Column Stills

 

These days when people talk of technology it tends to be a very narrow view centred on bits & bytes, the so-called information age.  Technology is merely knowledge on how to do something, and for the love of our spirits we can all give thanks to a significant breakthrough in distillation technology that came about in the early 1800s, principles of which are still used today.

 

It is interesting to note that malt whiskies that are all the rage these days is not a new concept and is actually the original category of whisky.  Blended whiskies however are a relatively recent innovation from the mid-1800s which became the universal class of whisky up till a couple of decades ago.  Unfortunately, it has now been unfairly relegated as an inferior grade of whisky over malts.  The re-commercialization of malt whiskies began in the mid-1950s, building up in popularity to what we see in the market today.

 

To understand the link between malt and blended whiskies, we need to understand the ingredients that make up whisky.  Whole grains such as barley, corn, rye or wheat are the raw materials used in whisky production.  Dry raw grain is composed of stable complex sugar which is not readily fermentable.   Hence, a process is needed to break down these complex sugars into simple sugars that can be fermented to produce alcohol.  This is achieved by the action of enzymes which are produced during malting.  Technically, malt is any germinated grain but in the whisky industry it only refers to germinated barley.

 

Grain whisky is made from the cheaper ingredients of unmalted barley, corn, rye or wheat with added malted barley for its enzymes.  Blended whiskies are simply the vatting of completed malt and grain whiskies from any number of malt or grain distilleries.

 

The transition from the predominant consumption of malt whiskies to blended whiskies was partly spurred on by the acute shortage of grape based alcoholic beverages such as brandy, grappa and wine. This was due to the phylloxera outbreak that hit Europe starting in the 1860s and became a watershed for the whisky world as demand for whisky as a substitute beverage exploded and ways were sought to meet this demand.  The whisky industry was helped both by the new Spirits Act of 1860 that allowed blending malt and grain whiskies and by the adoption of more efficient distillation technology.

 

A few decades before this calamity, Scotsman Robert Steins invented a vertical distillation process that looked nothing like the ubiquitous pot still that was until then used for all forms of distillation.  The pot still system worked in batches of alcoholic mash heated in a bulbous pot receptacle with its vapor funneled through a system of cooling coils condensing into a spirit which now contained a higher concentration of alcohol.  Steins’ vertical column still was refined by Irish Excise Man Aeneas Coffey who designed a two-column still that was better at conserving heat and thus more efficient.

 

This revolutionary system combined with the use of a wider range of ingredients was able to efficiently produce the new and cheaper category of whisky - grain whisky.  This whisky could be produced at very high alcoholic strengths but when reduced to about 40% was rather light and short on many of the robust flavours that malt whiskies offered.  The solution was to beef up the grain whiskies with the more powerful and rich tasting malt whiskies giving birth to blended whiskies.

 

Distillation was once the purview of alchemist and witches, the scientists of those days.  Knowledge of Physics and Chemistry are essential at designing an efficient system of distillation, nevertheless the science that goes into pot still distillation is relatively rudimentary.  After the mash is watered or sparged and then fermented to become alcoholic wash or beer, it becomes the charge that is poured in pot stills for distillation.  When the charge is heated in these pot stills alcohol which has a lower boiling point than water evaporates earlier and more vigorously; hence the vapour that is collected on condensation will have a higher concentration of alcohol than the original charge from which it came from.  In a double pot still distillation, the distillate from the first distillation becomes the charge in a second pot still.  As in the first distillation, the resulting distillate from the second pot still will contain yet a higher concentration of alcohol than its charge.

 

The principle used in column still distillation is far more complexed than that of pot stills.  It depends on the physics of latent heat and herein lies the brilliance of this technology.  Latent heat is the energy transferred when there is a change of state from gas to liquid and vice-versa.  When vapour condenses to a liquid, heat is given off.

 

Column Still distillation is a two stage process usually using two columns, the first stage being the Analyser and the second, the Rectifier.  The first stage is to strip the alcohol from the wash by the use of steam pumped in from the bottom of the first column as the wash is being poured in from the top of the still.  Perforated plates are fitted throughout the length of the Analyser to momentarily contain the wash as it trickles down the column.  This downward flow of wash is met by rising steam which heats the wash causing lighter elements in the wash to evaporate, essentially stripping it of alcohol.

 

The steam now mixed with alcohol vapours is then channeled to the bottom of the second still called the Rectifier.  As the steam rises in the Rectifier it condenses to form water and alcohol on shallow trays which are fitted throughout the height of the still.   This liquid is then heated by the release of latent heat from more condensing alcoholic steam coming from below.  Alcohol being more volatile evaporates more readily than water, hence the new mix of alcoholic steam leaving this tray will have a higher concentration of alcohol than that of the steam that condensed into it.  This newly created steam then condenses on the tray above, again releasing its latent heat and evaporating yet an stronger alcoholic mix than the trays below it and this process is repeated as we go up the still.

 

You might want to think of each tray as being a mini pot still, each being heated by the latent heat of condensation from the vapour released by the tray below and subsequently evaporating a higher concentration of alcoholic steam which condenses into the “pot still” above it and heating it in the process.

 

With the process of condensation and evaporation repeated upwards over as many as 70 trays in the rectifier, very high alcoholic concentrations can be achieved.  However, under standard conditions it is not possible to reach alcoholic strengths of 100% ABV as there is a molecular affinity between water and alcohol which sets an azeotropic level of 97.5% ABV.

 

Differences between Column Stills and Pot Stills

 

Pot stills is a batch distillation method, where the charge - or wash in the case of whisky production - is fed into the still for each distillation.  The residue or stillage has to be removed after each distillation so that the pot still can receive the next charge.  In column still distillation, a continuous flow of wash is piped in at the top of the Analyzer with the residue or spent wash flushed out continuously from the bottom of the still.  The wash put into pot stills is free from solids whilst the wash fed into column stills contains solids from the mash.

 

Column stills are industrial in nature and very efficient at producing large quantities of high ABV grade alcohol and they can operate continuously without shutting down for weeks.  Wash is fed down the top of the analyser and after being stripped and devoid of alcohol by steam, the residue or stillage which falls to the bottom of the column is then extracted.  For maximum efficiency, this process goes on uninterrupted and held at a “steady state” maintaining the same temperature, steam used, wash flow in and stillage extracted for the duration of the distillation process.  In terms of whisky production, it isn’t meaningful to compare the output of pure alcohol between column and pot stills.

 

Commercially, a distillate with a higher concentration of alcohol translates into larger quantities of the final bottled product, as the distillate is usually reduced or diluted to around 40% ABV - the standard bottling strength of most spirits.  There is however a tradeoff between achieving a high level of alcohol during distillation and the flavour intensity of the distillate.  That part of the distillate that is not alcohol is flavoured water which captures the characteristics of the raw materials used.  All things equal, a distillate that is lower in alcohol is more flavorsome.  The next consideration then is the quality of that flavor, though that is a different discussion.

 

Column stills are able to produce new make with very high levels of alcohol and these are referred to as neutral spirit since congener and other flavonoids have been stripped in the purification process.  European Community Regulation 110-2008 stipulates a minimum ABV of 96% for distillates to be considered neutral spirits.   Essentially, they need to be flavourless with little indication as to which raw materials were used in the ferment.  On the other hand when distillers want to capture the essence of the base ingredients and give the spirit its characteristic taste, the new make is distillated to a much lower % ABV.  For this reason, Bourbon by law must be distilled to less than 80% ABV giving this whisky class its distinctive personality.

 

Under the Scotch Whisky Association’s Regulations, Scotch Malt whisky can only be distilled in copper pot stills.  In Scotland, column stills are used for the production of grain whisky as well as other spirits such as vodka and gin.  In countries such as the USA, there is no requirement for any class of whisky to be distilled in pots stills nor do they need to be made of copper.

 

Sulphur which is found natural in the raw material can leave an odor as well as a sour taste in the finished product.  Copper has been found to be reactive with sulphur to form Copper Sulfate that finds its way into the stillage which is then removed.  For this reason many stainless steel column stills have combined elements of copper within these stills.

 

Increasingly, more distillers are experimenting with Lomond Stills, which is a hybrid of both the pot and column still.  These stills are batch produced and have a pot base but have incorporated a rectifier similar to column still systems on top of the pot still.  This gives the distillers the ability to create different styles of new make by adjusting the number of tray in the column.

 

In Pot Still operations which could be up to a three distillation process, there is significant emphasis and precision on the cut points that make up the heads, hearts (new make) and tails. In Column Still operations the new make constitutes the bulk of the output from the rectifier with a smaller component of unusable sulphured heads vented out as is the proportion of tails that is fed back into the analyser for re-distillation.

 

By

Lewis Mitchell

Cask & Dram Magazine

24 July 2016