Organism
Acetobacter aceti
An acetic acid bacterium that oxidises ethanol to acetic acid wherever cider meets air, and the organism behind most volatile acidity in cider.
- Kind
- Bacteria
- Binomial
- Acetobacter aceti
- Role
- Spoilage, Present throughout
What it does
- Oxidises ethanol in two membrane-bound steps — alcohol dehydrogenase to acetaldehyde, then aldehyde dehydrogenase to acetic acid — using dissolved oxygen as the terminal electron acceptor, which is why the reaction is strictly dependent on air.
- Esterifies part of that acetic acid with ethanol to give ethyl acetate, so acetification and solvent character arrive together and the fault is usually smelled before the acidity is tasted.
- Overoxidises acetic acid further to carbon dioxide and water once ethanol is exhausted, a trait that distinguishes Acetobacter from Gluconobacter and matters to anyone deliberately making vinegar.
- Survives in low numbers in almost every cidery — on fruit, in press cloths, in pipework, carried by vinegar flies — and requires only oxygen to become a problem.
Conditions it works in
What the organism tolerates and what suppresses it. These are the levers a maker actually has: temperature, acidity, air, alcohol and sulphite.
| Condition | What is recorded |
|---|---|
| Temperature | Grows across roughly 10 °C to 35 °C, with a marked optimum around 25–30 °C; warm cellars accelerate acetification sharply. |
| pH | Highly acid-tolerant, growing well below pH 3.5 and continuing as its own acid accumulates. |
| Oxygen | Obligately aerobic. This is the single most important fact about the organism: with no oxygen there is no acetic acid, and every practical control follows from it. |
| Alcohol tolerance | Grows readily at cider strength and well beyond; ethanol is its substrate, not its limit. |
| Sulphite tolerance | Moderately sensitive to free sulphur dioxide, but sulphite does not substitute for excluding air. |
What it produces
Compounds this organism makes. Which organism made a compound usually decides whether it reads as a feature or as a symptom.
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
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.
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.
Carbon dioxide
The other product of fermentation, which protects a cider from air while it is being made and, dissolved in the finished drink, changes both its texture and its perceived acidity.
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.
Faults it causes
Faults this organism is implicated in. Several are faults only against a particular expectation — the same activity is a signature elsewhere.
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.
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.
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.
Vinegar fly contamination
Small flies carrying acetic acid bacteria and spoilage yeasts directly into juice and cider, and depositing them wherever they land.
Oxidation
The cumulative effect of oxygen on finished cider: fruit aroma flattens, colour deepens towards amber, and a bruised-apple or sherry-like character replaces the fresh one.
Microbial haze
Cloudiness caused by a growing population of spoilage organisms, and therefore a symptom of something worse rather than a clarity problem in itself.
Where in the process it appears
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.
Oxygen management in fermentation
Giving the yeast the oxygen it needs early to build viable membranes, then excluding it once fermentation is under way and especially once it slows.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Barrel ageing
Holding cider in wood — which may mean an old neutral vat used simply as a vessel, or fresh or spirit-seasoned oak used as a source of flavour, and the two are not the same operation.
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.
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
Cellar storage
Holding packaged cider in conditions that let it change slowly and predictably, where stability of temperature matters more than the temperature itself.
Wild fermentation
Letting the microflora already present on the fruit and in the press house carry the ferment, in a succession rather than as a single organism.
Maceration
Holding milled pomace before pressing so that phenolics, aroma precursors and pectin have time to move out of the solid tissue and into the juice.
Microbial succession
The ordered handover of a spontaneous ferment from apiculate yeasts to *Saccharomyces* to lactic acid bacteria, and the spoilage organisms waiting at the end of it.
Bag-in-box
A collapsing laminate bladder in a card outer, the standard farmgate and draught format, which keeps air out during dispense but lets oxygen through the film over weeks.
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
Fruit washing
Removing soil, grass, stones and surface contamination from gathered fruit, usually in a water flume, before it reaches the mill.
Kegging
Filling cider into a sealed keg for gas dispense or into a vented cask for gravity and handpump service — two formats whose difference is not the container but whether the cider ever meets air.
Shaking and collecting
Bringing fruit down with a pole or letting it fall, then gathering it from the orchard floor — the traditional way most cider and perry fruit has always been harvested.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
Windfall management
Deciding which fruit that has reached the orchard floor can be used, and getting the rest out of the crop before it becomes a patulin problem.
Juice storage
Holding unfermented juice sound between pressing and fermentation, by chilling, sulphiting, gas blanketing, freezing or aseptic filling.
Pomace conditioning
Letting milled pomace stand before it goes to the press so that it drains better, presses faster and gives more juice.
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.
About Acetobacter aceti
Acetic acid bacteria are the last organisms in the cider succession and the only ones with no natural end point. Yeasts run out of sugar; lactic acid bacteria run out of malic acid; Acetobacter runs out only when the ethanol is gone, at which point the cider is vinegar. They are present in essentially every cidery in small numbers and are held in check by one thing: the absence of oxygen.
That makes acetification a design problem rather than a hygiene problem. The interventions that work are all about air — keeping vessels full, topping up as cider is withdrawn, minimising headspace, racking with as little splashing as the equipment allows, sealing barrels properly, replacing air with an inert gas where that is available, and controlling vinegar flies, which carry acetic acid bacteria directly to the surface of the liquid and are a documented vector rather than merely a nuisance.
The fault is also chemically compound. As acetic acid accumulates, some of it esterifies with the cider’s ethanol to give ethyl acetate, whose aroma threshold is far lower than acetic acid’s taste threshold. A cider heading towards acetification usually announces it as nail varnish and pear drop long before it tastes of vinegar. In small amounts a trace of volatile acidity is a normal part of many traditional ciders; the trajectory, not the presence, is what a maker judges.
It is worth adding that this organism is not only a spoiler. Cider vinegar is a deliberate product made by giving Acetobacter precisely the conditions a cider maker spends the year denying it: warmth, a large surface area and continuous air.
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.
- 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 oxidation in cider
- 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
- All organisms — Grouped by what each does in the ferment, and by what kind of organism it is.
- Compounds — The chemistry this microbiology produces.
- Troubleshooting — Work from the symptom in the glass back to the organism.
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.