Compound
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.
Also called CO2.
- Class
- Gases
- Formula
- CO2
- How often it matters
- Present in every cider
What it does in cider
- Is produced in equal molar quantity with ethanol, so a fermenting vessel generates a blanket of it that excludes oxygen for as long as the ferment runs.
- Dissolves to form carbonic acid, which is why a sparkling cider tastes sharper than the same liquid poured flat even though its acid content has not changed.
- Carries volatile aroma compounds out of the liquid as bubbles break, increasing aromatic intensity at the nose.
- Generates pressure in a sealed package in proportion to how much is dissolved and how warm the package is, which is a safety matter as much as a stylistic one.
- Strips aroma during vigorous fermentation, taking fruit-derived volatiles with it as it escapes.
How it is perceived
What the compound registers as, and at roughly what concentration. Perception is not a property of the molecule alone: sugar, tannin and carbonation all change where a threshold falls.
- prickle
- fizz
- sharpening
- foam
- lifted aroma
Perceptible as a faint prickle from around a gram per litre and as unmistakable sparkle from four or five; the same dissolved concentration reads as more aggressive in a cold drink than a warm one because bubbles come out of solution differently.
Descriptors it is responsible for
Sensory records that name Carbon dioxide as a cause. Each states the perception and the mechanism behind it.
- Bread dough — A raw, yeasty, faintly sour note from live yeast in suspension, characteristic of unfiltered and bottle-conditioned cider.
- Flat and lifeless — The absence of aroma and lift where both were intended — a fault defined by what is missing rather than present.
- Fresh yeast — The clean, faintly sulphidic smell of an active yeast population, met in young cider and during fermentation.
- Fermenting vat — The pungent, CO2-lifted smell of a ferment in full activity, met in very young cider straight from the vessel.
The structure it moves
| Dimension | What it is |
|---|---|
| Carbonation | How much dissolved carbon dioxide the drink carries, from perfectly still to fully sparkling. |
| Acidity | The sharp, mouth-watering quality that makes a cider taste fresh rather than flat. |
| Freshness | Whether the drink smells and tastes of live fruit or of time and air. |
| Body | How much weight and viscosity the drink has in the mouth. |
Measured figures
Shown as they were measured, with the context each was taken in. They are not averaged: a concentration recorded in one country's fruit in one decade is not a constant.
2 separate analyses of dissolved carbon dioxide. They are shown as they were measured, in their own contexts, and are not averaged — the same fruit grown somewhere else can genuinely give a different number.
Dissolved carbon dioxide1.0–2.5 g/L
context not recorded · Peter Mitchell / Cider and Perry Academy
The band a lightly sparkling or pétillant cider generally occupies. Still ciders sit below it; fully sparkling ciders sit at roughly twice it or more. These are working ranges from production practice, not category definitions, and legal or designation-specific carbonation rules differ by jurisdiction.
Dissolved carbon dioxide4.0–8.0 g/L
context not recorded · Peter Mitchell / Cider and Perry Academy
Fully sparkling cider. Pressure in the sealed bottle rises steeply with temperature at these concentrations, which is why bottle strength and storage temperature are treated as safety questions in commercial practice rather than as matters of preference.
Carbonation, which governs both texture and bottle pressure. Measured in grams per litre.
3 separate analyses of bottle pressure. They are shown as they were measured, in their own contexts, and are not averaged — the same fruit grown somewhere else can genuinely give a different number.
Bottle pressure1.5 bar
Pays d’Auge, Cornouaille and Perche appellations, France, 2020–2026 · Minimum required at 20 °C by the operative cahiers des charges · INAO
A floor a producer must clear, not a description of how these ciders drink. Three separate French appellations landed on the same number independently; the Cornouaille specification adds that 3 g/L of dissolved CO₂ satisfies it, which is the same condition stated the other way round.
Bottle pressure1.0 bar
Cidre Cotentin appellation, Manche, France, 2016–2026 · Minimum required at 20 °C by the cahier des charges homologated 14 October 2016 · INAO
The lowest minimum of the French cider appellations, equivalent in the same text to 2 g/L of CO₂. A Cotentin cider is allowed to be perceptibly less fizzy than a Pays d’Auge one and still carry its name, which is a style decision written into law rather than an accident of drafting.
Bottle pressure1.4 bar
Domfront appellation, Orne, France, 2025 · Minimum required at 20 °C by the cahier des charges homologated 7 October 2025 · INAO
The only one of these figures that applies to perry. It sits between the Cotentin and Pays d’Auge minima, and the specification pairs it with the lowest alcohol floor of any of them — 3% acquired against 3.5% for the ciders.
Gauge pressure at a stated temperature — what a specification requires, what a bottle must survive, and what decides how the bubble reads in the glass. Measured in bar.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Wild fermentation
Letting the microflora already present on the fruit and in the press house carry the ferment, in a succession rather than as a single organism.
Inoculated fermentation
Starting a ferment by pitching a chosen yeast culture so that one known strain, rather than the fruit’s resident population, does the work.
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.
Forced carbonation
Dissolving carbon dioxide into cider under pressure to a chosen level, giving complete control of the sparkle and none of the flavour a second fermentation would contribute.
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.
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.
Priming
Adding a measured, calculable quantity of fermentable sugar at bottling so that the fermentation which follows generates a predictable volume of carbon dioxide.
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.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
Filtration
Passing cider through a porous medium to reach a stated visual brightness, at a real cost in colloidal material, body and aroma.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
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.
Under-carbonation
A cider intended to sparkle that has little or no dissolved gas, usually because the bottle conditioning never started or the closure did not hold.
Flat and lifeless
A cider with nothing obviously wrong and nothing to say: aroma faded, acidity dulled, finish short — usually the cumulative result of over-processing or slow oxidation.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
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.
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.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
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.
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.
Described in full
- The unit
- A volume of carbon dioxide is the volume of gas, at standard conditions, dissolved in one volume of liquid. It is the unit cidermakers and brewers work in because it maps directly to pressure at a given temperature.
- Still, nought to one volume
- No perceptible bead. Sidra natural and most still perry sit here. Nothing needs to be held in: there is no meaningful pressure inside the container.
- Pétillant, one to two and a half volumes
- A light prickle rather than a mousse. French cidre bouché and much English farmhouse cider live in this band, at up to roughly two bar of gauge pressure at 20 °C.
- Sparkling, two and a half to four volumes
- The band most commercial cider is carbonated to. Roughly two to three and a half bar at 20 °C, which is why crown caps and pressure-rated bottles become necessary rather than optional.
- Fully sparkling, four to six volumes
- Traditional-method cider and Champagne-style sparkling perry. At six volumes the gauge pressure at 20 °C is around six bar, which needs a sparkling bottle and a wired or crown closure.
- Temperature matters more than intuition suggests
- The same dissolved gas exerts markedly less pressure cold than warm. A bottle safe in a cellar can be dangerous in a car in summer, and every pressure figure here is quoted at 20 °C for that reason.
- Where the bands come from
- They are descriptive conveniences, not legal categories. The boundary between pétillant and sparkling in particular is a matter of usage, and different traditions place it differently.
About Carbon dioxide
Fermentation makes as many molecules of carbon dioxide as of ethanol, and for most of the time a cider is being made the gas is doing protective work. A fermenting vessel is a vessel full of carbon dioxide, and oxygen cannot reach the liquid while the ferment is active. The moment fermentation stops, that protection stops with it, which is why the weeks immediately after a ferment finishes are the most dangerous in a cider’s life and why topping up and sealing become urgent exactly then.
In the finished drink, dissolved carbon dioxide does three separable things. It fizzes, which is a physical sensation. It forms carbonic acid, which is a taste — a sparkling cider genuinely is more acidic than the same cider flat, and the difference is enough to change how a blend should be balanced. And it carries aroma: bubbles rising and bursting transport volatiles into the headspace, so a sparkling cider smells stronger than a still one made from the same liquid.
The arithmetic of getting it there is simple and worth knowing, because getting it wrong is dangerous. Fermenting sugar yields close to half its weight in carbon dioxide, so roughly 4 g/L of priming sugar gives about 2 g/L of dissolved gas, near enough one volume, and adds roughly one atmosphere of pressure at cellar temperature. Pressure also rises with temperature, so a bottle conditioned to a comfortable pressure in a cold cellar can be well above it in a warm room. Bottles intended for conditioning are specified for the pressure they will carry, and using ordinary bottles for a fully sparkling cider is the single most common way a home cellar produces genuine injuries.
The gas is also a thief. Escaping carbon dioxide strips volatile aroma compounds from the liquid, so a hot, fast fermentation loses more of the fruit character the juice carried than a cool, slow one. This is one of the several reasons temperature control does more for a cider’s aroma than yeast selection does.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Alcohols
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.
Sugars
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.
Sugars
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.
Gases
Dissolved oxygen
Essential to a healthy yeast population at the start of fermentation and the principal enemy of a cider from the moment fermentation ends.
Acids
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.
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 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.
- What is malic acid and why does it matter in cider — Malic acid is the acid of apples and supplies almost all the sharpness a cider has. It also sets pH, which decides how well sulphite works and how exposed the cider is to spoilage, so it does far more work than its taste suggests.
- 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.
- 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.
- 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.
- 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.
Where to go next
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
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.
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.