Compound
Gallic acid
A minor phenolic acid in apple that matters chiefly as the reference standard every total-phenolics figure in cider is expressed against, and as a marker of oak contact.
- Class
- Phenolics
- Formula
- C7H6O5
- How often it matters
- Occasional
What it does in cider
- Serves as the calibration standard for the Folin–Ciocalteu assay, so total phenolics are reported as gallic acid equivalents whether or not any gallic acid is present.
- Enters cider from oak, where it is released by the hydrolysis of the wood’s hydrolysable tannins.
- Contributes a slightly sour astringency at the concentrations barrel maturation produces.
- Distinguishes wood tannin from fruit tannin analytically, since apple procyanidins yield no gallic acid on hydrolysis.
How it is perceived
What the compound registers as, and at roughly what concentration. Perception is not a property of the molecule alone: sugar, tannin and carbonation all change where a threshold falls.
- sour astringency
- dry
- inky
The structure it moves
| Dimension | What it is |
|---|---|
| Astringency | The drying, rough sensation left after swallowing. |
| Tannin | The phenolic material that gives cider structure, grip and ageing capacity. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Barrel ageing
Holding cider in wood — which may mean an old neutral vat used simply as a vessel, or fresh or spirit-seasoned oak used as a source of flavour, and the two are not the same operation.
Wood and botanical infusion
Steeping oak in chip, cube or stave form, or bark, root, herb and flower material, in a neutral vessel to flavour a cider without the vessel itself doing any of the work.
About Gallic acid
Gallic acid appears in two quite different guises in cider work. The first is as a unit of account. The Folin–Ciocalteu assay, which is how total phenolics are almost always measured, does not identify anything; it measures reducing capacity and expresses the result as the mass of gallic acid that would give the same reading. Every total-phenolics figure quoted in milligrams per litre as gallic acid equivalents is therefore a comparison, not an inventory, and two juices with identical GAE figures can have entirely different phenolic compositions.
The second is as an actual compound arriving from wood. Oak contains hydrolysable tannins — ellagitannins and gallotannins — that fruit does not, and their hydrolysis releases gallic and ellagic acids into the cider. This is a real chemical distinction between fruit tannin and wood tannin, and it is one of the ways an analyst can tell that a cider has been in a barrel.
Sensorially it is a modest contributor, adding a sour-edged astringency to the vanillin and oak lactones that dominate the aroma of wood contact. It is worth knowing about mostly so that a total-phenolics number is read for what it is.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Phenolics
Total phenolics
The single number used to summarise everything phenolic in a juice, useful for comparing fruit and misleading whenever it is used to predict how a cider will taste.
Phenolics
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
Esters
Oak lactones
The coconut-and-wood compounds of oak, whose concentration depends more on which species of oak the barrel was made from than on anything a cidermaker does.
Aldehydes
Vanillin
The vanilla compound released from oak lignin, and a good illustration of why fermenting in a barrel gives less wood character than maturing in one.
Phenolics
Tannin–protein complexes
What astringency physically is: long tannin chains cross-linking salivary proteins and precipitating them, which is also the mechanism behind fining, protein haze and the classic cider-and-cheese pairing.
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 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.
- Does cider contain antioxidants — Cider carries apple polyphenols, and tannic bittersweet ciders carry considerably more than pale ones made from dessert fruit. What that means for health is a clinical question this site does not adjudicate.
- Why does tannic cider feel drying rather than bitter — Because astringency is a physical event rather than a taste. Tannin cross-links the proteins that lubricate the mouth, and the loss of lubrication is what registers as drying; bitterness is a separate receptor event happening at the same time.
- Should i ferment cider in an oak barrel — A sound, full barrel is a fine maturation vessel; a leaky, half-empty or previously spoiled one is the fastest route to vinegar. Wood cannot be sterilised, so a barrel carries whatever lived in it before.
- Where does vanilla flavour in cider come from
- What are oak lactones
Where to go next
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
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.
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.