The World of Distillation
Also known as ethanol (C2H6O, with a boiling point of 78.37 °C); it is the intoxicating ingredient. Produced during fermentation and concentrated during distillation, this is the percentage we are talking about when we measure the strength of an alcoholic drink.
Also known as methanol (CH4O, with a boiling point of 64.7 °C); the lightest of alcohols and more volatile than ethanol. It is poisonous in quantity and discarded to a great extent with the heads. Due to a lower boiling point it is easier to collect.
SMALL AMOUNTS OF OTHER ALCOHOLS
Fusels, organic compounds produced when carbon and hydrogen atoms are rearranged during fermentation and distillation; a source of isoamyl (C5 alcohol), isobutyl (C4 alcohol) and isopropanol (C3 alcohol) compounds which can have characteristic rank odours, varying degrees of volatility and the propensity to produce pungent esters when they oxidise.
Highly reactive organic compounds produced during the dehydrogenation of alcohols; which can give a pleasant character on the nose and develop more complexity through the ageing process.
Organic compounds that react with water to produce acids; they have pleasant odours giving fragrance and flavour.
Similar to, but with a higher boiling point than alcohols; Natural phenols oxidise easily and undergo autoxidation during the ageing process. Simple natural phenols are acidic in nature, initially are sharp and spicy on the nose, but become more complex as they oxidise.
All of these latter compounds, commonly known as congeners, provide the spirit with its identity, its nose and its palate. If one is aiming at a neutral distillation, there would be less than 1 % of higher alcohols, fusels and congeners in the resulting distillate. However, the amount of congeners in a spirit can vary dramatically, with 30–35mg/ltr the average for vodka and as many as 2,500mg/ltr in a very old Armagnac, much of which is as a result of the distillation strength.
When producing spirits for consumption, some of the vapours are undesirable (such as Methanol) and will be rejected; depending on the end product, the distiller will take great care to collect only the required elements from the condensed vapours to make spirit. Remove them all, however, and the remaining congeners may not hold together properly. Controlling the distillation, in order to separate these congeners, and enable decisions about what to keep and what not to keep, is not easy.
When you consider that all the congeners have different boiling points and as the vapours rise they are condensed and collected at different moments, you begin to appreciate the art involved.
Many cultures assert that theirs was the first to distil alcohol as a beverage, but it would appear that there is insufficient historical documentation to bestow the honour definitively on any one group. It was neither a Greek nor a Roman invention and there is absolutely no evidence of any distillation in France before the 14th century. The Moors and distillation are almost synonymous with many believing that the Moors discovered its secrets around 900AD. However, they used the resultant alcohol for antiseptic and medicinal purposes rather than for imbibing. The Moors legacy lives on in today’s nomenclature: al-kohl whence alcohol and al-ambiqs anglicised to alembic.
On the other hand, monasteries in Ireland (oases of wisdom during the stagnation of the Dark Ages) may also have a claim. Legend has it that King Henry II of England found spirits produced in Ireland, which would date their spirit production from the 12th century.
Monks in Salerno were also involved in distillation around this time although in all likelihood they learnt the art from the Moors. Another legend gives the honour to the Chinese with travellers bringing news of the methodology to Egypt, which subsequently spread as alchemy (derived from al-khem).
Distillate became known generically as the water of immortality because of the preservative effect it had on anything of an organic nature. This translated into many different languages and eventually evolved into names with which we are familiar today including uisge beatha from the Gaelic, whence whisky; eau de vie from French; aqua vitae from the Latin and interestingly elixir from Old Roman. In English the expression became spiritual water and subsequently what we now refer to as spirits.
What we can say with conviction is that treatises on distillation were openly printed by 1500 in Germany and in 1651 a great deal of the collected practise was published by English physician John French in his book ‘The Art of Distillation’.
Ingredients & Process
The most important factors in the distillation process are the ingredients, the size & shape of distilling apparatus and the resultant distillate’s alcoholic strength.
For our purpose, here, we will simplify matters and limit the ingredients to what we will cavalierly call beer (a wash or mash made from grains) or wine (a ferment made from fruit or vegetables). Both of these must firstly produce alcohol through the process of fermentation…a conversion of natural sugars, and those drawn from carbohydrates, to alcohol.
Under normal conditions this will achieve a maximum alcohol content of about 20% abv, at this point, the yeast reproduction cycle is inhibited by the levels of alcohols and fermentation ceases. For those of a technical persuasion…here we would say that ethanol fermentation is the breakdown of one glucose molecule into two pyruvates (the organic acid intersect in the aerobic respiration path); the two pyruvates are then broken down into two acetaldehydes; the two acetaldehydes convert to two ethanol…all very simple!
In order to extract the alcohol we can use the following methods of distillation:
THE POT STILL OR RETORT
Both alembics and retorts started as glass vessels with a neck pointing to the side at a downward angle which allowed the condensing vapour to drip for collection. Latterly, when we moved from capturing essential essences to larger scale production of potable spirits, the collection method had to grow and become more stable, thus the development of the copper alembic.
The copper did not add anything from a creative standpoint but it did lend a helping hand to proceedings, because it was available for shaping, was an excellent and even conductor of heat and did not corrode with acid contact. It was also found to help neutralise the acids in the ferment as well as any naturally occurring sulphurous elements in the vapours, unwanted by the distiller.
Early distillation used one vaporisation and one condensation, but it was found that purity was enhanced by further distillation of a selection of the condensate. In those days larger volumes simply meant more stills or repeated distillation. These days, this batch method is usually done at least twice to obtain spirit of the right quality, character and strength and we are able to manufacture stills of much greater size.
So, the alcoholic wash is placed in the pot and heated; the vapours rise up through the neck and are condensed and collected. The first distillation is essentially a light concentration resulting in alcohol between 20 –30 % abv depending on the original strength of the ferment and the size and shape of the still. These low wines are then distilled again and the distillate condenses in the following order:
- Head -
The first to appear are the most volatile such as methanol; these are discarded or mixed with either the ferment or the low wines ready to return to the pot with the next batch. However, the last of the heads can be light, elegant and delicate in character and some of them may be desirable.
- Heart -
Deciding when the heart has begun to reveal itself and when it has finished are vital. Collect too many heads and the result will be too volatile, start collecting the heart too late and the elegant, fruity, floral elements so desirable in the character of a quality spirit will be lost.
- Tail -
Gravity dictates that these are the last to appear as these are the heaviest compounds. They contain larger molecular combinations, some of which may actually be beneficial with rich, powerful aromas adding body to the distillate. On the whole, they are mostly undesirable, containing many fusels and other compounds that, if collected, would taint the final spirit.
In order to make a proper selection, an even heat is very important; too hot and all the vapours rush together in a blinding haze preventing any kind of discernment, too low and a dilute, somewhat bitter distillate will emerge. Thus the speed of distillation has an effect on both the character and the quality of the spirit; controlling it, gives the distiller the opportunity to separate the congeners and produce the type of spirit desired.
The size and shape of the still and its head are very important in deciding the character of the end product. If the pot is shaped more like a fat onion than an olive then the wash circulates violently inside like a tornado as it tries to escape whereas the olive type is a lot calmer resulting in vapours of different character rising more easily up to the neck. If the neck is longer and taller then only the lighter vapours containing the more elegant, delicate characteristics will be able to rise to the top and ultimately be condensed and collected. The heavier, richer elements will rise some of the way, but fall back to be re-vapourised. This continued process is called reflux and various adaptions including a small condenser are often used to increase the purity of the distillate.
Continuous or Column Distillation
A vertical cross-section of a continuous still would show linked columns, in the basic two column set they are known as an analyser and a rectifier and each is divided by perforated plates. The cool wash enters the still through a pipe, which snakes its way through the rectifier, playing a role in a heat exchange. This pipe empties the now warm wash into the top of the analyser, which then tumbles down through the plates. Meanwhile, steam is introduced at the bottom of the same column and when it and the ferment meet, the alcohol vaporises and this alcoholic steamy vapour rises to the top of the column. The wash, which remains in liquid form, continues its descent becoming progressively weaker in alcohols. The process continues until all the ferment has been distilled.
The steam and alcohol vapours rise up through the pipe, which leads them back to the bottom of the rectifying column where they continue to rise. Its journey takes it through the perforated plates in the rectifier and also into contact with the pipe taking the cool wash to the analyser; thus a heat exchange occurs whereby the vapours are cooled and the wash warmed. This is an efficient way of achieving two desired elements: warm wash to charge the analyser, and a start to the vapours condensing. As the vapours rise, the congeners begin to separate which continues until the vapours hit a point at which the spirit can be collected.
This point can actually be anywhere in the rectifier allowing various styles of spirit to be produced by the same still. Again, good control of the heat source, which in most cases is steam, enables a proper separation of all the elements and a collection of the desired style of spirit.
There are many variations of continuous stills and they are known by many different names. They may have many rectifying columns or just one, they may sit on top of pot stills (sometimes known as a hybrid or lomond) and they frequently allow for the heads and the tails to be recycled into the ferment or be drawn off.
In general, to produce lighter spirits such as vodka, light rum and neutral grain bases, the apparatus may be as a high as a four-storey building with possibly four or more rectifying columns and one analyser. A column with a high reflux ratio may have fewer stages, but it produces a large amount of liquid, giving a wide column that can hold up a large liquid volume. Conversely, a column with a low reflux ratio must have a large number of stages or plates, thus requiring a taller column. A very small column still with just one rectifier and perhaps as few as three perforated plates might be used in the Armagnac region to produce a heavier, richer, more powerful spirits with earthy notes. Vacuum columns can be used where the wash enters under pressure (such as with grappa or acquavite production) ; here less heat is required and a more aromatic distillate can be obtained.
Today complex computer programmes have been devised that allow simulation of distillation columns, the calculation of the collection or equilibrium points and the reflux ratios which means that the total operation of the process can be computerised. What is sure is that the alcohol concentration desired can be achieved by maximising the potential alcohol from the ferment at the lowest cost.