Fermentation
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.
Also called Restarting a stuck ferment, Fermentation rescue, Reinoculation.
- Stage
- Fermentation
- Traditional in
- No single tradition — used wherever it suits
- What it most changes
- Not recorded as moving a sensory dimension
- Safety
- Carries a safety consideration — see below
Never attempt a restart in a sealed or pressure-rated closed vessel, and never bottle a cider whose fermentation is not genuinely complete or deliberately stabilised. A ferment that resumes in a closed container generates carbon dioxide with nowhere to go, and pressure can rise fast enough to burst glass or expel a closure with force. Restart only in a vented vessel with ample headspace and an airlock or loose cover, confirm by assay rather than by a stable gravity reading that fermentable sugar is genuinely exhausted before packaging, and treat any bottled batch suspected of refermenting as a hazard — chill it and handle it with eye protection rather than opening it warm.
What it is
A stuck fermentation is one that has ceased with fermentable sugar still present and no intention on the maker’s part that it should stop. Restarting it means establishing a new, viable yeast population in a liquid that has already defeated one — a liquid containing ethanol, depleted of nitrogen, possibly cold, possibly carrying sulphite, and offering none of the conditions in which dried yeast normally establishes. The technique that works is not a bigger pitch but a staged one: a healthy starter is built in favourable conditions and then acclimatised by adding the stuck cider to it in increments, so that the population adapts to the cider rather than being dropped into it.
Why it is used
- A cider left with unfermented sugar and no active yeast is unstable, vulnerable to spoilage organisms that can use what Saccharomyces left, and unsafe to bottle.
- Correct diagnosis before intervention avoids the common outcome in which a second pitch fails for exactly the reason the first one did, doubling the dead yeast in the vessel and making a third attempt harder.
- Where the cause is simple — the vessel has gone cold, or the yeast needs nitrogen — the remedy may involve no new yeast at all, and identifying that saves the character of the ferment.
How it works
- Diagnosis comes first and separates the plausible causes: nitrogen exhaustion, temperature below the strain’s working range, ethanol above its tolerance, residual or added sulphite, an osmotically difficult sugar concentration in a very high-gravity juice, or a genuine chemical inhibition.
- Where temperature is the cause, warming the vessel gently into the strain’s range and rousing the settled yeast back into suspension is often sufficient, and no new yeast is required.
- Where nitrogen is the cause, adding nutrient and rousing may restart the existing population, though a population that has been dormant for a long period may no longer be viable enough to respond.
- Where a new population is needed, a starter is built with a strain chosen for ethanol tolerance and low nutrient demand, grown up in a small volume of dilute juice or water with nutrient until it is fermenting actively, and then acclimatised by adding stuck cider a portion at a time and waiting for activity to resume before each further addition — so that ethanol and any inhibitors rise slowly enough for the population to adapt.
- A keeved ferment that has slowed or stopped is a different case entirely: it has arrested because it was deliberately starved of nitrogen, and treating it as a fault by feeding and reinoculating destroys the style it was made to produce.
The chemistry and the organisms
What is actually being changed, and by what. Each entry says what that compound or organism does in cider generally; this page is one place it does it.
Compounds involved
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.
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.
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.
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.
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.
Hydrogen sulphide
The rotten-egg gas a nitrogen-starved yeast produces, detectable at concentrations too small to measure easily, and removable only if it is caught before it becomes something worse.
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.
Zygosaccharomyces bailii
A preservative-resistant spoilage yeast that refements sweetened cider and juice, and one of very few organisms able to grow through sorbate and benzoate at cider strength.
Brettanomyces anomalus
The second Brettanomyces species regularly recovered from cider and beer, less studied than B. bruxellensis but capable of the same phenolic chemistry.
What it is done with
Thermometers and temperature measurement
Temperature drives fermentation rate, yeast stress and the aromatic compounds that result, and the only reading that means anything is one taken in the liquid — not on the outside of the vessel and not in the room.
The hydrometer and the trial jar
A weighted glass float that sinks to a depth set by the density of the liquid, read against a scale on its stem; with a starting and a finishing reading it gives the sugar consumed and an estimate of alcohol produced.
What can go wrong
Faults that begin at this step, or that this step makes more likely. Each one is set out with its causes and whether it can be put right.
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.
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.
Bottle over-carbonation
More dissolved carbon dioxide in the bottle than intended or than the glass is rated for — a presentation problem at the mild end and a physical hazard at the severe one.
Gushing
Cider that erupts from the bottle on opening, either because it is over-pressurised or because something in it is nucleating the dissolved gas violently.
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.
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.
More on restarting a stuck fermentation
The instinct on finding a ferment stopped is to pitch more yeast into it, and that is the intervention most likely to fail. The cider that stopped a ferment will stop another one just as readily, and dried yeast rehydrated in water and tipped into an alcoholic, nitrogen-poor liquid meets the worst possible combination of osmotic and ethanol stress at the moment it is least able to withstand it. The failure is usually silent — no visible activity, another few kilograms of dead cells on the lees, and a cider now slightly harder to rescue than before. Everything that works about a proper restart follows from taking the opposite approach: build the population somewhere hospitable, then bring the hostile environment to it gradually.
Diagnosis is where the effort belongs, and the monitoring record is what makes it possible. A ferment that decelerated smoothly and stopped near dryness has probably finished, and a gravity reading in a perry near its sorbitol floor may look stuck when it is not. A ferment that stopped abruptly at a temperature well below the strain’s range is a temperature problem and needs warmth and rousing rather than yeast. One that stopped with a sulphurous smell and plenty of sugar left has run out of nitrogen. One that stopped at high alcohol in a high-gravity juice has hit the strain’s ethanol tolerance and needs a tolerant strain properly acclimatised. And one that was keeved has done exactly what it was supposed to. Each of these calls for a different response, and choosing the wrong one wastes the population that is still alive.
The acclimatised starter is the method that works when a new population is genuinely required. A tolerant strain is rehydrated properly and grown in a small volume of dilute juice with nutrient until it is visibly fermenting; then stuck cider is added, a modest fraction at a time, with each addition allowed to ferment before the next. Rousing the vessel to redistribute yeast and drive off dissolved carbon dioxide helps, as does bringing the cider to a workable temperature and adding nutrient if nitrogen is implicated. Where sulphite is the inhibitor, time and aeration reduce free sulphur dioxide but the position may simply not be recoverable, and a maker is sometimes better served by accepting the cider as it is, stabilising it deliberately and treating it as a sweet cider than by fighting a ferment that will not restart.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Fermentation
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
Fermentation
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
Yeast selection
Choosing which cultured strain to pitch, on the basis of the temperature, nitrogen, alcohol and aroma behaviour that separates one commercial yeast from another.
Fermentation
Yeast rehydration
Reviving active dried yeast in warm water before pitching, so the cells restore their membranes without being ruptured by the sugar concentration of juice.
Fermentation
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.
Fermentation
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.
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 long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- 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.
- What yeast should i use for cider — CiderHQ does not recommend brands. The choice is between a neutral, reliable strain that lets the fruit show, an aromatic wine strain that adds its own esters, and no addition at all. Alcohol tolerance, cold tolerance and nitrogen demand are the properties worth comparing.
- How do you stop cider from fermenting — By removing the yeast, by chilling, by filtering it out, by pasteurising, or by a combination — and in practice a home maker cannot reliably stop a ferment mid-way with chemicals alone. Sorbate prevents yeast multiplying but will not stop an active ferment.
- Does cider contain sulphites — Most does. Sulphur dioxide is added to protect juice and finished cider, and fermentation itself produces a small amount even when none is added. European labels must declare it above 10 mg per litre.
- 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.
Where to go next
- How cider is made — The whole sequence, stage by stage, with the choices open at each one.
- Cider science — The chemistry and microbiology the methods on this page rest on.
- Troubleshooting — What goes wrong, how to recognise it, and whether it can be reversed.
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 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.
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.