Organism
Acetobacter pasteurianus
The film-forming acetic acid bacterium of traditional vinegar production, and a common cause of surface growth and volatile acidity in cider.
Also called Acetobacter pasteurianum.
- Kind
- Bacteria
- Binomial
- Acetobacter pasteurianus
- Role
- Spoilage, Surface film, Present throughout
What it does
- Oxidises ethanol to acetic acid by the same two-step membrane-bound route as A. aceti, dependent throughout on dissolved oxygen.
- Forms a surface pellicle on cider exposed to air, positioning the population where oxygen is most available and making the growth visible before the acidity is severe.
- Produces ethyl acetate alongside acetic acid, giving the combined vinegar-and-solvent aroma characteristic of acetification.
- Tolerates higher acetic acid concentrations than many acetic acid bacteria, which is why it is the traditional organism of surface-culture vinegar making.
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 15 °C to 35 °C, with acetification accelerating markedly above 25 °C. |
| pH | Very acid-tolerant; unaffected by anything in the cider range. |
| Oxygen | Obligately aerobic and dependent on a liquid surface or on dissolved oxygen from ingress. |
| Alcohol tolerance | Grows at cider strength; ethanol is its substrate. |
| Sulphite tolerance | Moderately sensitive, but not reliably controlled by sulphite where air is available. |
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.
Faults it causes
Faults this organism is implicated in. Several are faults only against a particular expectation — the same activity is a signature elsewhere.
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.
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.
Vinegar fly contamination
Small flies carrying acetic acid bacteria and spoilage yeasts directly into juice and cider, and depositing them wherever they land.
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.
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.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
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.
Cellar storage
Holding packaged cider in conditions that let it change slowly and predictably, where stability of temperature matters more than the temperature itself.
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.
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 pasteurianus
Acetobacter pasteurianus does the same chemistry as A. aceti and differs mainly in how it arranges itself. It readily forms a film at the surface, which historically made it the organism of the Orleans process and other surface-culture vinegar methods, where a mat of bacteria floats on the liquid with air above and alcohol below.
In a cidery that same habit is a warning sign. A greyish, slightly slimy film on cider in a part-empty vessel may be a film yeast, may be acetic acid bacteria, or may be both, and the distinction matters because the second is doing far more damage. A maker who finds surface growth should check volatile acidity rather than simply skimming it off.
The two Acetobacter records here are separated because both names appear in cider literature and quality reports, not because a maker needs to tell them apart. They respond identically to the only control that matters, which is excluding 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 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.
- 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.
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.
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.