Glossary · Faults
Diacetyl
Definition
A compound produced by lactic acid bacteria from citrate and by yeast as a fermentation intermediate, smelling of butter, butterscotch or hot popcorn.
- Subject
- Faults
In more detail
At low levels it adds a rounded, creamy note that is part of the expected character of many malolactic ciders. At high levels it dominates and reads as artificial butter. Because pears carry more citric acid than apples, perry undergoing malolactic conversion is more prone to excess.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Fault
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Chemistry
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Fermentation
Malolactic fermentation
The bacterial conversion of malic acid into lactic acid and carbon dioxide, which lowers total acidity, raises pH slightly and softens the drink. It is a bacterial process, not a yeast fermentation.
Microbiology
Lactic acid bacteria
The bacteria that convert malic acid to lactic acid and, in other conditions, produce the disorders of ropiness, mousiness and acrolein bitterness. *Oenococcus*, *Lactiplantibacillus* and *Lactobacillus* species dominate in cider.
Chemistry
Citric acid
A minor acid in apples but a significant one in pears, where it makes up a larger share of total acidity and behaves differently from malic acid under bacterial attack.
Perry
Perry
A fermented alcoholic drink made from the juice of pears, and specifically from perry pears rather than dessert pears. It is not cider made with pears added: it is a separate drink with its own fruit, chemistry and problems.
Where to go next
- Cider glossary — The whole vocabulary, by subject and A–Z.
- Comparisons — Terms that are only really clear when set against the thing they are not.
- Search — Finds terms by their aliases and by their spellings in other languages.
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.