Fermentation
Secondary fermentation
Any fermentative event that follows the primary ferment — residual sugar refermenting, a deliberate second alcoholic fermentation, or the malolactic conversion of the maturation phase.
Also called Second fermentation, Refermentation.
- Stage
- Fermentation
- Traditional in
- No single tradition — used wherever it suits
- What it most changes
- Carbonation up, sweetness down
- Safety
- Carries a safety consideration — see below
Any secondary fermentation in a sealed container generates pressure, and an unintended one is unbounded because nobody has calculated the sugar available. Bottles refermenting on residual sugar can exceed the strength of standard glass and burst, or expel a closure with force. Cider intended for bottle should be either genuinely fermented out, deliberately primed with a known quantity of sugar in appropriate pressure-rated bottles and closures, or stabilised before packaging — and any batch suspected of refermenting should be chilled, stored in a contained space and handled with eye protection.
What it is
Secondary fermentation is the general term for microbial activity occurring in a cider after the primary alcoholic fermentation has ended. It covers three distinct phenomena that share only their position in the sequence: a further alcoholic fermentation of sugar that was left behind or added back, whether that happens in tank, in bottle or unbidden in a supposedly finished product; a deliberate second fermentation carried out to generate carbon dioxide for sparkle; and the malolactic conversion carried out by lactic acid bacteria on malic acid rather than on sugar. Whether a given secondary event is a technique or a fault depends entirely on whether it was intended and whether the container can accommodate the gas.
Why it is used
- Deliberate secondary alcoholic fermentation is the traditional means of carbonating cider, producing dissolved carbon dioxide within the closed container rather than injecting it.
- Malolactic conversion softens a high-malic cider and adds lactic and diacetyl-derived character, and in many traditions is simply what happens during maturation rather than something chosen.
- Recognising unintended secondary fermentation for what it is — rather than as a haze or a sediment problem — is what prevents a batch reaching the trade in bottles that will over-pressurise.
- Understanding that the yeast lees deposited by a secondary fermentation are part of the product explains why some styles are disgorged, some are decanted and some are deliberately roused.
How it works
- A second alcoholic fermentation requires three things simultaneously: fermentable sugar, a viable yeast population and conditions the yeast can work in — remove any one and no secondary event occurs, which is the basis of every stabilisation technique.
- In a sealed container the carbon dioxide produced cannot escape, so it dissolves into the cider under the pressure it generates; the relationship between the sugar fermented and the pressure reached is what makes priming a calculation rather than a guess.
- Malolactic conversion is not a sugar fermentation at all: lactic acid bacteria decarboxylate L-malic acid to L-lactic acid, releasing carbon dioxide, which is why a cider undergoing it can appear to be gently refermenting.
- Sulphite-tolerant organisms including Zygosaccharomyces bailii and Saccharomycodes ludwigii can ferment residual sugar in ciders where Saccharomyces has been controlled, which is why a back-sweetened cider held on a nominal stabilising dose can still referment.
- Rising temperature is the most common trigger for an unintended secondary event, since it revives a dormant population in a cider that had appeared stable through a cold winter.
What it changes
The direction this step pushes the finished drink in, dimension by dimension. A direction, not a measurement: how far it moves depends on the juice, the temperature and how the step is carried out.
| Dimension | Direction | Why |
|---|---|---|
| Carbonation | Raises | Carbon dioxide generated inside a closed container has nowhere to escape and dissolves into the liquid under its own pressure, which is a different and finer condition from gas injected into a still cider. |
| Sweetness | Lowers | A secondary alcoholic fermentation consumes the residual sugar that was giving the cider its sweetness, which is why an unintended one turns a medium cider dry as well as fizzy. |
| Fermentation character | Raises | Yeast active in the package and then autolysing on the lees contributes bready and savoury notes that a cider filtered and force-carbonated does not develop. |
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
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.
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.
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.
Malic acid
The acid of apples, which supplies almost all the sharpness a cider has and, through pH, decides how vulnerable that cider is to everything that could spoil it.
Lactic acid
The softer acid that replaces malic when malolactic fermentation runs, halving the acid a cider carries and changing its texture as much as its sharpness.
Diacetyl
The butter compound, made mainly by lactic acid bacteria and perceptible at very low concentration, which is a defining part of some traditional ciders and an obvious fault in others.
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.
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.
Oenococcus oeni
The acid-tolerant lactic acid bacterium that carries out most deliberate malolactic fermentation, converting malic acid to lactic acid after the yeast has finished.
Lactiplantibacillus plantarum
A versatile lactic acid bacterium, renamed out of Lactobacillus in 2020, capable of malolactic conversion and of a range of faults depending on conditions.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
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.
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
Airlocks, bungs and headspace management
An airlock is a one-way water trap that lets carbon dioxide out of a vessel while keeping air, insects and dust from coming in — a small component that solves the largest single problem in small-scale cider making.
Choosing a fermentation vessel: material and shape
Vessel material decides oxygen ingress, flavour contribution and how well the thing can be cleaned; vessel shape decides temperature behaviour, lees depth and how much surface the cider presents to whatever is above it.
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.
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.
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.
Yeast haze
Cloudiness from yeast cells that have not settled out, usually because the strain flocculates poorly or the cider has not been left alone long enough.
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Lactic off-flavours
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
Styles it produces
Categories in which this step is characteristic or required. Some name it in their definition; for others it is simply how they have always been made.
Bottle-conditioned cider
Cider that completes a fermentation inside its sealed bottle, generating its own carbonation and, in most cases, leaving a yeast deposit behind.
Sparkling cider
Cider carrying enough dissolved carbon dioxide to produce a persistent bubble, by bottle fermentation, tank fermentation or injection.
Pétillant cider
Lightly carbonated cider in which the gas is felt as a prickle rather than a stream of bubbles, the level most traditional bottle fermentation reaches on its own.
Bottle-conditioned perry
Perry that completes its fermentation in the sealed bottle, generating carbonation and lees character while the fruit’s sorbitol keeps a sweetness the yeast cannot remove.
Sidra espumosa
Sparkling Asturian cider made by a second fermentation under pressure or in bottle, presented as a fine sparkling drink rather than in the still poured tradition.
Sparkling perry
Perry carrying a persistent bubble, a presentation that suits pear’s aromatics and residual sorbitol and has a documented history going back to the seventeenth century.
More on secondary fermentation
The value of treating secondary fermentation as one topic is that it makes the underlying logic visible: sugar plus viable organism plus tolerable conditions equals fermentation, wherever the liquid happens to be. A cider that is bottle conditioned, a tank-conditioned cider carbonating under pressure, a sidra espumosa taken through a second fermentation and disgorged, and a batch of supposedly medium cider quietly refermenting on a warehouse pallet in July are the same event under different degrees of control. The techniques that produce sparkle deliberately and the stabilisation techniques that prevent refermentation are two applications of one piece of knowledge, and a maker who understands the phenomenon in general terms has less to remember than one who learns each case separately.
The malolactic case sits inside this category but works differently enough to be worth separating. Lactic acid bacteria acting on malic acid rather than sugar produce carbon dioxide as a decarboxylation product, so a cider undergoing malolactic conversion bubbles gently and can be mistaken for one refermenting on sugar — an important distinction, because the amount of gas a malolactic conversion can produce is bounded by the malic acid present, whereas a sugar fermentation is bounded only by the sugar. In much traditional English, Norman and Asturian practice the malolactic conversion simply happens during maturation without being initiated, and it is a defining part of the resulting drink; in tightly controlled modern production it is often suppressed. `malolactic-fermentation` covers it fully.
What a practitioner decides, in the end, is which of the three states a cider will be in when it is packaged. It can be fermented genuinely dry, with no fermentable sugar remaining, in which case nothing further will happen and any sparkle must be added deliberately. It can be packaged with a known, deliberate quantity of sugar and yeast in a container built for the resulting pressure, which is `bottle-conditioning` and `priming`. Or it can be sweet and stabilised, its sugar protected by filtration, pasteurisation or a stabilising treatment that must actually be effective against the organisms present — and the sulphite-tolerant Zygosaccharomyces bailii and Saccharomycodes ludwigii are the reason a nominal stabilising dose alone is not a plan. What a cider must never be is sweet, unstabilised, bottled and warm.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Carbonation
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.
Fermentation
Malolactic fermentation
A bacterial conversion of sharp malic acid into softer lactic acid, which lowers acidity, changes aroma, and in most traditional cider happens whether it was planned or not.
Carbonation
Tank conditioning
Running a second fermentation in a sealed pressure tank so the gas is generated by yeast but the sediment never reaches the bottle.
Carbonation
Traditional method
A second fermentation in the bottle the cider will be sold in, followed by riddling the deposit into the neck and expelling it, so the drink is both bottle-fermented and clear.
Carbonation
Priming
Adding a measured, calculable quantity of fermentable sugar at bottling so that the fermentation which follows generates a predictable volume of carbon dioxide.
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 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.
- How is sparkling cider made
- What is malolactic fermentation — Malolactic fermentation is a bacterial conversion of sharp malic acid into softer lactic acid, releasing carbon dioxide. It lowers total acidity and raises pH, and in cider it is often the source of a farmyard or buttery note as well.
- What is priming sugar — Priming sugar is a measured dose of sugar added at bottling so that the remaining yeast produces carbon dioxide inside the sealed bottle. Too much of it is the usual cause of burst bottles.
- 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.
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.
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.