Fault
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.
Also called VA, volatile acid.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Maturation, Fermentation, Harvest
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Fermentation, Maturation, Storage
- Signs
- 5 recorded
What it is
Volatile acidity is the portion of a cider’s total acid that will carry over in steam — in practice almost entirely acetic acid, with a contribution from its ester, ethyl acetate. Malic and lactic acid are fixed acids and do not count towards it. A small volatile fraction is present in every fermented drink and is part of normal complexity; volatile acidity becomes a fault when it rises far enough to be smelled as vinegar and felt as a hot, scouring finish that no amount of residual sugar covers.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- A sharp vinegar note in the nose that arrives before any fruit does
- Fruit character that seems to have been stripped out rather than covered up — Rising volatile acidity masks esters, so the cider reads as bare as well as sharp.
- A prickle at the top of the nose when the glass is first raised
On the palate
- A hard, hot burn at the back of the finish that lingers after the flavour has gone — Acetic acid is perceived as much as an irritant as a flavour, which is why the finish feels rough rather than simply sour.
How the batch behaves
- The same batch tasting progressively rougher over weeks in a part-empty vessel
The words for it: Vinegar, Glue. Each links to what produces it.
When it appears. Rises gradually rather than arriving. A batch that was sound in November and rough in March has usually been sitting in a part-empty vessel the whole time, and the change is continuous rather than an event.
Perceived intensity depends on pH as well as on concentration, because more of the acid is in its undissociated volatile form at low pH. Two ciders with the same laboratory figure can therefore differ noticeably in how vinegary they smell, which is why a bench figure and a tasting disagree more often here than for most faults.
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.
- Ethyl acetate taint — Ethyl acetate reads as solvent, nail varnish or pear drop and hits the top of the nose; volatile acidity reads as vinegar and burns at the back of the finish. They usually arrive together — the question is which dominates.
- Acetification — Acetification is the process, volatile acidity the measurement. A cider with raised VA from a stressed ferment has no acetic bacteria in it; one that has acetified has a surface film or a vinegar-fly history to go with the smell.
- Excessive acidity — Sharpness without any vinegar in the nose is fixed acid — malic or lactic — and is a blending problem. Volatile acidity announces itself before the glass reaches the mouth.
- Traditional volatile lift — Whether the acetic note arrives before or after the fruit. Lift sits behind the apple; a fault sits in front of it.
A trace of volatility is part of what people recognise in West Country farmhouse cider and in sidra natural, and there is no analytical line at which lift becomes fault. The practical test is order of arrival rather than presence: a cider whose fruit has disappeared behind vinegar has crossed it.
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 dominant contributor is acetic acid, produced when acetic acid bacteria — chiefly Acetobacter aceti and Acetobacter pasteurianus, with Gluconobacter oxydans active earlier on sugar — oxidise ethanol using dissolved oxygen. The conversion runs through acetaldehyde as an intermediate and requires air continuously; these are obligate aerobes and cannot work in a genuinely full, sealed vessel.
A second and quite separate route contributes to the same number. Yeasts, especially the apiculate species such as Hanseniaspora valbyensis that dominate the first days of a spontaneous ferment, produce acetic acid as a by-product of their own metabolism, and Saccharomyces cerevisiae itself produces more of it when stressed by high sugar, extreme temperature or nitrogen shortage. Lactic acid bacteria acting on residual sugar rather than on malic acid — a heterolactic route — add more again.
The perceived severity is not set by acetic acid alone. Acetic acid esterifies slowly with ethanol to give ethyl acetate, whose odour threshold is far lower, so a cider can smell strongly of solvent and vinegar at an acid concentration that is not itself extreme. The two are usually reported together because they usually rise together, and because the palate does not separate them.
Perceived intensity also depends on pH. At a low pH more of the acetic acid is present in its undissociated, volatile form and is therefore smelled more readily, so two ciders with the same analytical volatile acidity can differ noticeably in how vinegary they seem.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Air space left above the cider in a part-empty vesselThe single most frequent origin. Acetic acid bacteria need oxygen, and a headspace supplies it continuously. | Maturation | common |
| Slow or stuck fermentation leaving sugar and low alcohol for an extended period | Fermentation | common |
| Rotten or damaged fruit carrying a heavy acetic bacterial and vinegar-fly load into the mill | Harvest | common |
| Pumps, taps and transfer lines that splash and aerate at every racking | Maturation | occasional |
| Barrels or vessels that were previously spoiled and never properly cleanedWood is porous and holds an acetic population that colonises the next fill. | Maturation | occasional |
| Fermentation run hot, or with a nitrogen shortage, raising the yeast’s own acetic output | Fermentation | 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 |
|---|---|---|
| Let the batch go on to become cider vinegar deliberately | Reliable | A batch that is already substantially acetified is on its way to a genuinely useful product. Run it in a separate vessel and keep it out of the cellar, because an active acetic culture is a source of infection for everything else. |
| Blend the affected batch into a much larger volume of sound cider | Partial | Dilution works arithmetically, and is what a commercial cellar would consider first. Do a bench trial before committing the sound cider, because blending can spoil a good batch rather than rescue a bad one, and the fault will still be detectable if the ratio is wrong. |
| Seal the vessel, exclude air and stop the rise where it is | Partial | This does not reverse anything, but it prevents further conversion. If the fault is caught early the batch may still be usable young. |
| Mask the fault with sweetening | Unlikely to work | Sugar shifts the balance a little but does not touch the aroma, and the hot finish remains. It also introduces a refermentation risk if the cider is then bottled. |
| Remove acetic acid from the finished cider | Not possible | There is no domestic method for taking acetic acid back out of cider. Techniques used industrially are membrane processes on a scale and cost no home producer has access to. |
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 full and topped up, and move cider into a smaller container as soon as the volume no longer fills the one it is in.
- Fit airlocks that actually seal, and check the water in them rather than assuming it has not evaporated.
- Sort rot out of the fruit before milling rather than trusting fermentation to deal with it.
- Get the primary ferment started promptly and finished cleanly, so there is never a long period of low alcohol and available sugar.
- Rack quietly, with the outlet below the surface of the receiving vessel, rather than letting cider fall through air.
- Where sulphite is used, take the addition rate from the supplier’s instructions and from the juice pH, not from a figure carried in the head — the antimicrobial effect of sulphur dioxide changes sharply with pH.
Volatile acidity is the number a laboratory reports; acetification is the process that produces it. Keeping the two apart matters, because a cider can carry a raised volatile acidity from a stressed ferment without ever having been colonised by acetic acid bacteria, and the remedy for one is not the remedy for the other.
What makes it the defining spoilage risk of small-scale cidermaking is that everything about a farmhouse cellar favours it. Vessels are rarely completely full, wood is porous, fruit arrives off the ground, and the traditional slow ferment holds the cider at low alcohol for weeks. Every one of those is an invitation to an organism that only needs ethanol and air.
It is also the fault most often defended as character. A trace of volatility genuinely does lift a West Country cider and is part of what people recognise in traditional styles, and there is no analytical line at which lift becomes fault. What is not defensible is a cider whose fruit has disappeared behind vinegar, and the practical test is simple: if the acetic note arrives before anything else does, it has stopped being complexity.
The important thing to tell a home producer is that a vinegary cider is spoiled, not unsafe. Acetic acid is the acid of vinegar, and the batch will not hurt anyone. The loss is the season’s work, which is reason enough to take headspace seriously.
The compounds involved
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.
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.
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.
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
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.
Acetobacter pasteurianus
The film-forming acetic acid bacterium of traditional vinegar production, and a common cause of surface growth and volatile acidity in cider.
Gluconobacter oxydans
A sugar-preferring acetic acid bacterium abundant on damaged fruit and in fresh juice, which oxidises glucose to gluconic acid before fermentation begins.
Hanseniaspora valbyensis
The apiculate yeast most consistently reported as dominating the first days of a spontaneous cider fermentation, and a major contributor to its aroma.
Lactobacillus brevis
An obligately heterofermentative lactic acid bacterium, renamed *Levilactobacillus* in 2020, associated with volatile acidity, biogenic amines and mousiness rather than with clean malolactic conversion.
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.
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.
Fruit sorting
Taking rotten, mouldy and damaged fruit and foreign material out of the crop before it reaches the mill, which is the primary control on patulin in cider.
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.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
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.
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.
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.
Diagnosing a batch that has gone wrong
Something is wrong with my cider. How do I work out what?
How cider is made, start to finish
How is cider actually made?
How to taste cider and perry
How do I taste cider properly?
Making perry
How do I make perry, and what is different about it?
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?
Writing a tasting note that is worth reading later
How do I write a tasting note that is actually useful?
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.
- What is oxidation in cider
- 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.
- 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.