Fault
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Also called buttery character, butterscotch fault.
- Severity
- Quality — the drink is worse for it
- Where it starts
- Maturation, Fermentation
- Can it be fixed?
- Partly — it can be reduced, not removed
- When you notice it
- Fermentation, Maturation
- Signs
- 5 recorded
What it is
Diacetyl is a small ketone with a low odour threshold and an unmistakable smell of butter or butterscotch. In small amounts it can add a rounded, creamy note that some styles carry comfortably; above threshold it becomes a slick, buttery character that flattens fruit and leaves a filmy impression on the palate. In cider it comes mainly from lactic acid bacteria, and its presence usually says something about how the malolactic fermentation was handled.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- A clear smell of butter, butterscotch or buttered popcorn
- Fruit character that seems muffled underneath the butter
In the mouth
- A slick, coating feel on the tongue
On the palate
- A rounded, almost creamy sweetness in a cider fermented dry
How the batch behaves
- Butteriness that fades over months in a cider still in contact with yeast or bacteria — Both yeast and lactic bacteria reduce diacetyl if they are still active, which is why the timing of racking matters.
The words for it: Butter, Butterscotch. Each links to what produces it.
When it appears. During and shortly after malolactic fermentation. It can diminish afterwards if a live yeast population remains to reduce it, which is why the same cider tasted twice can differ.
It is more prominent in perry than in cider at equivalent concentrations, because citric acid in pear juice gives the same organisms an additional route to producing it. Individual sensitivity varies enough that a panel will commonly split on a borderline sample.
Cider and Perry Academy (Peter Mitchell), Campbell-Sills, El Khoury, Favier and others, Genome Biology and Evolution 7(6):1506–1518
What it is mistaken for, and how to tell
One observation per rival, chosen because it separates them rather than because it describes either. Several of these are not faults at all — a style feature, a normal outcome, or the fruit behaving as it does.
- Lactic off-flavours — Diacetyl is butter and butterscotch specifically; the wider lactic range is sour and dairy rather than buttery.
- Buttery character from oak — Whether the cider was in wood. Oak lactones give a creamy, coconut-adjacent impression rather than the distinct butter of diacetyl.
- Cooked character — Cooked character brings caramel and stewed fruit; diacetyl is butter without the sugar note.
At low concentration it is a recognised and sometimes wanted component of a cider that has been through malolactic fermentation, contributing a soft, rounded impression. It becomes a fault at the point where the cider tastes of butter rather than of apple.
What is actually happening
The chemistry or microbiology behind it. Understanding the mechanism is what makes the prevention make sense rather than being a list of rules.
The main route in cider runs through citric acid. Lactic acid bacteria that metabolise citrate produce pyruvate, and excess pyruvate is converted to alpha-acetolactate. Alpha-acetolactate is unstable and decarboxylates oxidatively outside the cell to give diacetyl. The oxidative step is spontaneous rather than enzymatic, so it happens whether the bacteria intend it or not.
The same bacteria then reduce diacetyl, through acetoin, to 2,3-butanediol, which is essentially odourless. This is a two-stage process with a peak in the middle. A malolactic fermentation stopped at the wrong moment — by racking, by sulphiting, by chilling — locks the cider at the peak, whereas one allowed to run to completion ends with most of the diacetyl reduced away.
Yeast contributes by a similar route during alcoholic fermentation, forming alpha-acetolactate as an intermediate of valine biosynthesis. A healthy Saccharomyces cerevisiae population reabsorbs and reduces the resulting diacetyl at the end of fermentation, which is why removing yeast too early leaves it behind.
Because the perception is threshold-based and the compound is potent, the difference between an attractive roundness and an obviously buttery cider is a small change in concentration.
How it happens
| Cause | Stage | How often |
|---|---|---|
| A malolactic fermentation halted part-way through | Maturation | common |
| Racking or sulphiting at the point of maximum diacetyl | Maturation | common |
| Lactic acid bacteria metabolising citric acid in a cider with high pH | Maturation | occasional |
| Yeast removed from the cider before it finished reducing its own diacetyl | Fermentation | occasional |
| Oxygen exposure during maturation, which drives the conversion of alpha-acetolactate | Maturation | occasional |
| A stressed alcoholic fermentation, raising the yeast’s intermediate output | Fermentation | occasional |
Can it be put right?
Most cider faults cannot be reversed. Where that is the answer it is given plainly — an honest 'this batch is what it is' is more useful than a procedure that will not work.
| Option | Effectiveness | What it involves |
|---|---|---|
| Leave the cider in contact with active yeast or bacteria for longer | Partial | The most reliable route where anything is still viable, because the organisms reduce diacetyl to odourless butanediol themselves. It requires patience and a population that has not already been removed. |
| Stir the lees to increase contact | Partial | Helps if there is still active yeast. Increases the risk of picking up reductive character and of disturbing sediment before packaging. |
| Blend into a larger clean volume | Partial | Threshold-dependent perception, so dilution can work where the fault is moderate. |
| Aerate the cider to drive it off | Unlikely to work | Diacetyl is not volatile enough for this to help meaningfully, and the oxygen introduced converts more alpha-acetolactate into diacetyl. The intervention can make the cider worse. |
| Remove diacetyl chemically | Not possible | There is no additive that removes it from cider. |
CiderHQ gives no additive dosage figures. An addition is a food-safety decision that depends on legal limits, on the juice in front of you and on what you are protecting against, and the right figure comes from your supplier’s or regulator’s own guidance rather than from a general reference.
Preventing it
- Where malolactic fermentation is wanted, let it run right through rather than stopping it as soon as the malic acid has gone.
- Confirm by measurement that conversion is complete before racking, sulphiting or chilling.
- Leave the cider in contact with a healthy yeast population until fermentation has genuinely finished.
- Keep oxygen out during maturation, since the key conversion step is oxidative.
- Manage pH so that lactic activity happens when it is wanted, not spontaneously and unpredictably.
Diacetyl is the fault that punishes impatience. Both the yeast and the bacteria that produce it also remove it, and almost every case in cider comes from taking the cider away from those organisms during the window in which the compound is at its peak.
It is also a good illustration of why measurement beats observation. The malic acid disappearing does not mean a malolactic fermentation has finished its work; the reduction of diacetyl continues afterwards, and a cider racked at the moment the malic acid is gone is racked at the worst possible time.
Perry is more exposed than cider on this point, because pear juice carries more citric acid than apple juice does, and citrate metabolism is the main bacterial route to diacetyl. A perry going through spontaneous lactic activity can develop a butteriness a cider would not.
At the low end the compound is not necessarily unwelcome — a faint creaminess can suit a soft, low-acid cider. What is not defensible is a cider where butter is the first thing the nose finds and the fruit is somewhere underneath it.
The compounds involved
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.
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.
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.
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.
Malic acid
The acid of apples, which supplies almost all the sharpness a cider has and, through pH, decides how vulnerable that cider is to everything that could spoil it.
The organisms involved
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.
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
Where in the process it arises
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.
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
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.
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
Oxidation
The cumulative effect of oxygen on finished cider: fruit aroma flattens, colour deepens towards amber, and a bruised-apple or sherry-like character replaces the fresh one.
Flat and lifeless
A cider with nothing obviously wrong and nothing to say: aroma faded, acidity dulled, finish short — usually the cumulative result of over-processing or slow oxidation.
Where this comes up in a guide
The link lands on the step where the fault arises rather than at the top of the pathway, because that is where the decision that causes it is taken.
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.
- How long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- What is oxidation in cider
- 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.
- Which yeast ferments cider — Saccharomyces cerevisiae finishes almost every cider fermentation, whether it was pitched or arrived from the press house. In a spontaneous ferment it is not the first organism present, only the one that survives the alcohol it makes.
- 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.
- What is racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.
Where to go next
- The fault finder — Describe what you can smell and see, and narrow it down from the signs.
- All faults — Grouped by where they come from and how serious they are.
- Cider science — The chemistry and microbiology these faults come out of.
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.
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.