What is distillation?
This first section is a basic guide to distillation, what it is and how it fit in in reference to whisky and other spirits.
Over the years I've been doing tastings and training and have always found people at different levels of interest and knowledge. Hopefully you'll find the information below engaging, fun and informative. I've included references at the bottom of this section and have attempted to provide correct and current information.
So, I think it's important to start with the basic definition and we can work forward from there.
The Collins Dictionary definition of distillation is:
1. the process of evaporating a liquid and condensing its vapour,
2. a concentrated essence, distillate,
This principle applies whisky, cognac, rum, gin, vodka, etc... the list goes on!
Now that you know what distillationis. how does it translate to Scotch whisky?
To keep it simple I won't go into the technical process (yet), but will point out that the liquid being evaporated to make whisky is beer, that's right, good old beer!
The beer does have some differences from your common garden ale. To start with it doesn't contain hops, and is made using barley. Before you wonder what it would be like with hops Admans Brewery do make an oak aged english spirit using Broadside (their top selling beer), which has an interesting taste. This un-hopped beer is called wash.
Alcohol in the wash is separated from water and waste material during the evaporation process.
Malt whisky is distilled twice in copper pot stills, the dimensions of which are unique to every distillery in Scotland. In a handful of distilleries the spirit is distilled a third time to give a higher strength but lighter spirit. For example, Auchentoshan distillery uses a triple distillation and Benrinnes, Mortlach and Springbank distilleries have a near-triple distillation.
Pot stills are large copper kettles which narrow at the neck and then curve, this is called the ‘lyne arm’ or lyne pipe’. They then enter the condenser which is often located in the open air outside the stillhouse. Traditionally the condenser was a coiled pipe of decreasing diameter immersed in a tub of cold water, known as a ‘worm’ condenser. Worms are now quite rare (they are still used in Talisker, Springbank and Edradour distilleries), far more common are ‘shell and tube’ condensers where the shell contains the vapour and the tubes within it contain the cooling water.
Pot stills work in pairs, the first distillation takes place in a wash still and the second distillation takes place in a smaller low wines or spirit still.
Wash from the fermentation is pumped into an intermediate vessel known as a wash charger before being pumped into the wash still to around two-thirds of its capacity.
Heat is applied to the wash from within by steam heated coils, or occasionally from below by solid fuel or gas. Distilleries which have directly fired stills, eg Macallan, have ‘rummagers’ fitted inside the stills. These are revolving arms which drag heavy copper chain-mail around the base of the still. Rummagers are fitted in order to prevent solid particles in the wash sticking to the bottom of the still and scorching.
The temperature is applied gently at first and then taken up to just below boiling point. This separates the alcohol from the water and waste material. The alcohol, and other compounds which are more volatile, vaporize first and pass up the still over the swan neck where they are condensed in the worm or condenser pipe.
As wash approaches boiling point it has a tendency to froth, this froth can rise up into the worm or condenser and pass into the spirit safe leading to undesirable characteristics in the low wines. A small window or sightglass located about two thirds of the way up the wash still allows the stillman to control this. Frothing is more likely to occur in a still which has been overcharged or with wash which has undergone a short fermentation.
When the wash still stops producing alcoholic vapours the distillation is stopped. The residual water, yeast and other material is known as ‘pot ale’ and is mixed with draff, the residue from the mash, to make pot ale syrup and draff pellets which are used as cattle feed.
The distillate from the wash is known as ‘low wines’ and is around 21% alcohol. As well as ethyl alcohol wash contains a large number of less pure alcohols and oils. These are mainly esters, aldehydes, furforal and other compounds of hydrogen, oxygen and carbon formed in the process of distilling the wash. Hundreds of these organic chemicals have been identified in malt whisky and chemists know that there are hundreds more present which have yet to be isolated and described. They are known as ‘congeners’ or ‘congenerics’. Although they are impure they give malt whisky its character and flavour.Low wines flow through the spirit safe and are collected in the low wines receiver. In the spirit safe samples of low wines are siphoned off and their temperature and specific gravity are measured by thermometers and hydrometers to find the strength of the alcohol.
The low wines from the first distillation are then pumped into the spirit still to be distilled a second time along with the foreshots and feints from the previous distillation. The second distillation is done in the same way as the first but to a slower timescale and with careful temperature control.
The second distillation is closely watched and carefully controlled by the stillman who will divide the distillate into three fractions, again by measuring the temperature and specific gravity of the distillate in the sample glasses. At no time will the stillman come into physical contact with the distillate - testing in the safe is done under lock and key, the lock is only removed under supervised conditions to allow cleaning of the safe. The separation of the 3 fractions is done by cut off points which are peculiar to the distillery in which they are used. During distillation regular checks are kept on the strength of the distillate until it approaches the cut-off point.
The early part of the distillation is known as ‘foreshots’ and contains the most volatile compounds. The strength of the foreshots varies from charge to charge but will be around the range of 70-75% alcohol. Foreshots are used to clean out the heavy oils clinging to the surfaces from the previous distillation, they will run for anything between 5 and 45 minutes depending on the distillery. Foreshots are very high in esters which are highly desirable in malt whisky, but also contain impurities which have to be distilled off before spirit can be collected.
The middle cut is known as ‘spirit’, this is the fraction which will go on to be filled into oak casks for maturation. The flow of the middle cut is maintained at a slow pace to ensure maximum reflux. The spirit cut may be stopped and feints collected as low as 57% alcohol depending on the distillery.
The last part of the distillation is known as ‘feints’ and can be distilled at a faster rate than either foreshots or spirit. Feints contain various oily compounds which would harm the aroma and flavour of the whisky.
The spirit is collected in the intermediate spirit receiver from which it will be pumped into the actual spirit receiver and filled into casks. Foreshots and feints are collected together in the feints receiver from where they are pumped back to the spirit still to be distilled with the next batch.
The watery residue left in the spirit still is known as spent lees and is run to waste. The still is then recharged for the distillation of the next batch.
Effect on flavour
Distillation is an important factor in determining the individual character of a malt whisky and leads to a huge variety of flavour components in the spirit produced. No single factor in distillation is paramount in affecting flavour and quality - flavour comes from a combination of many factors.
Every distillery in Scotland has its own unique type of still which it is most careful to retain throughout repairs and new fittings to the still. Some distilleries are so diligent in doing so that when they replace part of a still they may even recreate the same dent as was present in the previous part.
The reason distilleries are so careful to maintain the dimensions of their stills is because the shape and size of the still has a marked influence on the character of the individual malt whisky. For example, a tall still tends to produce a lighter spirit as it only allows the lighter, more volatile vapours to be collected while a short still tends to lead to a richer, creamier, oilier spirit as heavier vapours fall back as reflux and are distilled again. Boil balls, spherical chambers between the body of the still and the head perform a similar function.
Copper has traditionally been used to make stills as it is a good conductor of heat, does not react with alcohol and is malleable with a good resistance to wear. However, it has only recently been discovered that copper also has an influence on flavour and plays an important role in the quality of the spirit.
In general, low copper contact during spirit still distillation will lead to a heavier spirit character and high copper contact will lead to lighter spirit character.
The copper patina which forms on the copper surfaces exposed to spirit vapours plays an important role in the fermentation of congeners, especially the esterification of organic acids by alcohols. The length of time the charge stays in the still and the interplay of the various volatiles exposed to the copper affects the formation of the different reaction compounds which eventually come off the still. Interaction with the copper reduces and removes volatile compounds of sulphur which are derived from the yeast, particularly in the wash still. Reaction with copper is, therefore, essential for producing a clean, light spirit.
Copper contact and copper levels in spirit, can be affected by a number of factors. A still with a large charge will have less headspace and less reflux resulting in lower copper contact. A slow speed of distillation will lead to higher levels of copper in the spirit and to the production of a lighter character spirit. Also, long rest times between distillations leads to initially high copper levels in spirit with resulting lighter characters when distillation is restarted.
Spirit from worms tends to give a heavier character than spirit from condensers. Distilleries with condensers rather than worms tend to give higher copper content to the spirit.
The separation of the 3 fractions of the spirit distillation is of great importance in determining the flavour of the mature whisky. The cut points are peculiar to the distillery in which they are used and are consistently applied.
During pot still distillation the product changes continually as the batch process goes on, therefore, the characteristics which are present at the time of the cut will have a great effect on the flavour of the mature whisky. Apart from the peaty/smoky qualities which may be present, there are two distinct aromas which distil in the spirit cut. These are termed estery and feinty characteristics. The way in which they appear in the distillate and their intensities are shown in the attached diagram.
In the early period of pot still distillation the product starts off as strongly estery, indeed the major flavour constituents of whisky are esters. The most significant esters in terms of aroma are isoamyl acetate which imparts banana-like or pear drop characteristics and ethyl caprylate which has a fruity, almost citrus aroma. There are at least a hundred individual esters in whisky. Descriptive terms often used to describe estery characteristics include solvent, fruity, fragrant, pear-drops, banana-like, rose-like, and nail varnish remover.
During the initial period of distillation, the esteriness changes very little and the curve diagram may be taken as representing just one quality which decreases gradually as time goes on.
Estery character is in short supply in most Scotch malt distillates. Generally it is the Speyside malt whiskies which have the most dominant estery aromas and is the reason why they form the basis for most blends.
On the other hand, the term ‘feinty’ represents a whole family of aromas. As a result of mashing of cereals, their subsequent fermentation and boiling up during distillation, a wide range of chemical compounds are formed. Prominent among these are organo-nitrogen compounds which have aromas which may be described as ‘cooked’, ‘hot cereals’, ‘bread-like’ and ‘toasted’. These clearly acceptable aromas are an integral part of whisky flavour but must be kept under control.
Another group of chemical compounds in feints are the organo-sulphur compounds. These present a very wide range of aromas and their odour intensity can be very strong (for example, dimethyl trisulphide may be detected down to a few parts per trillion). Descriptive terms for organo-sulphur compounds include ‘sulphury’, ‘rubbery’ and egg-like.
Following the ‘feinty’ curve on the diagram, the feinty qualities tend to begin with biscuit-like aromas (formed out of particular combinations of nitrogen, sulphur compounds in certain quantities, together with other compounds such as organic acids, esters, fusel alcohols and so on). In new distillate there are about 300 or more chemical compounds which affect flavour to varying extents.
As time goes on the digestive biscuit-like aroma gives way to more cereal-like, porridge aromas and this changes to tobacco-like in due course. The changes are gradual and subtle. Later, a characteristic which is a distinct quality contributor to distillate takes over, this has clear honey-like qualities. In due course the organic acid ‘sweaty’ aromas take over (honey can possess a sweaty characteristic) and these change to fishy even ‘amine’-like characteristics. It is the unacceptable elements of late feinty character which obliges distillers to stop collecting spirit, even though the alcoholic strength of the distillate is still around 60-65% alcohol.
In new make malt whisky the balance of estery to feinty character should be about equal. Thus, on the diagram the two curves form a symmetrical ‘X’ during the spirit cut. If it is imbalanced in favour of feinty character, the whisky will be lacking in desirable quality (estery) and will require longer to develop during maturation. If the spirit cut has included too much of the ‘sweaty’, ‘fishy’ characters the product may not mature to a balanced one (one which is recognisably whisky and without unacceptable qualities). In such a case the distillate should be rejected for use as a blending malt at the time of new make spirit assessment.