Compound
Acetoin
The intermediate between diacetyl and odourless 2,3-butanediol, far less aromatic than the compound it comes from and a useful marker of how far lactic and acetic activity has run.
Also called 3-hydroxybutan-2-one.
- Class
- Ketones
- Formula
- C4H8O2
- How often it matters
- Specialist — met in particular circumstances
What it does in cider
- Sits midway in the reduction of diacetyl to 2,3-butanediol, so its accumulation shows that yeast has been working on diacetyl.
- Contributes a mild creamy or yoghurt-like note, needing far higher concentrations than diacetyl to be noticed.
- Is produced directly by lactic acid bacteria from citrate and by acetic acid bacteria oxidising 2,3-butanediol.
- Can be re-oxidised to diacetyl if a cider is exposed to air, so a cider that seemed to have lost its butter note can regain it.
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.
- cream
- yoghurt
- faint butter
A much higher threshold than diacetyl — typically reported in the tens to hundreds of milligrams per litre — so it is often present without being perceived.
Descriptors it is responsible for
Sensory records that name Acetoin as a cause. Each states the perception and the mechanism behind it.
- Butter — The buttery note of diacetyl, produced by lactic acid bacteria during malolactic conversion.
- Butterscotch — Diacetyl at higher concentration with a caramel edge, sitting at the boundary between character and fault.
- Cream — A soft, rounded dairy impression from acetoin and lactic acid without the sharpness of free diacetyl.
The structure it moves
| Dimension | What it is |
|---|---|
| Fermentation character | Aromas made by the ferment rather than carried in from the fruit. |
| Body | How much weight and viscosity the drink has in the mouth. |
Measured figures
Shown as they were measured, with the context each was taken in. They are not averaged: a concentration recorded in one country's fruit in one decade is not a constant.
Acetoin2.0–4.0 mg/L
Villaviciosa, Asturias, Spain, 2001–2002 · 4 samples · Gas chromatography with flame ionisation detection · Suárez Valles, Pando Bedriñana, Fernández Tascón, Querol Simón and Rodríguez Madrera, Food Microbiology 24(1):25–31
Measured in ciders that had completed malolactic fermentation, which is the stage that ordinarily raises acetoin. These are low figures, well under what a buttery cider would carry.
A buttery-smelling ketone made by both yeast and lactic bacteria, and the compound diacetyl reduces to. Measured in milligrams per litre.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Malolactic fermentation
A bacterial conversion of sharp malic acid into softer lactic acid, which lowers acidity, changes aroma, and in most traditional cider happens whether it was planned or not.
Lees ageing
Deliberately holding a cider or perry on its fine yeast deposit so that autolysing cells release material that changes texture and foam behaviour.
Oxidative maturation
Ageing a cider with a deliberate, measured oxygen supply so that tannins polymerise and soften and an aldehydic character develops on purpose, which is not the same thing as the oxidation fault.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Lactic off-flavours
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
Oenococcus oeni
The acid-tolerant lactic acid bacterium that carries out most deliberate malolactic fermentation, converting malic acid to lactic acid after the yeast has finished.
Lactiplantibacillus plantarum
A versatile lactic acid bacterium, renamed out of *Lactobacillus* in 2020, capable of malolactic conversion and of a range of faults depending on conditions.
Acetobacter aceti
An acetic acid bacterium that oxidises ethanol to acetic acid wherever cider meets air, and the organism behind most volatile acidity in cider.
Gluconobacter oxydans
A sugar-preferring acetic acid bacterium abundant on damaged fruit and in fresh juice, which oxidises glucose to gluconic acid before fermentation begins.
About Acetoin
Acetoin is the quiet middle term of a three-compound sequence. Diacetyl, strongly aromatic and perceptible at tenths of a milligram per litre, is reduced by yeast to acetoin, which needs perhaps a thousand times as much to be noticed; acetoin is reduced further to 2,3-butanediol, which is effectively odourless. A cider’s buttery character is therefore a function of where in this sequence it has got to.
The sequence is reversible at the top. Exposure to air, particularly with acetic acid bacteria present, oxidises acetoin back to diacetyl, so a cider that had lost its butter note during lees contact can develop it again in a poorly filled barrel. Where a butter note reappears after apparently being resolved, this is usually why.
For a maker, acetoin is most useful as evidence. Finding it in quantity says that lactic bacteria were active and that reduction has occurred — a snapshot of the microbiology of the cellar written into the analysis.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Ketones
Diacetyl
The butter compound, made mainly by lactic acid bacteria and perceptible at very low concentration, which is a defining part of some traditional ciders and an obvious fault in others.
Acids
Lactic acid
The softer acid that replaces malic when malolactic fermentation runs, halving the acid a cider carries and changing its texture as much as its sharpness.
Acids
Citric acid
A minor acid in apples and a much more significant one in pears, whose metabolism by lactic bacteria is the reason perry gains more butter and more vinegar from malolactic fermentation than cider does.
Acids
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
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.
- 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.
- Why does cider contain lactic acid — Because malolactic bacteria converted the malic acid into it. Malic acid has two acid groups and lactic acid has one, so roughly half the titratable acidity disappears and the cider tastes softer.
- What turns cider into vinegar — Acetic acid bacteria oxidising ethanol, in two steps that both consume oxygen. This is why acetification is an air problem: a full sealed vessel does not acetify however many bacteria it contains.
- Which bacterium carries out malolactic fermentation in cider — Oenococcus oeni, in cider as in wine. Genome studies find that the cider strains are genetically distinguishable from the wine ones and that the strain most basal to the whole species was isolated from cider.
- 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.
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 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.
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.