The Origin of the Campden Tablet

How many times have you seen the phrase "1 Campden Tablet per gallon gives 50 ppm SO₂"? But who invented this little thing and why? What's the origin of its name? And is that statement even true?

Well the story goes back about 100 years and originally it had nothing to do with wine or cider making; quite the reverse in fact. Back in the days of World War 1, the Long Ashton Fruit and Cider Research Station near Bristol in the UK started to look at methods of fruit preservation for the then national emergency, and in 1919 they set up an outstation near the plum growing areas in the Vale of Evesham, at Chipping Campden in Gloucestershire. (Ironically, Long Ashton finally closed in 2003 but Campden is still thriving as a food and beverage consultancy having parted company from its parent many years ago).

The scientists at Long Ashton / Chipping Campden had been experimenting with 'cold sterilisation' using various preservative chemicals to avoid the difficult and costly use of heat in conventional fruit canning and bottling. They turned to the use of sulphur dioxide which had long been known to inhibit yeast and mould growth and was commonly used (by burning sulphur candles) to sterilise cider and wine barrels. What they found is that if the fruit were lightly packed into containers and the empty space around it was filled with a sulphite solution of around 1000 ppm in concentration, it would prevent spoilage for many months. Of course you couldn't eat the fruit with all that sulphite in it, and it had to be removed by boiling it away before use, but it was a good way of capturing a glut of fruit quickly and easily instead of it going to waste. The technique was mostly intended for commercial producers but the idea was promoted to domestic users too. To make it practicable for households and small companies to use, the research station started to market a 1000 ppm solution of sulphur dioxide as the "Campden Fruit Preserving Solution" during the 1920's and 30's.

It didn't catch on much though, until war broke out in 1939. There was a glut of fruit in the summer of 1940 and understandably people wanted to preserve it, and they had no sugar due to rationing. All of a sudden Long Ashton and Chipping Campden were inundated with requests for the "Campden Fruit Preserving Solution" and they couldn't cope. They needed an easier method so that people could prepare the solution in their own home. Hence the Campden Fruit Preserving Tablet was born.

The idea was simple. Under the auspices of the Ministry of Food, a formula was circulated to manufacturing chemists and pharmacists all over the country so that defined tablets of sodium or potassium metabisulphite could be prepared and sold to the general public. When each tablet was dissolved in half a pint of water, it gave a 900 - 950 ppm sulphite solution which was sufficient to chemically preserve one pound of fruit. The specification was set such that each tablet should deliver "4 grains of sulphur dioxide". The "grain" is an obsolete measure that was still in common use in pharmacies in those days - 1 grain is nowadays defined as 65 mg. So each tablet needed to provide 260 mg of SO₂.

Now, the yield of SO₂ from sodium metabisulphite in acid solution is about 60% in good real-life conditions (though the theoretical yield is 67%). So in modern terms the tablets need to contain around 440 mg of the sodium metabisulphite salt. In practice, the tablets need fillers and binders for bulk and coherence so the weight of each tablet is always more than the bare 440 mg.

After the war this method of fruit preservation fell out of use but people started to turn to home winemaking from native fruits. A convenient low-dose regime of sulphur dioxide addition was needed to eliminate wild yeasts and moulds, much like burning a sulphur candle in a barrel but much more controllable. Some of the staff at Long Ashton had also been involved with the fruit preservation programmes, and they quickly realised that the Campden tablet was ideal for this too. Quite by chance it turned out that 1 Campden tablet per imperial gallon gave 260 mg of SO₂ in 4.54 litres which comes out as 57 ppm. Allowing for natural tolerances and maybe some further loss on storage, it became an easy shorthand to say that "1 Campden Tablet per gallon gives 50 ppm SO₂". Note that's only true for an imperial gallon though. For the smaller US gallon, it's more like 60 ppm.

So that was how staff at the Long Ashton Research Station invented the Campden tablet. What else did they invent? Well there's always "Ribena" of course, but that's another story!

Sources

Barker B and Grove O. 1924 Sulphur Dioxide as a Preservative for Fruit Long Ashton Annual Report p 97-108

Crang A. 1941 Preserving Fruit with the Campden Fruit Preserving Tablets Long Ashton Annual Report p 118-124

Leach M (editor) 1971. Preserving Fruit with Chemicals in "Home Preservation of Fruit and Vegetables" MAFF Bulletin 21 published by HMSO. 13th edition [This is the last edition in which cold sterilisation using Campden tablets is described. The 14th edition of 1989 no longer mentions it. Both editions are out of print]

Last edited by Andrew Lea December 2013

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