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Whisky off-flavours: sulphur, soap, plastic - where they come from

Sometimes something appears in whisky that spoils the pleasure: a note of rotten egg, soap, burnt rubber or plastic. These are so-called off-flavours, that is foreign, unwanted aromas that betray errors or the specifics of production. Where do they come from? The answer lies in fermentation and distillation, and a key role is played by the copper of the still and the moment of cutting the spirit. Interestingly, the line between a fault and a feature can be fluid: the same note in one context is an error, in another a desired character. In this post we will explain where sulphur, soap and plastic in whisky come from, how distilleries fight them and how to recognise these notes in the glass. It is knowledge that will make you a more conscious taster.

What off-flavours are

Off-flavours are foreign, unwanted aromas and flavours that should not dominate in well-made whisky. They can result from errors at various stages of production, from fermentation, through distillation, to maturation, or from a faulty cask. Not every off-flavour, however, means a catastrophe: some in small concentration are tolerated, and even desired in certain styles. The problem begins when a foreign note becomes too intense and covers the proper character of the drink. Recognising off-flavours is a valuable tasting skill, because it helps understand what went wrong, and also appreciate the craft of distilleries that manage to avoid them. It is worth remembering that many off-flavours arise naturally during production, and the art lies in controlling, removing or softening them, rather than in completely eliminating every trace, which is often practically impossible.

Sulphur - egg, matches, rubber

One of the most common off-flavours is sulphur. Sulphur compounds give notes associated with rotten egg, a struck match, gas, a pencil eraser, burnt rubber or stagnant water. Where do they come from? Natural sulphur compounds form already during fermentation, as a product of yeast metabolism, and also during distillation and maturation. In small concentration they are tolerated, and even add a certain meatiness to whisky, but in excess they clearly spoil the flavour. Sulphur is one of the most characteristic and recognisable off-flavours, and at the same time one of the hardest to tame. Distilleries devote much attention to keeping sulphur compounds under control. Fortunately there is a natural ally in the fight against sulphur, built into the distillation process itself, which we will tell about in the next section. It is precisely this that means most whisky does not reek of sulphur.

The role of copper

A key tool in the fight against sulphur is copper, from which stills are traditionally built. During distillation copper reacts with the volatile sulphur compounds present in the alcohol vapours, binding them and forming harmless compounds that are then easily removed. The more contact of the spirit with copper, the cleaner, less sulphury the spirit. That is why the shape and size of the still and the length of contact of the vapours with copper have such a large influence on the character of the whisky. Distilleries that want a light, clean spirit ensure maximum contact with copper; those that prefer a heavier, more meaty style limit it. Copper is thus not only a construction material, but an active participant in the process, a natural filter of sulphur. Understanding its role explains why stills around the world are still made of copper, despite its cost, rather than of cheaper metals.

Soap and fatty notes

Another off-flavour is soapy and fatty notes. Whisky can take on a character resembling soap, tallow or fat, which can be perceived as a fault. These notes usually relate to particular fractions of the spirit, rich in fatty acids and heavier compounds, especially in the later part of distillation. Soapy, fatty characters have been described in the raw fractions of the spirit as one of the less desirable aspects. Controlling these notes depends on skilful conduct of the distillation and the right moment of the cut. The soapy note is not as common as sulphur, but when it appears, it can clearly lower the quality of the drink, giving an impression of impurity and heaviness. It is another example of how subtle decisions at the distillation stage translate into the final flavour of whisky in the glass. A good distiller knows how to avoid them, cutting the spirit at the right moment.

Plastic and sweaty notes

The characteristic notes of plastic, sweat and other rather unpleasant-sounding aromas relate to the so-called feints, that is the later part of distillation. It is precisely then that heavier, more volatile compounds enter the spirit, giving notes of plastic, sweat, leather, tobacco or honey. Some of them, like leather or tobacco, are desirable in small amounts, but plastic and sweaty characters are rather faults. Interestingly, many of these notes mellow during years of maturation in the cask, which fortunately applies especially to the least pleasant, sweaty-plastic ones. This shows how important the moment of cutting the feints is, and how time can repair some of the imperfections of a young spirit. A plastic note in a young whisky does not therefore always mean a permanent fault, although in excess it remains a problem even after years of maturing.

The distillation cut

Many off-flavours are decided in one key moment: during the cut of the spirit. Distillation gives three fractions: the foreshots, that is the first part, the heart, that is the middle, cleanest part, and the feints, that is the last part. The foreshots and feints contain more unwanted compounds and off-flavours, which is why they are usually cut off and recycled, keeping only the heart. It is precisely the decision of where exactly to cut, separating the heart from the feints, that decides how many foreign notes, like plastic, soap or sweatiness, end up in the final drink. Cutting too late into the feints will introduce these faults, too early or too wide will in turn give rawness. It is one of the most important decisions of the distiller, requiring experience and feel. We write more about this in our post on the distillation cut.

Compounds from fermentation

Not all off-flavours are born only in the still, some arise already during fermentation. Yeast, turning sugars into alcohol, produces along the way a whole range of by-products. They include higher alcohols, called fusel, organic acids and the esters formed from their combination, as well as ketones, sulphur compounds and phenols. Many of them are desirable and build the complexity and fruitiness of whisky, especially esters with pleasant, fruity notes. Others, in excess, can become off-flavours. Which compounds and in what proportions form depends on the type of yeast, the length and temperature of fermentation. That is why fermentation, often underrated, has an enormous influence on the final profile of whisky, including the presence or absence of foreign notes. It is the first stage at which the aromatic character of the future drink is shaped, long before the cask.

When a fault, and when a feature

Here we come to the most interesting paradox: the line between an off-flavour and a desired feature can be fluid and depends on context. The same note of sulphur, which in one whisky is a fault, in another gives prized meatiness. Smoky, phenolic notes, which in beer would be a fault, in peated whisky from Islay are the essence of the style and the reason we love it. Leathery or tobacco accents from the feints, undesirable in excess, in small amounts add character. Much depends on the concentration, balance and expectations of a given style. What for one person is a defect, for another is a hallmark and an asset. That is why assessing off-flavours is not black and white. The key is the question not only of whether a given note is present, but whether it fits the style, is under control and does not dominate, spoiling the balance of the whole.

How cask and time soften faults

The good news for distilleries is that many off-flavours mellow or disappear during maturation in the cask. Time and contact with wood act like a corrective treatment:

The table shows that off-flavours have specific sources and specific ways of softening. Copper removes sulphur already during distillation, the right cut sifts out the worst fractions, and long maturing in oak smooths the rest. That is why whisky needs time: the cask not only adds flavour, but also repairs the imperfections of a young spirit.

How to recognise an off-flavour

Recognising off-flavours is a skill that can be trained. Smell the whisky attentively and ask: does any note seem foreign, unpleasant, out of place? You will recognise sulphur by the smell of rotten egg, matches or burnt rubber. Soap by a fatty, soapy note. Plastic by a characteristic, artificial, chemical accent. Remember context, however: before you deem something a fault, consider whether it is not a feature of the style, like smoke in peated whisky. It helps to compare different whiskies side by side, because contrast highlights unusual notes. Note your observations, to build memory and feel. Over time you will learn to distinguish real faults from intended characters. It is a skill that deepens tasting and lets you consciously assess the quality of the drink, rather than just drink it. We write more about clean spirit in our post on new make spirit.

The key points in a nutshell

Off-flavours are foreign, unwanted notes in whisky, such as sulphur, soap or plastic, arising at various stages of production. Sulphur, giving a smell of egg or rubber, is born in fermentation and distillation, and is removed mainly by the copper of the still. Soapy and plastic notes come from particular fractions of distillation, especially the feints, which is why the right cut of the spirit to the heart is so important. Fermentation too creates compounds that in excess become faults. Context is key: the same note can be a fault or a feature of the style, like smoke in peated whisky. Time in the cask softens many off-flavours. Want to learn to recognise these notes and record your impressions? Keep notes in the GustoNote app. See also our posts on the distillation cut and new make spirit.