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Diagnosis guide

When the fermentation is not doing what it should

My fermentation is doing something strange. What is going on?

In short

Almost every fermentation worry has more than one cause, and the causes need different responses. A ferment that never started may have no viable yeast, or may be too cold, or may be in juice carrying a preservative — and warming a preserved juice achieves nothing while warming a cold one fixes it.

So work from an observation rather than from a symptom name. The useful ones are cheap: a gravity reading taken twice, the temperature of the liquid rather than the room, whether the surface has anything on it, and what the juice was before you started.

Two things are worth separating first, because they look identical and only one is a problem. A ferment that has finished and one that has stopped short both show a stable gravity and no airlock activity. Which you have depends on the number, and in a perry it depends on knowing that sorbitol never ferments at all.

Who this is for. Anybody watching a fermentation and unsure whether what they are seeing is normal. It assumes a hydrometer; almost nothing here can be settled without one. (no experience assumed · 8 steps)

Worth reading first
Where scale changes the answer

At domestic scale a stalled batch is a disappointment and the remedies are all available. Above a few hundred litres the arithmetic changes: restarting a large stuck volume needs a stepped starter built up over days rather than a sachet, and the temperature of the mass takes far longer to move than a demijohn’s. The diagnosis is the same; the response is slower and has to be planned.

Three ferments that all stoppedSpecific gravity against time for a cider that finished, a cider that stalled and a perry that finished, drawn to show that stopping tells you nothing on its own.Cider, finishedCider, stalled at 1.020Perry, finished at 1.0121.0501.0201.000Week 0Week 8All three stopped. One of them has a problem.
Specific gravity against time for a cider that finished, a cider that stalled and a perry that finished, drawn to show that stopping tells you nothing on its own.
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.

The path

Each step says what happens, why it matters downstream, and what you are actually deciding. The links under a step are where the depth is.

  1. 1. Nothing has happened at all

    Three or four days with no airlock movement, no foam and no change in gravity. Before anything else, confirm it with the hydrometer rather than the airlock — a lid that is not sealing lets gas out around the rim and the airlock stays still while the ferment runs perfectly well.

    If the gravity really has not moved, there are four ordinary causes and they are distinguishable. *A preservative in the juice — sorbate or benzoate — which stops fermentation by design; check the ingredients list rather than the front label. Too cold, below about 8 °C, where most yeast will not begin. Dead or badly rehydrated yeast, which is the commonest cause when a sachet was tipped into cold juice. And too little viable population*, which a wild ferment can take a week or more to overcome without anything being wrong at all.

    A fifth, rarer cause worth knowing: a very heavy sulphite addition will suppress a pitched yeast as well as the wild population. That is a dose problem rather than a yeast problem and it resolves with time as the sulphite binds.

    Why it matters: Three of the five want completely different responses, and warming a preserved juice or repitching into a sulphite excess wastes both the yeast and the week.

    What to measure

    • Specific gravity now, against the reading at pitching — Whether anything has actually happened. The airlock does not answer this; a lid that is not sealing makes an active ferment look dead. Work it out.
    • Temperature of the liquid — Whether the yeast is simply too cold to begin. Below about 8 °C most strains will not start.

    Go deeper

    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.

    • 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 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.

  2. 2. It started, and it is crawling

    Gravity is moving, but slowly. The first question is whether that is a problem: a deliberately cool fermentation at 10 °C is expected to take months, and slowness there is the method working rather than failing. The same rate at 18 °C is not.

    Where it is genuinely slow for its temperature, nitrogen is the usual answer. Apple juice is frequently short of what yeast can actually use, cider apples tend to sit lower than dessert fruit, and a ferment that began vigorously and then slowed abruptly at a third to halfway through has almost always run out. The confirming sign is a smell: a nitrogen-starved ferment commonly produces hydrogen sulphide alongside the slowdown.

    Other causes are worth ruling out because they need different action. A high starting gravity slows a ferment throughout by osmotic stress. A very clear juice — heavily settled, or keeved — has had solids removed that the yeast used as nucleation sites and nutrient. And a wide temperature swing does more damage than a low steady temperature, because the yeast keeps having to re-adapt.

    Why it matters: The single commonest cause has a remedy that must be applied during fermentation, and the window closes as the population declines.

    What to measure

    • A gravity series rather than a single reading — Whether it is slow, stopped, or simply slower than you expected. One reading cannot distinguish those. Work it out.
    • Liquid temperature, taken in the vessel — What the yeast is experiencing. An active ferment runs warmer than the room; a finished one runs at room temperature.

    Tools for this step

    Go deeper

    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.

    • Free amino nitrogen concentration correlates to total yeast assimilable nitrogen concentration in apple juice

      Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123 · peer-reviewed literature · passage verified 2026-08-24 · covers 2014–2015

      Open access; read in full on 2026-08-24. The paper that gives cider its own nitrogen numbers instead of borrowing wine’s. A hundred and eight apple samples from Virginia over two seasons, with the two fractions of assimilable nitrogen measured separately — which matters, because the finding is that apple juice is short of ammonium specifically, and that the wine practice of measuring both fractions may be unnecessary for apples while the wine practice of assuming there is enough nitrogen is badly wrong.

    • 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.

  3. 3. It has stopped, and there is still sugar

    Two readings several days apart, unchanged, at a gravity above about 1.005. That is a stuck ferment — as against a finished one, which is the same observation at a lower number.

    Except in three cases where it is not stuck at all. *A perry commonly finishes well above 1.000 because sorbitol contributes to the reading and never ferments; a perry at 1.010 and stable may be complete. A keeved cider stopped because the keeve removed the nitrogen deliberately, which is the entire point. An arrested fermentation* was stopped on purpose by chilling, racking or sulphiting.

    Where it is genuinely stuck, the causes are the ones from the previous step taken to their conclusion — nitrogen exhausted, temperature dropped, alcohol reached the strain’s tolerance — plus one more: a ferment that ran hot early can produce enough of its own stress products to stall itself later.

    Restarting is one of the few genuinely reliable remedies on this site, and it works by building a fresh population up to the cider rather than by tipping yeast in. A sachet added directly to a stuck, alcoholic cider mostly dies.

    Why it matters: Feeding, warming or repitching a perry that has simply finished creates problems that were not there, and it is the commonest misreading in cider-making.

    Hazard

    A stuck cider still contains fermentable sugar and a yeast population that stopped rather than died. Do not bottle it. Whatever paused the ferment — cold, most often — can be undone by a warm week, and the result is a refermentation in sealed glass with no ceiling except the sugar available. Either restart it and let it finish in a vented vessel, or stabilise it, before it goes anywhere near a bottle.

    What to measure

    • The finishing gravity, read against what this fruit should reach — Whether it stopped short or arrived. A cider that ferments dry lands near 0.996–1.000; a perry does not. Work it out.
    • Apparent attenuation — How far it went as a proportion. Read against perry expectations for a perry — a low figure there is not evidence of a stall. Work it out.

    Go deeper

    • Restarting a stuck fermentation — The stepped starter, which is what makes the remedy work.
    • Sorbitol — Why a dry perry does not read 1.000, and never will.
    • Keeving — A deliberate stop, and how to tell it from an accidental one.
    • Making perry — The whole set of places perry diverges from cider, of which this is the one that misleads most.

    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.

    • Free amino nitrogen concentration correlates to total yeast assimilable nitrogen concentration in apple juice

      Boudreau, Peck, O’Keefe and Stewart, Food Science & Nutrition 6(1):119–123 · peer-reviewed literature · passage verified 2026-08-24 · covers 2014–2015

      Open access; read in full on 2026-08-24. The paper that gives cider its own nitrogen numbers instead of borrowing wine’s. A hundred and eight apple samples from Virginia over two seasons, with the two fractions of assimilable nitrogen measured separately — which matters, because the finding is that apple juice is short of ammonium specifically, and that the wine practice of measuring both fractions may be unnecessary for apples while the wine practice of assuming there is enough nitrogen is badly wrong.

    • Research on pear juice composition and sorbitol in perry

      Various journals and institute reports · peer-reviewed literature · registered as competent for this subject

      The evidence base for the single most important chemical difference between cider and perry: pears carry substantial sorbitol, which yeast does not ferment, so a fully fermented perry retains sweetness a fully fermented cider cannot.

    • 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.

  4. 4. It had stopped, and now it is going again

    A cider that was stable and has begun producing gas again has been given something it did not have: warmth, nutrient, oxygen, or a fresh population. In a vessel this is usually harmless and sometimes welcome. In a sealed bottle it is a pressure problem.

    The commonest trigger is spring. A cider that stalled in a cold cellar in January has yeast that is dormant rather than dead, and the first warm week gives it back the temperature it needed. That is why a batch bottled in February can be bulging in May.

    A second trigger is racking, which introduces oxygen and rouses the population. A third is malolactic fermentation, which is a different organism converting a different acid and produces gas without touching the sugar at all — so a gently gassy cider whose gravity is not falling may be doing that rather than restarting.

    Why it matters: The same observation is unremarkable in a carboy and a hazard in a crate, and the difference is only whether it is sealed.

    Hazard

    A refermentation in sealed bottles has no ceiling except the sugar available, and it is not producing to a target. Chill the batch immediately, which lowers the pressure substantially; move bottles crated rather than by the neck; keep them out of any warm room; and vent behind a barrier with eye protection if a bottle has to be opened. Do not warm a suspect bottle to make it easier to open.

    What to measure

    • Gravity, to see whether sugar is actually being consumed — Whether this is a restarted alcoholic fermentation or a malolactic one. Malolactic produces gas and barely moves the gravity. Work it out.
    • Pressure the carbonation would reach at the warmest storage temperature, if it is in bottles — Whether the glass can hold what an uncontrolled refermentation is producing. Work it out.

    Tools for this step

    Go deeper

    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.

    • 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.

  5. 5. It smells of sulphur

    Three quite different things smell of sulphur and they need different responses, so the first job is to say which. *Rotten egg is hydrogen sulphide, produced by stressed yeast, usually mid-ferment and usually where nitrogen is short. Struck match, sharp and prickling at the back of the nose is sulphur dioxide, from an addition rather than from the yeast. Cooked cabbage, flint or a general staleness that lifts when poured* is reduction, from a cider that has had no contact with air.

    The distinction that matters most is time. Hydrogen sulphide caught early can be dealt with by racking with deliberate aeration — splashing the cider — and the smell goes. Left for weeks it reacts onwards to mercaptans, which smell of drains, rubber and garlic, and those do not respond to aeration at all. That window is the single reason to act on a sulphur smell rather than wait.

    A fourth possibility in bottles: light strike, from clear or green glass left under light, which produces a sulphury note that has nothing to do with the ferment.

    Why it matters: One of these has a short window and a reliable remedy; one is permanent; one resolves on its own. Treating them alike wastes the window on the first.

    Tools for this step

    Go deeper

    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.

    • Yeast Assimilable Nitrogen Concentrations Influence Yeast Gene Expression and Hydrogen Sulfide Production During Cider Fermentation

      Song, Gibney, Cheng, Liu and Peck, Frontiers in Microbiology 11:1264 · peer-reviewed literature · passage verified 2026-08-24 · covers 2016

      Open access; read in full on 2026-08-24. This is the paper CiderHQ cites against the folk rule that low nitrogen causes sulphide and adding nutrient cures it. Two Saccharomyces strains fermented the same Cornell juice at three nitrogen levels: one strain produced no detectable hydrogen sulphide at any level, and in the strain that did, the middle nitrogen treatment produced the most — twice the low treatment and six times the high. A rule that is true on average is being applied to individual batches where it can be exactly backwards, and the correction is worth a page of its own.

    • 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.

  6. 6. It is sweeter, or sharper, than expected

    *Sweeter than expected has three ordinary explanations. The ferment stopped short, which the gravity will show. The fruit was a perry pear, and sorbitol is doing it. Or the cider is dry and tastes* sweet because it is full of ripe-apple esters — fruit aroma reads as sweetness to most drinkers, and a bone-dry cider can be described as sweet by people who would fail a blind sugar test.

    *Sharper than expected likewise. The blend was acid-dominant, which is a fruit decision rather than a fault. Or malolactic fermentation has not* happened, so the acid is still malic rather than the softer lactic. Or — and this is the one to rule out first because it is the only one that gets worse — volatile acidity is rising, in which case the sharpness comes with a vinegar note in the nose before the glass reaches the mouth.

    The test that separates the last from the others costs nothing: smell it. Fixed acid has no aroma. If the sharpness has a smell, it is not a blending problem.

    Why it matters: Two of these six are ordinary properties of the fruit, three are process outcomes, and one is a spoilage that is still progressing.

    What to measure

    • Gravity, before concluding anything about sweetness — Whether there is sugar there at all. Perceived sweetness and residual sugar diverge more than people expect. Work it out.
    • Titratable acidity, and separately pH — How sharp it actually is, and whether it is microbiologically exposed. The two answer different questions and are routinely confused. Work it out.

    Go deeper

    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.

    • Research on pear juice composition and sorbitol in perry

      Various journals and institute reports · peer-reviewed literature · registered as competent for this subject

      The evidence base for the single most important chemical difference between cider and perry: pears carry substantial sorbitol, which yeast does not ferment, so a fully fermented perry retains sweetness a fully fermented cider cannot.

  7. 7. Something has appeared on it, or in it

    *A film on the surface.* A flat white or cream mat is film yeast; a fuzzy or coloured growth is mould. Both need a headspace and neither can form in a genuinely full sealed vessel, so whichever it is, the finding is that the vessel was not full. Film yeast leaves the cider stale and thin; a ring at the liquid line with a vinegar smell is acetification, which is worse.

    *A haze that was not there before.* A haze arriving in a previously bright cider is the one to take seriously, because a static haze is almost always pectin or yeast and a growing one is usually microbial. Pectin, starch, protein and metal hazes look identical and are told apart by simple tests rather than by looking.

    *A deposit.* Expected in anything unfiltered and in every bottle-conditioned cider. The useful question is not how much there is but whether it is still increasing.

    Why it matters: Almost everything visible in a fermenting or maturing cider is normal, and the two that are not — a surface film and a growing haze — are both consequences of something specific rather than of bad luck.

    Go deeper

    • Film yeast growth — The commonest surface growth, and what its presence proves about the vessel.
    • Microbial haze — The haze that is a warning rather than a cosmetic finding.
    • Starch haze — The one with a kitchen test — a drop of iodine settles it in seconds.
    • Why some cider is cloudy — The several causes, and how to tell which you have.
    • Topping up — The operation that prevents most of this.

    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.

  8. 8. When nothing explains it

    Some combinations do not resolve to one cause, and a diagnostic that always produces an answer is producing some wrong ones. A cider that is slow, slightly sulphury and finishing high is showing three consequences of one nitrogen shortage. A cider that is hazy, gassy and sharpening is showing a microbial population doing several things at once. Both are single stories. But a cider that is flat, oxidised and mousy has been through three separate failures, and treating it as one will fix none of them.

    The honest procedure when the picture will not resolve is to record it rather than to force it. Note what you observed, when, at what temperature, and what you had done in the previous fortnight. A fault that recurs next season with the same record attached is diagnosable; one that recurs without is not.

    Why it matters: Cider is annual, and a maker gets perhaps forty attempts. An unexplained batch with a complete record is worth more than a confidently misdiagnosed one.

    Go deeper

    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.

    • 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.

    • 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.

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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.