Cider science

Why tannin is both bitter and astringent

What is the difference between bitterness and astringency in cider?

In short

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.

Both come from the same class of compounds — condensed tannins, the procyanidins built from catechin and epicatechin units. What decides which sensation predominates is chiefly the average chain length. Short oligomers bind bitter receptors; long polymers are better at binding protein.

The ratio is therefore a property of the fruit and of what has happened to the cider since, and it changes over time as tannins polymerise during maturation.

At a fixed 750 milligrams per litre in model cider, astringency rises with procyanidin chain length while bitterness peaks in the middle and falls away.

Described in full

Shape: A horizontal axis of increasing average degree of polymerisation, marked with four positions: monomer and dimer at the left, tetramer in the middle, and long polymer at the right. Two labelled bands run across it — an astringency band that thickens steadily from left to right, and a bitterness band that is thin at the left, thickest at the tetramer, and thin again at the right.
Astringency: Increases with the degree of polymerisation. Longer chains carry more sites to cross-link salivary proteins, and it is the loss of lubrication in the mouth that reads as drying rather than as taste.
Bitterness: Peaks at the medium chain length. In the study behind this figure, the tetramer fraction was the most bitter of the four tested; both the shorter and the longer fractions were less so. Bitterness is a receptor event, and a molecule can be too large to fit the receptor while remaining perfectly able to bind a protein.
The fixed condition: Every point on this axis is 750 milligrams of procyanidin per litre in the same model solution of water, ethanol, fructose and malic acid. Concentration is held constant so that the axis shows chain length alone. The companion study found the chain-length effect is not identical at every concentration, so this figure describes one concentration.
The side effect: A note beneath the axis records that in the same experiments the perception of sweetness and sourness also moved with procyanidin concentration, although the fructose and the malic acid were identical in every sample. Tannin does not only add its own sensations; it changes the reading of the others.
What this is not: A model solution is not cider. It contains no pectin, no protein, no carbonation and none of the hundreds of compounds a fermentation makes, and every one of those can shift how a phenolic behaves on the palate.

Two different sensory systems

Bitterness is one of the recognised basic tastes, transduced by a family of receptors expressed on taste cells. It has a defined time course: it rises within a second or two of the stimulus reaching the tongue, peaks, and fades as the compound is cleared.

Astringency is transduced quite differently. Tannins in the mouth bind to proline-rich proteins in saliva, forming complexes that aggregate and precipitate. The salivary film that normally lubricates the oral surfaces is lost, and the resulting increase in friction between the tongue, palate and cheeks is detected by mechanoreceptors. There is no astringency receptor, and the sensation is properly described as a mouthfeel rather than a taste.

The two consequently behave differently. Bitterness fades; astringency builds, because each mouthful removes more salivary protein than the gland can immediately replace, and it peaks after swallowing rather than during. This is why a tannic cider seems to become drier over a session, and why palate cleansing between samples matters when tannic ciders are being assessed.

What decides the ratio

Factors that shift the balance between bitterness and astringency.
Factor	Direction	Mechanism
Short procyanidin chains (low degree of polymerisation)	Towards bitterness	Small molecules bind bitter receptors efficiently and precipitate protein poorly
Long procyanidin chains (high degree of polymerisation)	Towards astringency	Multiple binding sites per molecule cross-link salivary proteins effectively
Maturation	Bitterness falls, astringency rises then softens	Tannins polymerise over time; very large aggregates eventually precipitate out altogether
Ethanol	Reduces astringency	Interferes with tannin–protein binding and increases tannin solubility
Residual sugar and polysaccharides	Reduces both	Sugar suppresses bitterness perception; polysaccharides compete for tannin binding
Acidity	Increases perceived astringency	Low pH promotes protein precipitation, and acid contributes its own drying sensation
Carbonation	Increases perceived astringency	Carbonic acid adds to the acid effect and sharpens the palate
Individual sensitivity	Varies substantially	Bitterness sensitivity differs between people for partly genetic reasons; salivary flow rate differs and directly affects astringency

Why the total tannin figure is a weak predictor

Classification systems and analytical reports usually quote total phenolics or total tannin as a single figure, because that is what the standard assays measure. But the sensation depends on the distribution of chain lengths, not on the total, and two juices with identical total phenolics can be very differently perceived.

The Long Ashton classification acknowledges this among its own limitations: it measures total tannin, not the distribution, and the distribution is what decides the split. The same limitation applies to every scheme that uses a single tannin threshold.

This becomes a genuine research problem in the finished cider rather than in the juice. The relationship between measured phenolic content and perceived astringency in finished cider is not strong, because ethanol, sugar, polysaccharides, pH and proteins all modify the interaction. It is one of the areas this site treats as unresolved rather than settled.

What is established and what is notThat chain length governs the bitterness–astringency split is well supported by model-solution studies and by fractionation work. That a phenolic measurement on a finished cider predicts how astringent a panel will find it is not; correlations reported in the literature are moderate at best.

Where the tannin comes from and how it is controlled

Procyanidins are concentrated in the skin and the tissue just beneath it, so extraction is a processing variable. Finer milling exposes more tissue; maceration — leaving the milled pomace to stand before pressing — extracts more; higher pressing pressure extracts more from the later fractions. A producer aiming for a softer cider mills coarsely, presses gently and takes only the free-run and early juice.

Removal is also possible. Fining agents that bind tannin — proteins, and polyvinylpolypyrrolidone in modern practice — reduce phenolic content selectively, and are used where a cider is judged too bitter or too astringent. Both remove aroma and colour along with the tannin, which is the cost.

Time does the work without additions. Over months, polymerisation shifts the profile towards astringency and then, as aggregates grow large enough to fall out of solution, reduces both. A heavily tannic cider that is unapproachable at six months can be balanced at two years, which is the technical basis for ageing traditional bittersweet-based ciders.

Also answered on this page

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

Is astringency a taste?
Why does cider dry my mouth?
What makes one tannic cider bitter and another drying?
Does cider get less bitter with age?
Can tannin be removed from cider?

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 does bittersweet apple mean in a cider tasting note — The soft, low-acid, faintly earthy ripe-fruit character of traditional West Country cider fruit, inseparable from its phenolic weight.
What is still unknown about cider — A great deal, and rather more than the confident tone of most cider writing suggests. Four gaps are large enough to affect what can honestly be said on almost any page of this site.
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 is the difference between a bittersweet and a bittersharp apple — Both are high in tannin; the difference is acid. Under the Long Ashton scheme a bittersweet sits below roughly 0.45% acid and a bittersharp above it, so a bittersharp brings sharpness as well as bitterness to a blend.
Why do cider apples taste so bad if you bite one
What is maceration in cider making — Maceration is letting milled pomace stand before pressing, so that enzymes soften the tissue and phenolics and aroma move from the skins into the juice. It raises both colour and tannin, and raises the risk of oxidation with them.

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.

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.
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.
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.