Fault

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.

Also called turning to vinegar, acetic spoilage.

Severity: Serious — the batch may not be salvageable
Where it starts: Maturation, Harvest, Storage
Can it be fixed?: Yes, if caught in time
When you notice it: Fermentation, Maturation, Storage
Signs: 5 recorded

What it is

Acetification is what is happening when a cider is turning to vinegar: a population of acetic acid bacteria growing at the surface where cider meets air, oxidising ethanol to acetic acid and consuming the alcohol as it goes. Unlike most faults it is a live, ongoing process with a visible organism, and it gets worse every day it is left. A cider that is acetifying has a trajectory, and the useful question is not what it tastes like now but how fast it is moving.

What you notice

Grouped by sense, because that is how the fault presents itself rather than how it works.

To look at

A dull, wrinkled skin or slimy film across the surface of the cider — A thick, gelatinous mat is the *Komagataeibacter* cellulose pellicle — the vinegar mother.
Small flies around the bung or airlock

On the nose

A smell of vinegar rising off the vessel when the lid is lifted, not only from a poured glass

How the batch behaves

The cider tasting sharper each time it is checked over a few weeks

On the palate

Falling alcoholic strength alongside rising sharpness — Acetification consumes ethanol, so the drink thins as it sours.

The words for it: Vinegar. Each links to what produces it.

When it appears. Needs air and time. It is a maturation and storage fault far more often than a fermentation one, because an actively fermenting vessel is under a blanket of its own carbon dioxide.

What is known about detecting it

The acetic bacteria responsible are obligate aerobes and cannot work in a genuinely full, sealed vessel, so the presence of the fault is itself evidence that air reached the cider. That is the most useful diagnostic fact about it: the question is not whether the organisms were present but where the oxygen came from.

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.

Volatile acidity — Volatile acidity is a number; acetification is a colonisation. Look at the surface and the vessel: a film, a ring at the liquid line, or flies mean acetification rather than a stressed ferment.
Film yeast growth — Both make a surface film. Film yeast smells stale and slightly of solvent and does not make the cider vinegary; an acetic film comes with the vinegar note.
Deliberate cider vinegar — Intent, and the vessel. A batch being made into vinegar is kept separately and warm; a batch that acetified was meant to be cider and was in a vessel that was not full.

Oxygen, wild organisms and residual sugar each open a route to a specific group of faults, and each has its own prevention.

Described in full

Layout: Three parallel columns, each running downwards: the condition, the organism or reaction it permits, the faults that follow, and the practice that prevents it.
Oxygen: Air in a part-empty vessel or at every transfer. It permits acetic acid bacteria to work, and it drives chemical oxidation of phenolics independently of any organism.
What oxygen produces: Volatile acidity and, at higher concentration, frank acetification — cider turning to vinegar. Alongside it, oxidation gives sherry, walnut, bruised-apple and cardboard notes as acetaldehyde and browning products accumulate.
Wild organisms: Present on the fruit and in old wood. Brettanomyces and certain lactic bacteria decarboxylate hydroxycinnamic acids to volatile phenols; some lactic bacteria also form the tetrahydropyridines behind mousiness.
What wild organisms produce: Phenolic character — leather, smoke, sticking plaster — which is a signature at low levels and a fault above them. Mousiness is not detectable on the nose and appears only as a lingering aftertaste, which is why it is so often missed.
Residual sugar: Any fermentable sugar left in a sealed container is a fuel supply for whatever yeast survives filtration or arrives afterwards.
What residual sugar produces: Refermentation in the bottle, and with it over-carbonation, gushing on opening and, in the worst case, a bottle that fails under pressure. This is a safety matter, not only a quality one.
Prevention: Keep vessels full and cool, and minimise transfers, for oxygen. Sulphite and sanitation for wild organisms. For residual sugar, either stabilise and filter, or leave the cider genuinely dry — the only condition that cannot referment is one with nothing left to ferment.

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.

Acetic acid bacteria oxidise ethanol in two enzymatic steps at the cell membrane: alcohol dehydrogenase converts ethanol to acetaldehyde, and aldehyde dehydrogenase converts acetaldehyde to acetic acid. Both steps pass electrons to the respiratory chain and both require molecular oxygen as the terminal acceptor. This is why the reaction is confined to the air interface and why a completely full, sealed vessel arrests it.

Acetobacter aceti and Acetobacter pasteurianus are the species that dominate an acetifying cider, tolerant of the alcohol present and able to keep working as the acid accumulates. Gluconobacter oxydans behaves differently: it prefers sugar to ethanol, so it is a juice and early-ferment organism, producing gluconic acid and contributing to a batch that starts wrong rather than to one that sours late.

Komagataeibacter species secrete extracellular cellulose, building the tough floating mat traditionally called the mother. The mat is not the cause of the souring — the bacteria are — but it is diagnostic, because it holds the population at exactly the oxygen-rich interface where it works fastest, and it survives being disturbed.

The whole system is self-accelerating in a warm cellar. Growth rates rise with temperature, the film increases the effective surface area, and every racking that splashes reintroduces the oxygen the population needs.

How it happens

Causes, with the stage each originates at and how often it is the explanation.
Cause	Stage	How often
A vessel left standing part-empty for weeks or months	Maturation	common
A barrel or vessel carrying a resident acetic population from a previous spoiled fillWood cannot be reliably sterilised at domestic scale, and a barrel that has made vinegar once will do it again.	Maturation	common
Vinegar flies carrying bacteria directly into an open or poorly sealed vessel	Maturation	common
Rot, bruising and ground-lying fruit bringing a heavy load in with the juice	Harvest	occasional
Warm cellar temperatures accelerating a population that would otherwise stay marginal	Storage	occasional
A dispensing arrangement that lets air into a container of finished cider over days	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.

Corrective options and how well each actually works.
Option	Effectiveness	What it involves
Commit the batch to vinegar in an isolated vessel	Reliable	The most honest use of a batch that has gone far. Keep it physically separate from the cider store, and use separate equipment for it.
Seal the vessel completely and fill the headspace so the process stops	Partial	This genuinely halts further conversion because the bacteria cannot work without oxygen. It does not reduce the acetic acid already made. Caught in the first days it can save a batch; caught after months it cannot.
Blend into a much larger sound volume after arresting the process	Partial	Worth a bench trial. Only attempt it once the vessel is sealed and the population is no longer active, or the infection travels with the blend.
Skim the film and carry on	Unlikely to work	The visible mat is a fraction of the population. Skimming removes the sign and leaves the organism, and the film returns.
Reverse the souring and recover the alcohol	Not possible	The ethanol has been consumed. Nothing available to a home producer puts it back, and any process claiming to concentrate or strip the batch is not something to attempt in a domestic kitchen.

On dosages

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 every vessel genuinely full; a floating lid or a smaller container beats a half-filled demijohn every time.
Screen and seal openings, and keep flies out of the cellar during and after pressing.
Retire wooden vessels that have acetified rather than trying to recover them for cider use.
Clean and sanitise pumps, hoses and taps between transfers, since the residues in them are where the population survives between batches.
Keep the cellar cool and stable; acetic bacteria work far more slowly at low temperature.
Take any sulphite addition rate from the supplier’s guidance for the juice pH in front of you rather than from a general figure.

More on this

Cider and vinegar are two stages of the same journey, and the only thing standing between them is air. That is a genuinely useful way to hold the fault in mind, because it makes the prevention obvious: everything that keeps oxygen away from the liquid keeps the cider a cider.

The distinctive feature of acetification is that it is visible. Most cider faults are inferred from taste; this one grows a skin. A pale, dull film that wrinkles when the vessel is moved is worth looking at closely, because a thin bloom is more likely to be film yeast, while a tough, rubbery, gelatinous mat is a cellulose pellicle and means acetic bacteria are established.

It is also the fault where speed of response decides the outcome. A vessel caught within days of the film appearing, sealed and topped up, may produce cider that is sharp but sound. The same vessel left another month will not. Nothing about the process pauses while a decision is being made.

Deciding to make vinegar deliberately is not a defeat, but it does need to be done somewhere else. An active acetic culture in a cellar is an ongoing source of infection for every other vessel in it, carried on flies, cloths, hoses and hands.

The compounds involved

The organisms involved

Where in the process it arises

Faults it is confused with

These present similarly. What separates them is set out on each page.

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.

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.
How do i sterilise cider bottles — Wash them clean first, then sanitise with a no-rinse sanitiser or a sulphite solution, and fill while still wet with it. Bottles that look clean but have dried deposits inside are the usual source of bottle spoilage.
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.
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.
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.