Juice treatment
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
Also called SO2 addition, Sulphur dioxide treatment.
- Stage
- Juice treatment
- Traditional in
- No single tradition — used wherever it suits
- What it most changes
- Oxidative character down, fermentation character down
- Safety
- Carries a safety consideration — see below
The correct addition depends on juice pH, and a level appropriate to a sharp juice can be substantially under-effective in a low-acid bittersweet one — which is why pH is measured before the decision rather than after it. Maximum permitted levels in the finished drink, and the threshold above which sulphites must be declared on the label, are set by law and differ between jurisdictions. Sulphite sensitivity is a recognised adverse reaction in some people, notably some with asthma. Handling metabisulphite powder releases an irritant gas that should not be inhaled, and it is dissolved in a ventilated space. CiderHQ does not publish addition rates: they are pH-dependent and jurisdiction-dependent, and a figure without its context is worse than none.
What it is
Sulphiting is the addition of sulphur dioxide to juice, normally as potassium metabisulphite dissolved in a little of the juice itself, occasionally as gas or as a solution of the salt. It does three separable things: it inhibits bacteria and the more fragile wild yeasts, it inactivates polyphenol oxidase and so slows enzymatic browning, and it scavenges oxygen and binds acetaldehyde. It is the most widely used intervention in juice treatment and also the most widely misunderstood, because the quantity added tells you almost nothing on its own — what governs the effect is how much of it exists as molecular SO2, and that depends on the pH of the juice.
Described in full
- What is plotted
- The vertical axis is molecular sulphur dioxide as a percentage of free sulphur dioxide. The horizontal axis is juice pH, from 3.0 to 4.0. The curve is calculated, not sketched: the molecular fraction is one divided by one plus ten to the power of pH minus 1.81.
- Why only the molecular form counts
- Free sulphur dioxide is a mixture of molecular SO₂ and bisulphite. Only the molecular form is antimicrobial. Measuring free sulphite therefore tells you how much you added, not how much is working.
- The shape of the curve
- It falls steeply and then flattens. At pH 3.0 about six percent of the free sulphite is molecular; by pH 4.0 it is about six tenths of one percent. The active fraction falls roughly tenfold across one pH unit.
- What that means in practice
- To hold the commonly used protective level of 0.8 mg/L molecular SO₂ needs about 13 mg/L free sulphite at pH 3.0, about 50 mg/L at pH 3.6, and about 125 mg/L at pH 4.0.
- Why bittersweet juice is the hard case
- Low-acid bittersweet fruit ferments at a high pH, exactly where sulphite is least effective, and where the sulphite dose required approaches or exceeds legal and sensory limits. This is a large part of why sharp fruit is blended in.
- The 1.81 in the expression
- It is the first acid dissociation constant of sulphurous acid, expressed as pKa. The equation is the Henderson–Hasselbalch relationship applied to that equilibrium, which is why the curve is a property of the chemistry rather than an empirical fit.
- A caution
- Bound sulphite is not shown at all. A substantial fraction of any addition binds irreversibly to acetaldehyde and other carbonyls and never contributes to the free figure, so the dose added always exceeds the free level measured afterwards.
Why it is used
- It narrows the microbial field before fermentation, suppressing acetic acid bacteria, lactic acid bacteria and the least ethanol-tolerant wild yeasts while leaving Saccharomyces comparatively able to establish.
- It inhibits polyphenol oxidase, which preserves both the phenolic material and the fresh varietal aroma that oxidative browning would otherwise remove.
- It binds acetaldehyde and consumes dissolved oxygen, protecting juice during handling, settling and storage.
- It gives a maker working with fruit of uncertain condition a margin that fruit selection alone cannot provide.
How it works
- In solution, sulphur dioxide sits in a pH-dependent equilibrium between molecular SO2, bisulphite and sulphite. Only the molecular form crosses microbial cell membranes readily, and it is therefore the antimicrobial fraction.
- The proportion existing as molecular SO2 falls steeply as pH rises, so the same addition to a sharp Asturian or English sharp juice and to a low-acid bittersweet juice produces very different antimicrobial effect — often several-fold different.
- A large part of any addition binds almost immediately to acetaldehyde, sugars and keto-acids and becomes unavailable; only the free fraction is doing work, and the bound fraction still counts towards total SO2 for labelling and legal purposes.
- Against polyphenol oxidase, sulphite acts both by reducing the quinones formed back to phenols and by inhibiting the enzyme itself, so browning slows on two fronts.
- Its effect is selective rather than sterilising: it shifts which organisms dominate rather than clearing the field, which is why sulphited juice still ferments.
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 |
|---|---|---|
| Freshness | Preserves | Suppressing oxidase activity keeps the volatile esters and aldehydes responsible for fresh-fruit aroma in the juice rather than allowing them to be lost with the browning reaction. |
| Oxidative character | Lowers | Sulphite reduces the quinones formed by polyphenol oxidase back to phenols and binds acetaldehyde, so the browning and the aldehydic note that go with it do not develop. |
| Fermentation character | Lowers | The non-Saccharomyces yeasts responsible for much of the ester and volatile complexity of a spontaneous ferment are more sulphite-sensitive than Saccharomyces, so their contribution is reduced. |
| Phenolic character | Preserves | Phenolic material that would otherwise oxidise, polymerise and precipitate stays in solution, so the finished cider retains more of the tannin the fruit carried. |
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
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.
Acetaldehyde
The compound sitting one step short of ethanol, which smells of bruised apple and sherry, binds most of the sulphite added to a cider, and is the chemical signature of oxidation.
Polyphenol oxidase
The copper enzyme that turns cut apple brown within seconds, and the reason a cidermaker has to decide, at the press, whether to let the juice oxidise or to stop it.
Chlorogenic acid
The most abundant single phenolic in apple juice, the preferred substrate of the enzyme that browns it, and the precursor of one of the volatile phenols behind farmyard character.
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
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.
Organisms involved
Acetobacter aceti
An acetic acid bacterium that oxidises ethanol to acetic acid wherever cider meets air, and the organism behind most volatile acidity in cider.
Acetobacter pasteurianus
The film-forming acetic acid bacterium of traditional vinegar production, and a common cause of surface growth and volatile acidity in cider.
Gluconobacter oxydans
A sugar-preferring acetic acid bacterium abundant on damaged fruit and in fresh juice, which oxidises glucose to gluconic acid before fermentation begins.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
Hanseniaspora valbyensis
The apiculate yeast most consistently reported as dominating the first days of a spontaneous cider fermentation, and a major contributor to its aroma.
Kloeckera apiculata
The anamorph name for Hanseniaspora uvarum, still in wide use in cider writing, and often used loosely as a collective term for all apiculate yeasts.
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
What it is done with
Measuring sulphur dioxide
The reference method is aeration-oxidation, which strips sulphur dioxide out of an acidified sample and traps it for titration; the quicker Ripper method titrates the sample directly with iodine and over-reads in cider because other substances react too.
The pH meter and its calibration
A pH meter reads the activity of hydrogen ions through a glass electrode, giving the number that governs microbial safety and sulphur dioxide effectiveness — and it is worthless without regular calibration and proper probe storage.
Drawing a sample without spoiling the batch
Every sample is a small hole made in the protection around a batch: something goes in, air goes in with it, and cider comes out — so the technique is about drawing a representative sample while putting nothing back and letting in as little air as possible.
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.
Excess sulphur dioxide
A struck-match or burnt-match aroma and a prickle at the back of the nose from too much free sulphur dioxide, with a real health consideration for sulphite-sensitive people.
Oxidation
The cumulative effect of oxygen on finished cider: fruit aroma flattens, colour deepens towards amber, and a bruised-apple or sherry-like character replaces the fresh one.
Enzymatic browning
The rapid darkening of milled fruit and fresh juice as polyphenol oxidase converts phenolics to quinones, taking colour and some tannin structure with it.
Acetification
The active conversion of a cider’s ethanol into acetic acid by acetic acid bacteria at an air interface — the process, running in the vessel, that produces volatile acidity.
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.
Mousiness
A retronasal taint of mouse cage, stale popcorn or crackers that appears only after swallowing — and that a substantial fraction of people cannot detect at all.
Film yeast growth
A pale, powdery or wrinkled film of aerobic yeast growing on the surface of a standing cider, consuming alcohol and acid and producing acetaldehyde.
Colour loss
A cider left noticeably paler than it should be, usually because fining, filtration or sulphite has removed the phenolic material that gave it colour.
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.
Modern mainstream cider
The industrially produced, consistent, carbonated cider that accounts for most of what is sold worldwide, generally made partly from concentrate and finished to a fixed specification.
Single-varietal cider
Cider made wholly or overwhelmingly from one apple cultivar, presented so that the fruit’s own character is the subject of the drink.
Sweet cider
Cider in which sugar is the leading sensation, whether retained from an arrested fermentation, added after it, or produced by keeving.
Wild-fermented cider
Cider fermented by the yeast population already present on the fruit and in the cellar, without an added cultured strain.
Sidra natural
The still, dry, unfiltered cider of Asturias and the Basque Country, wild-fermented from local high-acid fruit and poured from a height to raise a momentary sparkle.
Sidra de nueva expresión
A modern Asturian category of filtered, clean, dry cider made with controlled fermentation and presented in a wine format rather than in the poured tradition.
Recorded figures
Shown with the place, period and method each was taken under, and never averaged: the same step run in another cellar genuinely gives a different number.
pH3.00–3.80 pH
Cider juice generally, 2026 · Not a measurement of a sample: the range over which the molecular fraction of sulphur dioxide falls by roughly an order of magnitude, from the dissociation constant of sulphurous acid. · Andrew Lea
A property of the chemistry rather than of any juice. At pH 3.0 something like six per cent of free sulphur dioxide is in the active molecular form; at pH 3.8 it is under one per cent. This is why a sulphite dose calculated without a pH reading is a guess.
Acid strength, which governs microbial risk and sulphite effectiveness. Measured in ph.
More on sulphiting
The single most important thing about sulphite in cider is that the number on the bag is not the number that matters. Sulphur dioxide in solution partitions between molecular SO2, the bisulphite ion and the sulphite ion, and the position of that equilibrium is set by pH. Only molecular SO2 passes readily into a microbial cell, so it is the fraction that does the antimicrobial work, and it makes up a rapidly diminishing share of the total as pH climbs. Cider is where this matters more than in almost any other fermented drink, because cider juice spans an unusually wide pH range: a sharp English or Asturian juice and a low-acid Herefordshire or Norman bittersweet juice can sit far enough apart that the same addition gives protection in one and very little in the other. A maker who sulphites by volume without measuring pH is not dosing, they are guessing.
The second complication is binding. A substantial part of any addition combines within hours with acetaldehyde, with keto-acids and with sugars, and the bound fraction is no longer antimicrobial. It still counts as total SO2 for legal and labelling purposes, so a cider can simultaneously carry a high total figure and be effectively unprotected. This is why free and total SO2 are measured separately, and why juice from bruised, oxidised or partly fermented fruit — which is already rich in acetaldehyde — consumes far more sulphite to reach the same free level than sound fresh juice does. Sulphiting late, after browning has begun, is expensive and only partly effective.
Practice diverges sharply along traditional lines, and the divergence is not simply modern-versus-old. Asturian sidra natural and the French cider AOCs are built around spontaneous fermentation by the fruit’s own microflora, and heavy sulphiting is incompatible with that: it removes precisely the non-Saccharomyces yeasts and lactic acid bacteria whose activity defines the style. Much English commercial practice, by contrast, treats sulphiting at the press as routine, because it is producing a consistent product from fruit of variable condition on a schedule. Modern North American craft practice tends to sulphite as a matter of course and inoculate afterwards, while a growing minority works unsulphited and accepts the risk. German Apfelwein practice sits between. None of these is the default from which the others deviate.
When it goes wrong it goes wrong in both directions. Too little, in a low-acid juice, and the protection is nominal: acetic acid bacteria establish, volatile acidity climbs, and lactic acid bacteria may produce mousiness or, in the presence of glycerol, the acrolein bitterness that no amount of blending will fix. Too much, and the ferment is sluggish or sticks outright, sulphidic notes appear, the drink smells of struck match, and the sensory penalty persists because bound SO2 releases slowly. The subtler cost is that a well-sulphited juice ferments into a cleaner but narrower cider: the aromatic complexity that the early wild yeasts contribute is simply not there. That is a legitimate trade, but it should be made knowingly rather than by habit.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Juice treatment
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
Stabilisation
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Fermentation
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.
Juice treatment
Juice storage
Holding unfermented juice sound between pressing and fermentation, by chilling, sulphiting, gas blanketing, freezing or aseptic filling.
Juice treatment
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Juice treatment
Patulin control
Managing the mycotoxin produced by rot fungi in damaged apples, which is controlled by fruit selection rather than by any treatment applied to juice.
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 does cider ferment — Yeast consumes the sugar in apple juice and produces roughly equal masses of ethanol and carbon dioxide, along with the esters and higher alcohols that give cider much of its aroma. In a wild ferment several yeast species take turns before *Saccharomyces* finishes the job.
- How do cider makers keep patulin out — By sorting rotten fruit out before milling, because patulin comes from moulds growing on damaged apples. Fermentation reduces what gets through, so fermented cider carries no limit where juice does.
- How do i sterilise cider bottles — Wash them clean first, then sanitise with a no-rinse sanitiser or a sulphite solution, and fill while still wet with it. Bottles that look clean but have dried deposits inside are the usual source of bottle spoilage.
- 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.
- 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 does my cider taste like vinegar — Acetic acid bacteria have reached the cider and, given air, are converting its alcohol into acetic acid. The cause is almost always oxygen — an unfilled vessel, a leaking bung, or a slow transfer.
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.
European Food Safety Authority scientific opinions
EFSA · regulator · retrieved 2026-08-24
Registered for the food-safety questions cider genuinely raises: patulin in juice from rotten fruit, sulphite sensitivity, and the toxicology behind additive limits.
Regulation (EU) No 1169/2011 on the provision of food information to consumers
European Union · legislation · passage verified 2026-08-24
The instrument behind the "contains sulphites" declaration and the alcoholic-strength labelling rule. Read on 2026-08-24 in the assimilated text maintained on legislation.gov.uk, which is the version in force for Great Britain and keeps the EU numbering. Two provisions matter to cider and both are widely misquoted: the sulphite threshold is 10 mg/L rather than a percentage, and a labelled cider strength carries a tolerance of a full percentage point either way — twice what still grape wine is allowed.