective quantity. A cider that was sharp and angular before conversion is rounder and broader afterwards, and diacetyl produced by the same bacteria adds a buttery note that reinforces the impression.

Whether this is wanted depends entirely on the style. It is standard and deliberate in a great deal of traditional English, French and Spanish practice, where the aim is a rounded cider with a settled acidity. It is suppressed in fresh, fruit-driven ciders, where the sharpness is the point and the buttery note would be out of place. The suppression is achieved by sulphiting, by cold, by filtration, or by a combination.

Volatile acidity, and the line between character and fault

Volatile acidity means the acids that can be distilled off, which in practice means acetic acid together with the ethyl acetate that usually accompanies it. It is produced by acetic acid bacteria oxidising ethanol, and the reaction requires oxygen; a properly filled, sealed vessel does not develop it.

A trace of acetic acid is present in essentially every cider and contributes to complexity. Above a low threshold it becomes recognisable as vinegar, and ethyl acetate, which smells of nail varnish and solvent, is generally detected before the acetic acid itself. The threshold at which this crosses into fault depends on style: Asturian and Basque sidra natural carry higher volatile acidity than most other traditions and are judged against that expectation.

The distinction that matters technically is whether the process is still running. A cider with elevated volatile acidity and no live acetic bacteria is stable; one with an active population will continue to acetify, and blending it into sound cider spreads the problem rather than diluting it away.

Why perry acidity is a different question

Pear juice carries both malic and citric acid, in proportions that differ by variety. A single total-acid figure therefore sums two acids that taste differently and behave differently: citric is brighter and shorter, malic longer and sharper. The same figure can describe two quite different palates.

Citric acid is also metabolised by some lactic acid bacteria, and the products include diacetyl and, under some conditions, acetic acid. A perry undergoing malolactic conversion can therefore develop volatile acidity from a route that does not exist in cider.

On top of that, perry pears are frequently low in acid overall, which leaves the juice at a pH where spoilage organisms are comfortable. Low acidity in perry is therefore a microbiological problem as much as a sensory one, and it is part of why perry is reckoned harder to make than cider.

Sourness and acidity are not the same measurementPerceived sourness tracks titratable acidity more closely than it tracks pH, but neither predicts it exactly, because different acids taste differently sour at the same concentration and because sugar, tannin and carbonation all modify the perception.

Also answered on this page

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

What acid makes apples sour?
What does malolactic fermentation do to the taste?
What is volatile acidity?
Why does perry have citric acid?
Is a trace of vinegar in cider a fault?

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 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 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.
Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
How much acetic acid is normal in cider — A small amount is present in every cider and contributes lift. Sound Asturian sidra natural has been measured at 0.2 to 0.3 g/L, and a clean modern cider much above about 0.7 g/L is generally showing a fault rather than a style.
How much acid should cider have — Commonly quoted targets sit around 4 to 6 grams per litre as malic acid, but acid and pH are separate questions and neither predicts the other. Perceived sharpness follows the acid; microbial risk and sulphite effectiveness follow the pH.
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.
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.
Research on pear juice composition and sorbitol in perry
Various journals and institute reports · peer-reviewed literature · registered as competent for this subject
The evidence base for the single most important chemical difference between cider and perry: pears carry substantial sorbitol, which yeast does not ferment, so a fully fermented perry retains sweetness a fully fermented cider cannot.
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.
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.