Raw meat, rotten eggs, vegetal, struck match and rubber aromas are how sulphur compounds announce their presence. For many distillers it’s effectively a warning and the objective is to reduce the level through distillation and ageing. However, some distillers welcome a certain level of sulphur compounds which can add body and complexity. Moreover, sulphur compounds can also yield tropical fruit notes, for example, so what matters is attaining a level and range that suits the house style.
Sulphur compounds originate during fermentation, when yeast metabolises (digests) sugars and protein in the wort, to grow and reproduce. Metabolising nutrients produces ‘waste matter’ which yeast cells expel into the wort. The ‘waste’ from sugar is alcohol and flavour compounds including esters (fruit notes), while metabolising protein results in sulphur compounds. This includes around 10 to 12 (possibly more) individual compounds. Identifying them is a challenge, as sulphur compounds are measured in parts per billion and parts per trillion, and the technology to do this conclusively is still in development.
The level of sulphur compounds decreases during distillation, as a result of the vapours (which contain sulphur compounds) coming into contact with copper. A ‘matrix’ on the surface of the copper is able to retain sulphur compounds, exactly how is still being researched and the greater the contact between the vapours and copper, the greater the reduction of sulphur compounds. The level of contact is partly determined by the rate of distillation. Heating the still gently promotes a slower rate, which means the vapours are less dense and progress more gradually along the neck, consequently experiencing greater copper contact. Turning up the heat increases the distillation rate, resulting in denser vapours ascending the neck more rapidly, which reduces the level of copper contact.
“Our distillation rate is very slow as our objective is lowering the level of sulphur compounds as much as possible to produce a light, elegant spirit. Lowering the level of sulphur compounds reduces pungency, but also reveals lighter notes such as esters (fruityness) and sweetness which are otherwise hidden beneath sulphur compounds,” says Peter Holroyd, Kingsbarns distillery manager.
The level of sulphur compounds considered desirable depends of course on the house style.
“At Craigellachie we’re looking for a meaty, sulphur note, alongside high levels of fruity esters, which are masked by meaty notes in the new make spirit. When Craigellachie ages and sulphur compounds reduce, the fruity notes appear and give us pineapple chunks in syrup,” says Stephanie MacLeod, Dewar’s master blender.
During ageing, the level of sulphur compounds reduces most rapidly in Bourbon barrels, absorbed by the interior layer of char.
“Within 24 months the simpler, more pungent sulphur compounds such as vegetal notes have been absorbed by the char of a Bourbon barrel. More complex sulphur compounds which show in the new make spirit as meatier and tropical fruit notes remain for longer, adding savoury richness rather than beefiness,” says Stuart Watts, site director, William Grant & Sons.
Sherry casks have a toasted (as opposed to char) layer, with a smaller capacity to absorb sulphur compounds. But there are mitigating factors.
“Sherry casks contribute much richer flavours than a Bourbon barrel, which can mask sulphur compounds and give rise to more body, so Sherry casks can be better for a bigger, richer spirit with higher levels of sulphur compounds,” says Michael Henry, master blender, Loch Lomond.
Two supplementary influences are evaporation from the cask, with sulphur compounds part of the annual two per cent rate, together with oxidation breaking sulphur compounds down into subtler, less flavour-active forms.
All this raises the question of perception. Some people have a ‘blind spot’ that prevents them from detecting sulphur compounds. Others identify sulphur compounds at various levels, while some people are highly sensitive to all levels.