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Cider science

Sulphur dioxide, and why pH governs it

Why do cider makers add sulphite, and how much?

In short

Sulphur dioxide does three jobs: it inhibits unwanted microorganisms, it scavenges oxygen-derived oxidants, and it binds acetaldehyde into an odourless complex. It is the single most useful preservative available to a cidermaker, and it is used almost universally outside a few traditions that avoid it.

The critical point is that only a small fraction of the sulphite in solution is doing the antimicrobial work. Sulphur dioxide exists in equilibrium between molecular, bisulphite and sulphite forms, and only the molecular form is effective. That fraction is governed by pH, and it falls steeply as pH rises.

This is why a dose is calculated from the measured pH rather than taken from habit. A quantity that protects a juice at pH 3.2 will be substantially inadequate at pH 3.8, and a quantity that protects at pH 3.8 will be far more than necessary at 3.2.

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.

The three forms, and why only one works

Dissolved sulphur dioxide exists in three interconverting forms: molecular sulphur dioxide, the bisulphite ion, and the sulphite ion. The relative proportions are set by pH through the acid dissociation equilibria, and the changeover between the molecular form and bisulphite occurs well below the pH range of cider — which means that at any cider pH, the great majority of the free sulphite is bisulphite.

Only the molecular form passes readily through microbial cell membranes, so it is the only form that inhibits organisms effectively. The consequence is that a free sulphur dioxide figure, on its own, does not say how protected a cider is. What matters is molecular sulphur dioxide, and that is calculated from the free figure and the pH together.

The relationship is exponential rather than linear, because pH is a logarithmic scale. As a working rule, each increase of roughly 0.3 pH units approximately halves the molecular fraction, so the free sulphite must be roughly doubled to hold protection constant. Over the range of pH values cider actually occupies, this compounds into a difference of several times.

Free, bound and total

The binding reaction is why sulphite additions are not cumulative in the way they appear. Cider containing a lot of acetaldehyde binds much of an addition immediately, so a heavily oxidised cider consumes sulphite without becoming protected. Adding more to compensate raises total sulphur dioxide towards the legal ceiling without necessarily raising the free figure much at all.

The sulphite fractions and what each means in practice.
TermWhat it isWhy it matters
Molecular sulphur dioxideThe undissociated form, a pH-dependent fraction of the free sulphiteThe only antimicrobially active form; the figure a cidermaker actually targets
Free sulphur dioxideMolecular plus bisulphite plus sulphite — what has not reacted with anythingWhat is measured directly; converted to molecular using the pH
Bound sulphur dioxideSulphite that has reacted with acetaldehyde, sugars, quinones and other carbonylsNot antimicrobial and not antioxidant, but does neutralise acetaldehyde aroma
Total sulphur dioxideFree plus boundThe figure legal limits are expressed against, and what a laboratory reports

What it is used for, and when

Limits, labelling and sensitivity

Sulphites must be declared as an allergen on labels in the European Union and the United Kingdom where they are present above 10 milligrams per litre, expressed as total sulphur dioxide. This threshold is low enough that many ciders carry the declaration, including some to which nothing was added, because small quantities are produced by yeast during fermentation.

Legal maximum levels for total sulphur dioxide in cider are set by jurisdiction and differ between them; they are stated on the relevant regulatory pages rather than summarised here, because they change.

A minority of people react adversely to sulphites, most consistently among those with asthma, and the reaction is respiratory rather than a classical allergy. This is why the labelling requirement exists. It is worth stating that unsulphited is not synonymous with better made: a cider produced without sulphite has accepted a different set of risks, and the traditions that manage those risks well have other controls in place.

Dose by measurement, not by recipeA sulphite addition quoted without the pH it was calculated for carries no useful information. The same milligram figure represents very different levels of protection across the pH range that cider occupies.

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