Glossary · Faults
Film yeast
Also written flor, flowers of cider, mycoderma. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
Aerobic yeasts, chiefly Pichia and Candida species, that grow as a dull white or grey film on the surface of cider in a part-empty vessel, metabolising ethanol and acids.
- Subject
- Faults
- Also written
- flor, flowers of cider, mycoderma
In more detail
A film is a warning sign as much as a fault in itself: it means the vessel has an air surface, which is the condition acetic bacteria also need. Left alone, the film thickens, wrinkles and eventually sinks in flakes, and the cider beneath loses acidity and gains stale, flat character.
The remedy is mechanical and immediate — remove the film, rack the sound cider into a full clean vessel under sulphite, and correct whatever left the headspace.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Fault
Film yeast growth
A pale, powdery or wrinkled film of aerobic yeast growing on the surface of a standing cider, consuming alcohol and acid and producing acetaldehyde.
Microbiology
Pichia membranifaciens
A film-forming yeast that grows as a skin on the surface of cider left in contact with air, consuming ethanol and acid and leaving the cider thin.
Microbiology
Film yeasts
A functional grouping rather than a taxon: the oxidative yeasts that form a skin on cider exposed to air and consume its alcohol and acid.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Fermentation
Ullage
The empty space between the surface of the cider and the top of its container, and by extension the volume lost that creates it. Ullage in a maturing cask is where oxidation and acetification start.
Faults
Acetification
The oxidation of ethanol to acetic acid by acetic acid bacteria, turning cider progressively into vinegar. It requires oxygen and is the classic fault of a slack, part-empty cask.
Faults
Mother of vinegar
The gelatinous cellulose mat produced by acetic acid bacteria at a liquid surface. In a vinegar barrel it is the working culture; in a cider cask it is proof that the batch is being lost.
Fermentation
Topping up
Refilling a maturing vessel with similar cider to eliminate the headspace, so that no air surface remains for oxidative and acetic spoilage.
Faults
Flat
Lacking both carbonation and freshness — a cider that has lost its gas and, usually with it, its aromatic lift and acidity. In a still cider the fault is the lifelessness, not the absence of bubbles.
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.
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.