Sensory descriptor
Butter
The buttery note of diacetyl, produced by lactic acid bacteria during malolactic conversion.
Also called Buttery, Diacetyl.
- Aroma family
- Dairy and lactic
- Does it indicate a fault?
- It depends on the style it is met in
What causes it
Diacetyl — 2,3-butanedione — is formed by oxidative decarboxylation of α-acetolactate, an intermediate of bacterial citrate and pyruvate metabolism, and lactic acid bacteria produce it during and after malolactic fermentation. Oenococcus oeni and Lactobacillus species are the principal producers in cider, and the amount depends on citrate availability, oxygen, temperature and how long the bacteria are left in contact after the malic acid is gone. Yeast can subsequently reduce it to acetoin and then to 2,3-butanediol, both far less aromatic, so a cider left on active lees after malolactic conversion will lose butter character over time.
Diacetyl’s threshold in cider is in the region of a tenth of a milligram per litre. Below it the compound rounds the palate without being identifiable. Just above it, butter is a recognised part of the character of traditional malolactic ciders. Several times above it, the perception becomes buttered popcorn or butterscotch and every framework records it as a fault.
The compounds responsible
Where the molecule is established. Some descriptors in this lexicon name a compound the compound register does not yet hold a record for; those are named in the cause above instead.
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.
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.
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.
The organisms that make it
Where a specific organism is implicated rather than the fruit. Naming one answers a different question from naming a molecule: what is in the cellar, rather than what is in the glass.
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.
What raises it
Processes that increase this character, or preserve it where it would otherwise be lost.
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.
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.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
The fault it points at
Meeting this descriptor above a certain intensity is how the fault below is usually noticed first.
Fault
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Where it is usually met
Fruit and styles this character is recorded in. A record here is where the descriptor is commonly found, not a claim that every example shows it.
About Butter
Butter is the note that tells a taster malolactic fermentation has run, and in traditional English and French cider that is normal and generally deliberate: malolactic conversion softens acidity, raises pH and improves microbial stability once complete. A cider made in that idiom is expected to show it.
It is unwelcome in ciders sold on fresh fruit character, where it competes directly with the esters, and in those cases producers block malolactic conversion with sulphite, filtration or low temperature. The judgement is style-dependent; the chemistry is not.
Others in the dairy and lactic family
Butter, butterscotch, yoghurt, cream.
Butterscotch
Diacetyl at higher concentration with a caramel edge, sitting at the boundary between character and fault.
Cheesy
A rancid, sweaty note from short-chain fatty acids, chiefly butyric and isovaleric, produced by bacteria.
Cream
A soft, rounded dairy impression from acetoin and lactic acid without the sharpness of free diacetyl.
Soured milk
A sharp, unclean dairy note from uncontrolled lactic activity, recorded as a fault in every style.
Yoghurt
A tangy fermented-dairy note from lactic acid with acetaldehyde, indicating vigorous bacterial activity.
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 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.
- What is sidra natural — Sidra natural is the still, dry, unfiltered cider of Asturias and the Basque Country, bottled without added carbonation and poured from a height to rouse it. It is tart, funky and made to be drunk in small measures poured one at a time.
- What is the difference between french cidre and english cider — French cidre is usually sweeter, weaker and more carbonated, because fermentation is arrested while sugar remains. English cider is more often fermented dry and finished still or lightly sparkling, and is stronger for it.
- 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.
- 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.
Where to go next
- The sensory model — The thirteen structural dimensions and the twenty aroma families, in full.
- Compounds — The chemistry behind the words.
- Troubleshooting — When a perception turns out to be a problem, this is where it is diagnosed.
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.
A Somerset Pomona: The Cider Apples of Somerset
Liz Copas, The Dovecote Press, 2001. ISBN 9781874336877 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against Open Library: Liz Copas, The Dovecote Press, 2001, ISBN 9781874336877. The author was Long Ashton’s cider pomologist, which is why this work is registered for Somerset cultivar identity at all — it is the nearest thing to a successor to the station’s own descriptions. No copy was opened. Not digitised in any open collection.