Fault
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
Also called bottle refermentation, secondary fermentation in package.
This is the fault most likely to injure someone. A glass bottle in which fermentation restarts can burst without warning, and the injuries people receive are to eyes and hands from flying glass. Warning signs are a bottle that hisses far more than expected, a crown cap that has domed upward, a plastic control bottle gone hard, fresh sediment or a haze in bottles that were clear, or any bottle from a batch that has already burst. Handle a suspect batch as follows: chill the bottles first, because cold reduces both the yeast activity and the pressure; do not shake, carry, or invert them; move them in a closed crate rather than by the neck; and vent them one at a time behind a barrier such as a closed cupboard door or a heavy cloth, wearing eye protection, with the bottle pointed away from you and from anyone else. Never store a suspect batch in a warm room, an airing cupboard, a conservatory, or a car. If a batch cannot be handled safely, the safest course is to chill it thoroughly and dispose of the bottles unopened, wrapped and in a rigid container. Nothing on this page should be read as a reason to add sugar, juice or yeast to a sealed bottle in any circumstances.
- Severity
- Safety — there is a real hazard here
- Where it starts
- Packaging, Blending, Storage, Stabilisation
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Packaging, Storage
- Signs
- 6 recorded
What it is
Unwanted refermentation is a fermentation that resumes after the cider has been packaged. It requires two things present together: fermentable sugar, and a viable organism able to use it. Deliberate bottle conditioning is the same process under control, with a known sugar addition and a bottle rated for the pressure it will produce. Unwanted refermentation is that process with an unknown sugar level and often an unsuitable container, and it is the principal genuine safety hazard in home cidermaking.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
To hear
- Bottles that hiss sharply or foam over when opened, when they were meant to be still
To look at
- A haze developing in bottles that were clear at packaging
- Fresh sediment appearing in the bottom of a bottle over weeks
- Crown caps domed upward, or a plastic bottle gone rock hard — A plastic bottle filled from the same batch is the single most useful early-warning device a home producer has.
In the mouth
- A prickle on the tongue in a cider bottled still
On the palate
- The cider tasting drier than it did at bottling
When it appears. In the container, days to months after filling, and almost always after a warm spell. A batch bottled in a cold cellar in February can be unremarkable until the first warm week of the year.
The two conditions required are fermentable sugar and a viable yeast population, so the diagnostic question is which of the two was assumed absent. Back-sweetening without stabilisation supplies the first; a keeved or nutrient-limited cider supplies the second while looking finished.
Peter Mitchell / Cider and Perry Academy, Andrew Lea
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.
- Bottle over-carbonation — Cause rather than symptom. Over-carbonation from too much priming sugar was measured wrongly; refermentation was not measured at all, because there was sugar nobody had accounted for.
- Deliberate bottle conditioning — Whether the carbonation was calculated. A conditioned bottle was primed to a target; a refermenting one was not, and its ceiling is whatever sugar remains.
- Gushing — Gushing is what refermentation looks like on opening. The underlying question is why there was fermentable sugar and viable yeast in a sealed container.
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 chemistry is straightforward and that is precisely the problem. Yeast converts each unit of fermentable sugar into roughly equal molar quantities of ethanol and carbon dioxide. In an open vessel the gas leaves; in a sealed bottle it dissolves until the liquid is saturated at that temperature, and thereafter it builds pressure in the headspace. Because a bottle has fixed volume, there is no mechanism by which the process stops short of the sugar running out or the bottle failing.
The organism need not be Saccharomyces cerevisiae. Brettanomyces and Dekkera species ferment sugars that ordinary yeast leaves and work slowly enough to be missed at bottling. Zygosaccharomyces bailii is notoriously resistant to preservatives and osmotic stress. Saccharomycodes ludwigii tolerates sulphite well. Lactic acid bacteria and Zymomonas mobilis also produce gas from residual sugar.
Sugar can be present without the maker realising. A ferment that stalled near dryness may retain more than a hydrometer suggests, particularly where unfermentable sorbitol in perry or residual pentoses distort the reading. Back-sweetening obviously adds sugar. So does topping up with juice, adding fruit, or blending in a batch that was not fully dry.
Temperature governs everything. A cider that sits quietly through a cold winter can restart in spring, and bottles moved from a cool store into a warm kitchen or a car boot in summer can go from stable to bursting. Pressure also rises directly with temperature for a given dissolved gas content, so warming a bottle raises the pressure twice over.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Bottling before fermentation was genuinely completeAlmost always diagnosed by a hydrometer that was not consulted often enough. | Packaging | common |
| Back-sweetening without stabilising the cider | Blending | common |
| A stuck fermentation restarting when the weather warmed | Storage | common |
| Bottles stored somewhere warm | Storage | common |
| Sulphite-tolerant or preservative-tolerant spoilage yeasts surviving the stabilisation used | Stabilisation | occasional |
| Sorbate used without sulphite, or filtration that was not fine enough to remove yeast | Stabilisation | 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 |
|---|---|---|
| Return the cider to a vessel and let it finish fermenting | Reliable | The proper fix. Decant the bottles into an open-topped or airlocked vessel, let the fermentation complete, confirm stability by gravity, and only then rebottle. Nothing about this step should happen in a sealed container. |
| Move the bottles somewhere cold immediately | Partial | The first action for a batch already in glass. Cold slows or stops the yeast and reduces the pressure at a given gas content. It buys time; it does not make the bottles safe. |
| Vent the bottles carefully to release pressure | Partial | Chill first, do not shake, and follow the handling described in the safety note. Venting reduces pressure but does not remove the sugar or the yeast, so pressure returns. |
| Leave them and hope the sugar runs out first | Unlikely to work | Whether it does depends on how much sugar there was, which is exactly what is unknown. This is a gamble with glass. |
| Add more preservative to the sealed bottles | Not possible | It cannot be done, and attempting to open, dose and reseal pressurised bottles is more dangerous than the original problem. |
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
- Confirm dryness by gravity readings that are unchanged over several days before bottling, not by taste or by elapsed time.
- Treat any sweet cider as unstable until it has been deliberately stabilised by filtration, pasteurisation or an appropriate preservative regime worked out from supplier and regulatory guidance.
- Bottle in glass rated for pressure if there is any chance of carbonation; ordinary still-wine bottles are not made for it and screw-top or twist-off bottles are a particular risk.
- Fill one plastic bottle from every batch and keep it beside the glass ones as a pressure gauge.
- Store bottles cool and stable, and never in a warm kitchen, a conservatory, a car or in direct sun.
- Label every batch with its bottling date and gravity so its history is not a matter of memory.
Unwanted refermentation is the one cider fault that can hurt somebody, and it deserves to be understood as a mechanical problem rather than a flavour one. A sealed glass bottle is a pressure vessel with no relief path, and yeast will keep producing gas until the sugar is exhausted regardless of what the container can take.
The reason it catches people out is that the two ingredients are both easy to have without knowing. Sugar can remain in a cider that reads dry, particularly in perry where sorbitol confuses the hydrometer, and viable organisms survive treatments that were believed to have removed them. Neither is visible at bottling.
The plastic bottle trick is worth adopting universally. One plastic bottle filled from the same batch, capped and stored alongside the glass, gives a direct reading of what is happening inside the sealed bottles: soft means nothing is happening, firm means gas is being produced, rock hard means the glass bottles are under pressure that needs dealing with. It costs nothing and it converts an invisible risk into an observable one.
The distinction from bottle conditioning is entirely one of control. A conditioned cider has a measured sugar addition, a known starting gravity, a bottle rated for the pressure and a maker who calculated it. Everything that makes bottle conditioning a fine technique is absent when fermentation restarts by accident.
The compounds involved
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.
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.
Sucrose
The disaccharide of apple juice and the sugar most often added to it, split into glucose and fructose by the yeast’s own invertase before any of it is fermented.
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.
Sorbitol
The unfermentable sugar alcohol that pears carry in quantity and apples carry only in trace, and the single reason a fully fermented perry keeps a sweetness a fully fermented cider cannot.
Potassium sorbate
The soluble salt in which sorbic acid is actually added, used to hold sweetness in a bottled cider, and dangerous to use without sulphite because of what lactic bacteria do to it.
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.
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.
Saccharomycodes ludwigii
A large, sulphite-resistant yeast that refements sweet cider in bottle and is one of the classic causes of unwanted secondary fermentation.
Dekkera bruxellensis
The yeast behind 4-ethylphenol, and the organism that a cider tradition may regard as its signature or its ruin depending on where and how it is made.
Brettanomyces anomalus
The second Brettanomyces species regularly recovered from cider and beer, less studied than *B. bruxellensis* but capable of the same phenolic chemistry.
Zymomonas mobilis
The bacterium behind cider sickness — *maladie de la framboise* — a fermentative spoilage of low-acid sweet cider that produces gas, haze and a raspberry-like odour.
Where in the process it arises
Bottling
Transferring finished cider into glass, where the dominant variable is how much oxygen the liquid picks up in the few seconds it takes to fill and close each bottle.
Bottle conditioning
Carbonating cider by letting a second fermentation finish inside the sealed bottle, so the gas is generated where it is going to stay.
Priming
Adding a measured, calculable quantity of fermentable sugar at bottling so that the fermentation which follows generates a predictable volume of carbon dioxide.
Back-sweetening
Adding sugar, juice or concentrate to a cider that has fermented dry — a straightforward adjustment that leaves the drink microbiologically unstable until something is done about it.
Sterile filtration
Membrane filtration at a rating that retains viable yeast and bacteria, which is what allows a sweet cider to be shelf-stable without heat or preservative — provided the filling that follows is aseptic.
Pasteurisation
Heating cider enough to inactivate the organisms that would spoil it, either in the sealed package or in-line before filling, at a measurable cost in fresh aroma.
Bottle pressure management
Matching the pressure a carbonated cider will actually generate to glass that can contain it, taking account of temperature and of any fermentable sugar left in the bottle.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
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.
Excessive sediment
More deposit in the bottle or vessel than the presentation intends, ranging from a normal conditioning yeast layer to a loose sludge that clouds every pour.
Microbial haze
Cloudiness caused by a growing population of spoilage organisms, and therefore a symptom of something worse rather than a clarity problem in itself.
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?
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?
Working safely: pressure, fruit and gas
What is actually dangerous about making cider?
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 did my cider bottles explode — Because fermentable sugar was still present, or too much priming sugar was used, and the pressure exceeded what the bottle could hold. Never bottle a cider whose gravity is still falling, and never use bottles not designed for pressure.
- Why did my cider stop fermenting — The usual causes are a shortage of yeast-available nitrogen, a temperature that has dropped, too much sulphite at the start, or a yeast that has reached its alcohol limit. Check the gravity before assuming anything is wrong: many ciders simply finish.
- Why does my cider gush out when i open it — Either the cider is over-carbonated, or it is warm, or nucleation sites such as yeast and haze are releasing the gas all at once. Chilling the bottle thoroughly before opening solves most cases that are not genuine over-carbonation.
- Why is there sediment in the bottom of my cider bottle — In a bottle-conditioned cider the sediment is yeast and is expected — pour carefully and leave the last centimetre. In a filtered cider it means something has grown since bottling.
- 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.
- Which yeast ferments cider — Saccharomyces cerevisiae finishes almost every cider fermentation, whether it was pitched or arrived from the press house. In a spontaneous ferment it is not the first organism present, only the one that survives the alcohol it makes.
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 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.
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.