Measuring sulphur dioxide

Sulphur dioxide added to juice or cider immediately begins reacting with carbonyl compounds — acetaldehyde above all, and also sugars, quinones and other species — forming stable adducts. Bound sulphur dioxide is still present, still counts towards a legal total and towards a label declaration, and is largely useless as a preservative.

What protects the cider is the free fraction, and within that only the small molecular portion whose proportion is set by pH. So a maker needs the free figure to know whether the cider is protected, and the total figure to know whether a regulatory limit or a declaration threshold is being approached. Neither number substitutes for the other.

The bound fraction also grows over time and grows faster in juice that is high in acetaldehyde — which is typically juice that has been oxidised or that has fermented under stress. A cider that keeps needing more sulphite is usually a cider that is producing binding partners faster than it is being dosed.

The apparatus is a glass train: a reaction flask holding the acidified sample, a condenser or trap containing dilute hydrogen peroxide, and a pump or aspirator drawing gas through the assembly. Acidifying the sample converts sulphite and bisulphite into dissolved sulphur dioxide gas, and the airflow strips it out and carries it to the trap.

In the trap the hydrogen peroxide oxidises the sulphur dioxide to sulphuric acid. Titrating that acid with a standard alkali gives the amount of sulphur dioxide that was carried over, and hence the free figure. To obtain total sulphur dioxide the same procedure is run on a sample that has first been treated with alkali and left to stand, which releases the bound fraction, and then acidified.

The method’s virtue is specificity. Nothing else in cider ends up in the peroxide trap as a titratable acid in any quantity, so colour, phenolics and ascorbic acid do not interfere. Its costs are apparatus, time and a certain amount of glassware skill, which is why it lives in laboratories and in larger producers rather than on the average cider house bench.

Ripper titration is direct: acidify the sample, add starch indicator, and titrate with standard iodine solution until a persistent blue-black colour shows that iodine is no longer being consumed. The iodine oxidises sulphur dioxide, and the volume used gives the concentration.

The problem is selectivity. Iodine oxidises many things, and cider is full of oxidisable material — polyphenols in quantity, ascorbic acid where it has been added, and various reductones. All of these consume iodine and inflate the result, and the inflation is larger the more phenolic the cider is. In a heavily tannic traditional cider the error can be substantial, and it is always in the direction of reporting more protection than is really there.

The endpoint is also hard to see in a dark cider, for the same reason a titration endpoint is hard to see. Some practitioners run a blank on a decolourised or sulphite-free equivalent sample to subtract the interference, which improves matters but does not make the method a reference one.

Where the Ripper number is used at all, it is best used as a relative indicator on the same cider over time rather than as an absolute figure, and it should never be the basis for a claim on a label or for a regulatory declaration.