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.
Also called flor, cider flowers, surface film.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Maturation, Fermentation, Storage
- Can it be fixed?
- Partly — it can be reduced, not removed
- When you notice it
- Fermentation, Maturation, Storage
- Signs
- 6 recorded
What it is
Film yeast growth is the appearance of a whitish or cream-coloured skin on the surface of cider standing in a part-empty vessel. The organisms are oxidative yeasts that grow at the air interface, and they metabolise ethanol, organic acids and glycerol rather than sugar. The cider underneath loses alcohol, loses acidity, gains acetaldehyde and eventually tastes thin and stale. Traditionally called cider flowers, and closely related to the flor of certain fortified wines, where the same phenomenon is cultivated deliberately.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
To look at
- A pale, powdery or chalky film across the surface of the cider — Thin and dusty, breaking up when disturbed, unlike the tough rubbery mat of an acetic pellicle.
- A wrinkled or folded skin in a vessel that has stood a long time
On the nose
- A sherry-like, bruised-apple aroma developing in the cider
- Solvent or pear-drop notes alongside the staleness
On the palate
- The cider tasting flatter and less alcoholic than it did
How the batch behaves
- Falling acidity in a cider that has not been treated
The words for it: Cider shop, Lees. Each links to what produces it.
When it appears. On the surface of a cider standing under air, within days to weeks. It requires a headspace and cannot form in a full sealed vessel.
The presence of a film is proof that the vessel had a headspace, which makes it the most directly actionable finding on this list: whatever else is wrong, the cider was not topped up.
Various journals, Andrew Lea
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.
- Acetification — Smell the cider under the film. Film yeast leaves it stale and slightly thin; an acetic colonisation leaves it vinegary.
- Mould taint — Look at the growth. Film yeast is a flat white or cream mat; mould is fuzzy and often coloured.
- Microbial haze — Surface or body. Film yeast is on top; a haze is distributed through the liquid.
- The velo de flor of a deliberately aged cider — Intent, and whether the vessel was meant to be under air. A few traditions cultivate a surface film; almost no cider cellar does.
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 organisms are aerobic yeasts — Pichia membranifaciens, Candida species and other film-forming yeasts — that grow at the surface where oxygen is available and form a hydrophobic mat of cells that floats. They cannot grow in the body of the liquid, and they cannot grow at all in a full, sealed vessel.
Unlike fermentative yeasts they do not need sugar. They oxidise ethanol to acetaldehyde and beyond, and they metabolise organic acids including malic and lactic acid and glycerol. The consequence for the cider is a fall in alcohol, a fall in acidity, a rise in pH, and a rise in acetaldehyde — which is to say the cider becomes weaker, flatter, less well defended and stale-smelling all at once.
The rise in pH matters more than it first appears. As acidity falls, the cider becomes hospitable to lactic acid bacteria and to the organisms behind mousiness and ropiness, and any sulphite present becomes less effective. A film left to grow does not only produce its own fault; it opens the door to others.
The same phenomenon under control is flor, cultivated deliberately on sherry and on certain other wines to produce a specific oxidative character. The difference is entirely one of intent, vessel management and the drink being aimed at.
How it happens
| Cause | Stage | How often |
|---|---|---|
| A part-empty vessel with cider standing in contact with airThe necessary condition. Everything else is secondary. | Maturation | common |
| An airlock that has dried out or a bung that does not seal | Maturation | common |
| Little or no free sulphur dioxide during a long maturation | Maturation | common |
| A slow or stuck fermentation leaving a low-alcohol cider standing | Fermentation | occasional |
| Warm cellar conditions accelerating growth | Storage | occasional |
| Cider dispensed from a container over days, admitting air each time | Storage | 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 |
|---|---|---|
| Rack the cider from beneath the film into a full, sealed vessel | Partial | The correct action, and it works if done early. Draw from below the surface without disturbing the mat, fill the receiving vessel completely, and seal it. The organisms cannot continue without air. |
| Sulphite after racking | Partial | Helps prevent recurrence, at a rate taken from supplier guidance for the measured pH. It also binds some of the acetaldehyde produced, which improves the aroma. |
| Blend into a larger sound volume | Partial | Reasonable where the cider was caught early and is merely a little stale. Rack and seal first so the organisms are not carried across. |
| Skim the film and leave the cider as it is | Unlikely to work | The film reforms within days if the headspace remains. Skimming without addressing the air achieves nothing. |
| Restore the alcohol and acidity the film consumed | Not possible | What has been metabolised is gone. A cider that has carried a film for months is permanently thinner and flatter. |
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
- Keep vessels genuinely full; move cider into a smaller container rather than leaving headspace.
- Check airlocks regularly and keep them charged, and check bungs and lids seal properly.
- Maintain free sulphur dioxide appropriate to the measured pH where the style permits it.
- Finish fermentations promptly rather than leaving low-alcohol cider standing for months.
- Keep the cellar cool.
- Inspect the surface of standing vessels rather than only tasting from the tap.
Film yeast is the visible warning that a vessel has headspace, and it is worth acting on for that reason alone. The film itself is doing real damage — taking alcohol and acid out of the cider — but the more serious point is that the same air will support acetic acid bacteria, and a cider that has grown flowers is on a path towards vinegar.
Distinguishing it from an acetic pellicle is straightforward once the difference is known. Film yeast forms a thin, powdery, chalky bloom that breaks up when the vessel is moved. Acetic bacteria form a tough, rubbery, gelatinous mat that holds together. Both mean the same thing about the vessel; the second means the damage is further along.
The traditional name, cider flowers, records how familiar the sight once was in farmhouse cellars where vessels were routinely broached and left part-full. It also carries an implicit acceptance that a cider on the wane was normal, which is a different standard from the one a modern maker is working to.
It is the clearest possible illustration of the general principle that oxygen management is the whole of cellar practice. Every fault in this section — film yeast, acetification, oxidation, acetaldehyde — has the same prevention, which is a vessel that is full and sealed.
The compounds involved
Acetaldehyde
The compound sitting one step short of ethanol, which smells of bruised apple and sherry, binds most of the sulphite added to a cider, and is the chemical signature of oxidation.
Ethanol
The alcohol yeast makes from fruit sugar, which converts a perishable juice into a keepable drink and carries most of its aroma to the nose.
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.
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.
Glycerol
A syrupy three-carbon alcohol yeast produces as a side reaction of fermentation, which adds weight to a dry cider and is the raw material for one of its more obscure faults.
Ethyl acetate
The most abundant ester in cider, giving lift and pear-drop at low concentration and nail varnish at high, and the earliest audible warning that acetic bacteria are at work.
Dissolved oxygen
Essential to a healthy yeast population at the start of fermentation and the principal enemy of a cider from the moment fermentation ends.
The organisms involved
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.
Candida species
A large, historically artificial grouping of yeasts that appears throughout cider microbiology, containing organisms with little in common beyond the absence of a sexual stage.
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.
Wickerhamomyces anomalus
A tolerant, ester-heavy yeast common in fruit and juice, notable both as a producer of ethyl acetate and as a source of antimicrobial killer toxins.
Where in the process it arises
Topping up
Refilling a maturing vessel as evaporation and racking losses lower the level, so that no significant surface of cider is ever left in contact with air.
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.
Fermentation vessels
The container a cider ferments in — wood, stainless, plastic, glass or concrete — and how its permeability, thermal mass and resident microflora shape the result.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Cellar storage
Holding packaged cider in conditions that let it change slowly and predictably, where stability of temperature matters more than the temperature itself.
Faults it is confused with
These present similarly. What separates them is set out on each page.
Acetaldehyde excess
A bruised-apple, green-nut or sherry aroma from acetaldehyde, produced by oxidation, by film yeast, or left behind by a ferment that was interrupted.
Acetification
The active conversion of a cider’s ethanol into acetic acid by acetic acid bacteria at an air interface — the process, running in the vessel, that produces volatile acidity.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
Ethyl acetate taint
A solvent, nail-varnish or pear-drop aroma from ethyl acetate, the most abundant ester in cider and the one that turns from lift to fault over a narrow range.
Low acidity
A cider without enough acid to give it definition, tasting soft, heavy and dull — and sitting at a pH that leaves it exposed to spoilage organisms.
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.
How cider is made, start to finish
How is cider actually made?
What to measure, and what each number tells you
What should I be measuring, and what do the numbers mean?
When the fermentation is not doing what it should
My fermentation is doing something strange. What is going on?
Your first batch of cider
I have apples and no equipment. What do I actually do?
Planning a batch: fruit in, bottles out
How much fruit do I need, and how many bottles will I get?
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 my cider taste like vinegar — Acetic acid bacteria have reached the cider and, given air, are converting its alcohol into acetic acid. The cause is almost always oxygen — an unfilled vessel, a leaking bung, or a slow transfer.
- Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
- Why does my cider smell like nail varnish — That is ethyl acetate, formed when acetic acid combines with ethanol. In small amounts it reads as pear drops; above threshold it smells of solvent or nail varnish remover and usually accompanies rising volatile acidity.
- How many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
- 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.
- How should i store cider — Cool, dark and at a steady temperature. Light causes light-strike in clear glass, warmth accelerates oxidation, and temperature swings push cider past the closure of a bottle that is not tightly sealed.
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.
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.
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.