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Compound

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.

Also called SO2, Sulphite, Metabisulphite, Campden.

Class
Additives and processing aids
Formula
SO2
How often it matters
Present in every cider

What it does in cider

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.

On threshold

Free sulphur dioxide becomes perceptible as a struck-match pungency at levels that vary with pH, because it is the molecular fraction that is smelled — the same free figure is far more obvious in a low-pH cider than a high-pH one.

Descriptors it is responsible for

Sensory records that name Sulphur dioxide as a cause. Each states the perception and the mechanism behind it.

The structure it moves

Structural dimensions this compound contributes to. Direction and amount depend on concentration and on what else is in the drink; the dimensions themselves are set out in full under Sensory.
DimensionWhat it is
FreshnessWhether the drink smells and tastes of live fruit or of time and air.
Oxidative characterNutty, bruised-apple, sherry-like or cardboard notes from exposure to air.
Fruit characterHow strongly the drink smells of apple or pear, and of fruit generally.

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 sulphur 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.

Sulphur dioxide0.5–0.8 mg/L

context not recorded · Molecular (undissociated) SO2, calculated from free SO2 and pH · Andrew Lea

This is the target for the *molecular* fraction, not for free or total SO2, and confusing the three is the commonest error in sulphite practice. The free SO2 needed to reach it depends entirely on pH: a cider at pH 3.2 needs roughly a quarter of what a cider at pH 3.8 needs for identical protection.

Sulphur dioxide10.0 mg/L

context not recorded · European Union

A labelling trigger, not a technical or safety threshold. Above this concentration, expressed as total SO2, the presence of sulphites must be declared on the label as an allergen. It says nothing about whether the cider is adequately protected, and a cider below it is generally not protected at all.

Added or residual SO2, whose antimicrobial effect depends sharply on pH. Measured in milligrams per litre.

What forms it

Processes that put this compound into the drink, or increase how much of it is there.

What removes or limits it

Processes that reduce it, hold it below a threshold, or stop it forming in the first place.

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.

Organisms that produce it

Which organism is responsible usually decides whether the compound is a feature or a symptom.

Molecular sulphur dioxide against pHThe share of free sulphur dioxide that exists as the active molecular form, calculated across the pH range cider occupies.Molecular SO₂ as % of free02463.03.23.43.63.84.0Juice pH6.1%0.6%To hold 0.8 mg/L molecularneeds 13 mg/L free at pH 3.0and 125 mg/L free at pH 4.0
The share of free sulphur dioxide that exists as the active molecular form, calculated across the pH range cider occupies.
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.

About Sulphur dioxide

Dissolve sulphur dioxide in a cider and it immediately distributes itself between three species: molecular SO2, the bisulphite ion, and the sulphite ion. Only the first of these does the antimicrobial work. It is uncharged, so it crosses cell membranes freely, and inside the cell — where the pH is higher — it dissociates, acidifies the cytoplasm and reacts with essential thiols and cofactors. The charged ions cannot cross a membrane and are, for this purpose, inert.

The proportion in the active form is fixed by pH, through the first dissociation constant, which sits at about 1.81. The arithmetic is unforgiving. At pH 3.0 roughly six per cent of the free sulphur dioxide is molecular; at pH 3.5 about two per cent; at pH 3.8 about one per cent; at pH 4.0 well under one per cent. Since the target for meaningful protection is a molecular concentration of something like 0.5 to 0.8 mg/L, the free sulphur dioxide required to reach it rises from around 15 to 20 mg/L in a sharp cider to something over 100 mg/L in a cider at pH 4.0 — a level that would be organoleptically obvious and, in many jurisdictions, close to or above what the law permits. This is the whole reason a high-pH cider is hard to protect, and the reason a sulphiting decision cannot be made from a rate table without knowing the pH.

The second complication is binding. Sulphur dioxide reacts with carbonyl compounds, and the reaction with acetaldehyde is strong enough to be effectively permanent on cellar timescales. Bound sulphite is not molecular, not free, and not doing anything, but it is counted in total SO2 and therefore against any legal maximum. Juice from rotten or oxidised fruit is full of binders — acetaldehyde, pyruvate, ketonic compounds from mould metabolism, oxidation products of galacturonic acid — so it swallows large additions and shows almost no free SO2 the next morning. The useful conclusion is that sorting fruit is a sulphite-saving measure, and that a maker who responds to disappearing sulphite by adding more is paying twice for nothing.

What sulphur dioxide is actually used for varies with the stage. In juice it inhibits polyphenol oxidase and suppresses the wild yeast and bacterial population, which is what makes a predictable inoculated fermentation possible — and correspondingly, a maker who wants a wild ferment is choosing to forgo it, or to use a small addition that selects rather than sterilises. In finished cider it protects against refermentation, against acetic bacteria and against oxidation, both by scavenging oxidation products and by destroying the hydrogen peroxide generated when phenolics oxidise. It is also the necessary partner to sorbate, which does nothing to lactic bacteria and produces the geranium fault if they are left unchecked.

Two limits sit around all of this. The legal one is that maximum permitted concentrations are set by food additive law in each jurisdiction and are stated as total rather than free SO2, and that above 10 mg/L the label must declare sulphites. The human one is that a minority of people, most notably some with asthma, react to sulphites, which is precisely why the declaration exists. Neither of these is a reason to treat sulphur dioxide as sinister; both are reasons to use the smallest amount that does the job, and to make that judgement from the pH rather than from habit.

Related compounds

Compounds it is formed from, converted into, confused with, or routinely met alongside.

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.

Where to go next

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.