Understanding hydrogen sulphide and sulphur dioxide

The volcanic crater on Nysyros. (Photo: The Real Review)

Hydrogen sulphide (H2S) commonly occurs in wine and volcanic activity, as I was reminded on a visit to the active volcano on the Greek island, Nysyros. In wine, H2S can take many guises – in its pure form it’s the smell of rotten eggs, but when it interacts with the aromatic constituents of wine it can present as many diverse smells in the scatological realm.

A common mistake among lay wine drinkers is to confuse sulphur dioxide with hydrogen sulphide.

On Nysyros, I took a rented vehicle to the top of the volcano’s lip and walked down a track into the crater itself, which is a broad, flat area pock-marked by fumaroles and strange soil formations, shaded in patches by the pretty yellow colour of sulfur. As we approached the crater and then entered it, the rotten egg gas smell became stronger and stronger, but the strange thing was that in various places inside the cone it reminded me variously of chicken stock cubes, smoked chicken, chicken pooh and even smoked ham.

In its yellow powder form, elemental sulphur (or sulfur) is used in vineyards to dust the vine leaves, shoots and bunches to protect against fungal diseases. Sulphur rings, like little yellow mosquito coils, can be burnt inside barrels to keep them free of insects and moulds. In winemaking, sulphur dioxide (SO2) is commonly added as a liquid or gas to protect juice or wine from oxidation (by inhibiting the natural oxidative enzymes) and microbial attack (from undesired bacteria and yeasts). It’s also used as a sterilant to rinse bottles.

A common mistake among lay wine drinkers is to confuse sulphur dioxide (also known as sulphite) with hydrogen sulphide. Elemental sulphur doesn’t really have a smell so much as a sensation: it takes your breath away. Sulphur dioxide (SO2)’s olfactory effect is much the same as elemental sulphur. Hydrogen sulphide (H2S) is formed by yeasts reducing sulphur by adding hydrogen ions. (Oxidation is the addition of oxygen ions while reduction is the opposite reaction, the removal of oxygen ions or addition of hydrogen ions.) Hence a sulphide smell in wine is often referred to as ‘reduction’, or a ‘reductive’ smell.

It can take many guises, some more attractive than others – but people vary widely in their reactions to it. Some drinkers may enjoy a wine that others don’t.

Sulphidic odours are generally thought undesirable, but in some wines, and in controlled amounts, they can add extra complexity (eg. barrel-fermented white Burgundies and other chardonnays).

Pure rotten egg smell is disgusting to most people, as is the smell of faeces or farts. It’s one of the fascinating conundrums of wine that some sulphide influences can be found in the greatest white wines, such as certain Burgundies (eg. Coche-Dury, Ramonet) and chardonnays including Giaconda and Oakridge. Many winemakers actively chase these characteristics by using wild yeasts to ferment unclarified juice.

For further study you don’t need to sail to Nysyros as I did: New Zealand’s Rotorua is closer at hand and also offers sulfurous delights.

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