Fault
Stuck fermentation
A fermentation that has stopped before the sugar is gone and will not restart, leaving a sweet, low-alcohol cider that is vulnerable to everything.
Also called arrested fermentation, stopped ferment.
A stuck fermentation must never be bottled in glass on the assumption that it will stay stopped. Residual sugar plus any surviving or newly introduced yeast can restart in the bottle, and a sealed glass bottle has no way to release the carbon dioxide produced. Bottles burst, and flying glass causes serious eye and hand injuries. Before a sweet cider goes into any sealed container, either the sugar or the organisms must be removed: confirm stability by measurement, and stabilise by filtration, pasteurisation or an appropriate preservative regime worked out from supplier and regulatory guidance. If a batch has already been bottled and the sugar level is unknown, move the bottles somewhere cool immediately, keep them out of warm rooms and away from sunlight, and treat them as pressurised.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Fermentation, Juice treatment
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Fermentation
- Signs
- 5 recorded
What it is
A stuck fermentation is one that has ceased with fermentable sugar still present and shows no sign of resuming. It is distinct from a deliberately arrested fermentation, where the maker stops the ferment on purpose to retain sweetness, and from a sluggish one that is still moving. The immediate consequence is a cider that is sweeter and weaker than intended; the more serious consequence is a vessel of sugary, low-alcohol liquid that every spoilage organism in the cellar can use, and a bottle that may referment later.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
How the batch behaves
- Gravity readings that have not moved over several consecutive days — The definitive test. Airlock activity is not a measurement; a hydrometer is.
- The airlock silent while the cider is still noticeably sweet
To look at
- No yeast in suspension and a compacted bed at the bottom of the vessel
On the palate
- A sweet, thin taste with obvious sugar and little alcoholic warmth
On the nose
- Rising sulphurous or vinegary notes as other organisms take the opportunity
When it appears. Identified rather than observed: it is two identical gravity readings several days apart at a gravity well above dry. Nothing visible distinguishes a stuck ferment from a finished one.
The distinction between stuck and finished is not available from the liquid: it comes from knowing the starting gravity, the fruit and whether the juice was keeved. This is the clearest case on the site of a fault that cannot be diagnosed without a record.
Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123, Various journals and institute reports
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.
- Sluggish fermentation — Take two readings several days apart. A sluggish ferment is still moving, slowly; a stuck one is not moving at all. The remedies differ, and warming a sluggish ferment is usually enough.
- A perry that has finished — Sorbitol does not ferment and contributes to gravity, so a perry commonly finishes well above 1.000. A perry reading 1.010 and stable may be complete rather than stuck, and treating it as stuck creates problems that were not there.
- A keeved cider that stopped as intended — Whether the juice was keeved. A keeve removes nitrogen deliberately so that the ferment stops with sugar remaining — the stop is the objective, not the failure.
- A hydrometer reading taken warm — Temperature-correct before concluding anything. A reading taken on a cold sample reads high.
Described in full
- Shape
- Three curves on one pair of axes. Time runs left to right over roughly eight weeks; specific gravity runs top to bottom from 1.055 down to 0.995. All three curves flatten before the right-hand edge, and the flat sections are drawn with a repeated dot so that the horizontal run is countable rather than only visible.
- The finished cider
- A solid line falling from about 1.052 to just below 1.000 and then running flat for the last fortnight. This is the case every maker expects: the yeast took essentially all the fermentable sugar and stopped because there was none left.
- The stalled cider
- A dashed line falling from the same start, dropping more slowly, and flattening at about 1.020 in week three. It then runs flat for five weeks. The gravity has stopped moving exactly as decisively as the finished cider’s did, and a maker reading only the last two figures cannot tell the two curves apart.
- The perry
- A dotted line falling from about 1.055 and flattening at about 1.012. It has finished: there is no fermentable sugar left in it. The gravity sits high because a large part of what was dissolved was sorbitol, which yeast cannot ferment at all.
- What separates them
- Not the fact of stopping, and not the shape of the curve. It is where each one stopped, read against what was in the juice to begin with — a cider at 1.020 has sugar it did not take, and a perry at 1.012 has none. The reading that identifies a stall is the starting analysis, not the finishing one.
- Why the flat sections matter
- Two identical readings several days apart are the only evidence that a ferment has stopped, and all three curves supply that evidence. Two identical readings are therefore a statement that fermentation has ended and not a statement that it has succeeded.
- The scale is indicative
- Temperature governs the timescale entirely. A cold cellar ferment can take six months to draw the same shape, and a stall in a cold cellar is particularly easy to mistake for slowness.
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 commonest cause in cider is nitrogen exhaustion. Apple juice, particularly from traditional unfertilised orchards, is often low in yeast assimilable nitrogen, and a yeast population that has run out of nitrogen cannot maintain the sugar transporters it needs. Once those transporters are lost the cells stop taking up sugar even though sugar is abundant, and the condition is not reversed simply by waiting.
Temperature is the second cause and works in both directions. A cold shock — a cellar that drops sharply overnight — causes yeast to flocculate and settle out of suspension, effectively removing the population from the liquid. Excessive heat damages membranes and leaves cells unable to work at the ethanol concentration they have already produced.
Keeving is a deliberate version of the same phenomenon and shows how it works. In a keeved ferment, pectin methylesterase demethylates the pectin, calcium bridges it into a gel that floats the solids and the yeast into a chapeau brun, and the clarified juice underneath is left so nutrient-poor that fermentation slows to a crawl and stops naturally with sugar remaining. That is a controlled starvation, and an accidental stuck ferment is the same thing without the control.
Other contributors include an excessive sulphite addition before or during fermentation, a very high starting gravity, and inhibitory residues from cleaning agents or orchard sprays. Once fermentation has stopped, the cider is at its most vulnerable: low alcohol, residual sugar, and often no protective sulphite.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Yeast assimilable nitrogen exhausted before the sugar was | Fermentation | common |
| Cellar temperature falling sharply and dropping the yeast out of suspension | Fermentation | common |
| Fermentation run too warm, damaging the yeast population | Fermentation | occasional |
| A very high starting gravity, so ethanol reaches an inhibitory level before the sugar is used | Fermentation | occasional |
| Sulphite added at a rate that inhibited the yeast as well as the spoilage organisms | Juice treatment | occasional |
| Sanitiser or detergent residue left in the vessel | Fermentation | rare |
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 |
|---|---|---|
| Accept the cider as sweet and stabilise it | Reliable | A legitimate outcome, and what a keeved cider is. It requires a deliberate stabilisation decision — filtration, pasteurisation, or an appropriate preservative regime — because sweet unstabilised cider is exactly what referments in bottle. |
| Warm the cider gently to the yeast’s working range and rouse the sediment back into suspension | Partial | The first thing to try, and it often works where the cause was cold. Warm slowly rather than abruptly, and never in a sealed container. |
| Add yeast nutrient and rouse | Partial | Effective where nitrogen was the cause and enough viable yeast remains. Follow the product’s stated rate; nutrient blends differ, and adding more than the yeast can use feeds spoilage organisms instead. |
| Build a fresh, acclimatised starter and step the stuck cider into it | Partial | The recognised procedure: a healthy population is grown separately and the stuck cider added to it in stages so the new yeast is never shocked by the ethanol already present. Pitching dry yeast straight into the stuck vessel usually fails. |
| Bottle it and hope | Not possible | This is not a remedy, and it is dangerous. A stuck ferment with residual sugar can restart in a sealed glass bottle and generate enough pressure to burst it. See the safety note. |
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
- Assess juice nitrogen rather than assuming it is sufficient, and add nutrient early in fermentation at the supplier’s stated rate if it is needed.
- Keep fermentation temperature steady and within the range the yeast supplier specifies; insulate the vessel if the cellar swings.
- Choose a yeast whose alcohol tolerance comfortably exceeds the potential alcohol of the juice.
- Rehydrate dried yeast exactly as the manufacturer directs, so the population starts healthy.
- Take gravity readings on a schedule and write them down, so a slowing ferment is noticed while it is still moving.
- Rinse vessels thoroughly after cleaning, and use products intended for food contact.
A stuck fermentation is the point at which cidermaking stops being forgiving. Almost every other fault is a matter of flavour; this one leaves a vessel of sweet, weak liquid with no defences, and what follows is usually a second fault rather than a bad taste.
The diagnosis has to be made with a hydrometer. Airlock activity is a poor indicator — it stops when carbon dioxide saturation is reached and it continues when the vessel is merely warming — and the only reliable evidence is a gravity that has not moved over several days. Anyone diagnosing a stuck ferment by listening to the airlock is guessing.
The most useful thing to understand is that stuck fermentation in cider is usually a nutrition problem rather than a yeast problem. Cider juice from traditional orchards is genuinely poor in assimilable nitrogen, and that single fact accounts for a large share of stalled ferments, sulphide production and slow finishes. Buying better yeast does not address it.
It is worth separating from the deliberate arrest that produces a naturally sweet cider. Keeving achieves exactly this state on purpose, and a French cidre doux is a triumph of controlled starvation. The difference between the tradition and the accident is that the tradition knows what the residual sugar is and has a plan for the bottle.
The compounds involved
Yeast-assimilable nitrogen
The nitrogen a yeast can actually use, which apple juice is chronically short of — the shortage behind both stuck fermentations and rotten-egg aromas, and the shortage keeving deliberately makes worse.
Glucose
The sugar yeast takes first and the smallest of the three in apple juice, which is why the last sugar left in a slow ferment is almost never glucose.
Fructose
The dominant sugar of apple juice and the sweetest of the common sugars, which is why apple juice tastes sweeter than its total sugar figure implies and why a stuck ferment leaves sweetness behind.
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.
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
Ammonium nitrogen
The nitrogen form yeast takes up fastest and the one most nutrient additions supply, useful for rescuing a ferment and a poor substitute for a properly balanced juice.
The organisms involved
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
Saccharomyces bayanus
A name applied both to a hybrid *Saccharomyces* lineage and, loosely, to a whole class of commercial high-alcohol yeasts, and one of the least stable names in fermentation microbiology.
Saccharomyces uvarum
A cold-tolerant relative of *S. cerevisiae* recovered from spontaneous cider and wine ferments, associated with low-temperature fermentation and higher glycerol.
Where in the process it arises
Restarting a stuck fermentation
Diagnosing why a ferment has stopped with sugar remaining, then building an acclimatised starter and stepping the cider into it rather than pitching yeast into the problem.
Yeast nutrition
What a fermenting yeast population actually needs from apple juice — assimilable nitrogen, vitamins and membrane lipids — and what goes wrong when the juice cannot supply it.
Fermentation temperature control
Managing the temperature at which a ferment runs, which sets not only how fast it goes but which aromatics survive it and what the finished cider tastes of.
Gravity measurement
Measuring the density of juice or fermenting cider to follow sugar depletion and estimate alcohol, with the instrument limits and the perry complication that make the number less simple than it looks.
Keeving
Forming a floating pectin gel that lifts nutrients and solids out of the juice, so that the ferment starves before it finishes and leaves natural sweetness behind.
Arrested fermentation
Deliberately halting a ferment while sugar remains, to obtain natural sweetness from the fruit rather than from an addition — and accepting the instability that follows.
Faults it is confused with
These present similarly. What separates them is set out on each page.
Sluggish fermentation
A fermentation that is still moving but far more slowly than it should, extending the period during which the cider is weak, sweet and exposed.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
Nitrogen deficiency character
The set of characters a nitrogen-starved fermentation produces together — sulphide, a stalled or dragging ferment, harsh higher alcohols and a thin, hard cider.
Hydrogen sulphide
A rotten-egg or drain smell from hydrogen sulphide produced by stressed yeast, usually the first visible consequence of a nitrogen-short juice.
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.
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.
Carbonation: where the gas comes from and what it costs
How do I carbonate cider, and how do I do it without breaking bottles?
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?
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?
From sound cider to good cider
I can make cider without faults. How do I make it better?
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.
- How do i know when my cider has finished fermenting — Take two hydrometer readings several days apart: if the gravity has not moved, the ferment is over. Airlock activity is not a reliable test, because a slow ferment can produce gas too slowly to see.
- What temperature should cider ferment at — Most cider is fermented cool, commonly between about 12 and 18 °C. Cooler ferments keep more fruit aroma and run slower; above the low twenties the cider tends towards hot, solvent-like higher alcohols.
- 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 get rid of the sulphur smell in my cider — Racking with a little splashing usually blows off free hydrogen sulphide while it is still fresh. Once it has reacted into mercaptans the smell becomes rubbery and no longer responds to aeration.
- What does a nitrogen-starved cider taste like
- Why did my sweetened cider start fermenting again — Live yeast met the sugar that was added back. Sweetening is only stable if the yeast has been removed by sterile filtration, killed by pasteurisation, or held in check by sorbate together with sufficient sulphite.
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.
Cornell Cider Research and Extension programme
Cornell University, School of Integrative Plant Science · university · passage verified 2026-08-24
Runs cultivar trials in New York State and publishes juice chemistry for European cider varieties grown in a North American climate — the single most useful counterweight to treating English figures as universal.
The New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.
Institut Français des Productions Cidricoles (IFPC)
IFPC · research institute · retrieved 2026-08-24
The French technical institute for cider production. The authority for the French cultivar classification families, for keeving as an industrial process, and for the pectin and nitrogen chemistry that keeving depends on.