Compound
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.
Also called 2,3-butanedione, Butter flavour.
- Class
- Ketones
- Formula
- C4H6O2
- How often it matters
- Regularly encountered
What it does in cider
- Is produced by lactic acid bacteria during malolactic fermentation, particularly where citric acid is available alongside malic acid.
- Also arises in small quantity from yeast, as an intermediate in valine synthesis that leaks out of the cell and is normally reabsorbed.
- Is removed by yeast in contact with the cider, which reduces it to acetoin and then to odourless 2,3-butanediol — the mechanism behind leaving a cider on its lees.
- Registers at very low concentration, so it moves a cider’s character faster than most compounds present in similar amounts.
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.
- butter
- butterscotch
- popcorn
- cream
- rancid at high level
Among the lower thresholds in fermented drinks, in the region of tenths of a milligram per litre, and lower still in a delicate cider than in a tannic one where there is more to mask it.
Descriptors it is responsible for
Sensory records that name Diacetyl 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.
- Caramelised apple — Burnt-sugar sweetness over cooked fruit, from sugar breaking down under heat or high concentration.
- Yoghurt — A tangy fermented-dairy note from lactic acid with acetaldehyde, indicating vigorous bacterial activity.
- Acetone — A harsher, more chemical solvent note than ethyl acetate alone, from ketones alongside high volatile esters.
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. |
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.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
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.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Sterile filtration
Membrane filtration at a rating that retains viable yeast and bacteria, which is what allows a sweet cider to be shelf-stable without heat or preservative — provided the filling that follows is aseptic.
Pasteurisation
Heating cider enough to inactivate the organisms that would spoil it, either in the sealed package or in-line before filling, at a measurable cost in fresh aroma.
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.
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.
Pediococcus species
Homofermentative lactic acid bacteria that grow in tetrads, associated in cider with ropiness, diacetyl and slow spoilage during maturation.
Leuconostoc mesenteroides
A heterofermentative lactic acid bacterium common on fruit and early in fermentation, and a classic producer of the dextran that causes ropiness.
About Diacetyl
Diacetyl is a small four-carbon diketone with an aroma out of all proportion to its size and concentration. It reaches cider chiefly through malolactic fermentation, and particularly where the bacteria have citric acid to work on as well as malic acid — which is why perry, whose pears carry more citrate than apples do, can end up markedly more buttery than a cider given the same treatment.
Its concentration is not a one-way accumulation. Yeast in contact with the cider reduces diacetyl to acetoin and then to 2,3-butanediol, neither of which smells of much, and the reduction is efficient enough that a cider left on its lees after malolactic loses most of the butter it developed. This is the same mechanism brewers use as a diacetyl rest, and it means the timing of racking and filtration decides how much survives: rack or filter early and the butter is locked in, leave the cider on yeast and it fades.
Whether it should fade is a style judgement. A traditional West Country or Normandy cider with a soft buttery note alongside its lactic acid and its ethyl lactate is showing its maturation regime, and the character is expected. The same note in a fresh, aromatic, fruit-driven cider is a fault, and one that suggests bacterial activity nobody intended. The compound is identical; only the expectation differs.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Ketones
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.
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.
Esters
Ethyl lactate
The slow-forming ester of lactic acid and ethanol, which accumulates after malolactic fermentation and contributes the soft, milky roundness of a long-matured traditional 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.
- 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.
- 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.
- Why does my cider taste of butter — Diacetyl, produced by lactic acid bacteria and by yeast, smells of butter or butterscotch. A trace can add richness; above threshold it reads as artificial and stale.
- What is lactiplantibacillus plantarum doing in cider
- How is cider matured
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.
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.
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.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.