Alcohol distillation involves three key steps: fermentation, distillation and finishing. With these steps, alcohol is created, concentrated and made palatable to human consumption. While there is of course a huge quantity of complex chemical and technological operations involved in the overall process, many are only the concern of a master distiller and are, for the most part, irrelevant to gaining a working understanding of distillation. Here we are going to cover the basic processes.

Fermentation

As distillation is effectively a means of concentrating a component of a liquid, to make an alcoholic spirit, you first need alcohol. How do you produce alcohol? Simple:

Sugar + Yeast = Alcohol & CO2

Alcohol can be made from pretty much any ingredient in which there is either sugar or starch. Different ingredients require different treatments before fermentation can get underway, depending on whether you have simple sugars present, or whether you need to convert starches into sugar.

“Simple” Sugars

If the chosen ingredient has lots of ready-to-ferment sugars, then the yeast can get to action straight away. Each will contain a different combination of fermentable sugars, which are glucose, fructose, sucrose, maltose, and maltotriose, which dictate whether something is 100% fermentable or only 60% fermentable. When considering what to ferment, the brewer or distiller needs to consider not only what kind of sugars are present in the ingredient, but the concentration of sugars as well. For instance, honey is composed of almost exclusively sugars that are 100% fermentable, whereas grapes are roughly 15% sugar, all of which is fermentable. Common ingredients used in the production of spirits that have readily available sugars are:

• Sugar Cane
  • Sugar Cane can be manipulated in a number of different ways to produce primarily three variations that will result in hugely different spirits:
    • Sugar Cane Juice: the purest form, where sugar cane is run through two rollers that extract the pure sugar liquid.
    • Sugar Cane Honey: when sugar cane juice is harvested, it can be cooked up in various ways to produce a syrup of varying viscosities and flavour profiles.
    • Molasses: a byproduct of the sugar refining process, molasses are very thick, dark and treacle-like.
• Fruit
  • The most accessible and easy to ferment, fruit requires pressing to extract the sweet liquid prior to fermentation. Of course, depending on the type of fruit and the concentration of sugar therein, you may need significant quantities to produce a viable liquid for fermentation. Common fruits used in distillation range from grapes, orchard fruits such as apples and pears, or even green tomatoes. There is potential for an almost infinite variety of flavours of spirits produced from fruits.
• Honey
  • An immensely valuable liquid, honey is a fantastic source of fermentable sugars, from which the Mead is the most common alcoholic product. Few 100% honey spirits are produced owing to the cost of base ingredients.

With industrial society, many processed foods contain fermentable sugars which, when steeping in water, can provide a novelty base for fermentation.

Ingredients Requiring Conversion

The vast majority of spirits are produced from ingredients that contain large quantities of starch which, in order to be fermentable, require conversion into sugars. Starch is a long chain molecule comprising short chain sugars strung together. The enzyme amylase breaks down starch into its constituent sugar chains, which can then be fermented. Funnily enough, amylase is naturally occurring in certain grains, and is produced during germination. All that is required, therefore, is to trick the grains into starting the germination process, so anylase can be produced and those sugars unlocked. This process is known as “malting” and is a necessary when using barley and wheat, along with other grains.

The first step of malting is steeping the grain in water to ensure it is nice and damp, before leaving at a cool temperature. This tricks the grain into germination, triggering the production of enzymes. However, if the germination cycle was allowed to run until completion, the grain’s supply of enzymes would be exhausted, so it is stopped by drying out the barley with hot air. Although this in itself does not affect the flavour of the grain, the hot air can be flavoured in a way—most typically with peat, which contain phenols that coat and permeate the grain resulting in a smoky, savoury taste. The addition of peat can also come after the grain has already been dried out, wherein the peat flavours coat the grain as above.

Once the grain contains ample levels of the required enzymes, it is time to mill it into a coarse flour commonly called “grist”. Now, the grist is mixed with hot water (at around 70C) to make a watery porridge-like mix and left for around an hour. This process is called “mashing” and the grain’s ability to convert starch to sugar is called its “diastatic power”. During this time, the enzymes get to work on the starch, breaking it down into a sweet liquid called “wort.” Once the liquid has been separated from the grist, fermentation is ready to begin.

Barley and wheat are the main sources of the enzymes required to breakdown starch, so when choosing starch-rich ingredient in which these enzymes aren’t naturally occurring, a small percentage of malted barley or malted wheat will be added to initiate this process during mashing.Common ingredients that require conversion include:

• Grain
  • Cereals are the most important base ingredient for high quality spirits, which each impart their specific characteristics on the final products to greater or lesser extents. The most significant grains are:
    • Barley: the gold standard of grain, found at the heart of the best spirit in the world, Scotch single malt whisky. Distiller prefer a strain of barley that is easy to mash, with high diastatic power and low nitrogen, meaning two-row barley is preferred. Although many distillers do not think the type of barley used affects the final spirit, many are turning toward ancient and historic strains, and have been experimenting with single-field barleys to communicate a sense of terroir.
    • Wheat: a key component in bourbon, wheat is also commonly used to produce vodkas with a creamy texture. Although hardly used as 100% of the mash bill outside of vodka, it is used in combination with barley, corn and rye to produce complex spirits.
    • Rye: relatively temperamental to grow, rye has historically been a very important grain in Polish and Russian vodkas, where it imparts a complex cereal flavour and noticeable spice. Rye whisk(e)y has often been regarded as capable of producing high quality, characterful spirits that, at their very best, are on par with the best single malts Scotch. Many new distilleries are turning their attention to this great grain, with a few in the UK now experimenting.
    • Corn: while corn does not require the same conversion process as other grains, it does require some enzymatic activity to release the highly fermentable sugars within. A lush, rich spirit, it is key in bourbon.
• Rice
  • An interesting little pocket of starch, rice grains also require conversion but are not malted the same way as barley or wheat. Rather, mold takes the place of amylase as the motor of conversion. “Koji” as it is known, is responsible for fermenting rice, along with soya beans in the production of miso paste. Spores of koji are scattered over steamed rice to break down the starches, after which it is mixed with water and yeast in order to kick off fermentation.
• Vegetables
  • Vegetables are important source of both sugar and starch in varying proportions. As they do not possess the necessary enzymes, malted grain is required to unlock the latent sugars in the starch. Certain vegetables have a long, important history in distillation, most notably potatoes and beets. Beets in particular have a critical place in spirit production. They are easy to grow, high yielding and not particularly prone to disease. With a good mixture of ready-to-access sugars and starch, they do not require as much energy as other starch-heavy vegetables. Potatoes, on the other hand, require more energy but are very cheap to grow, and have been historically utilised to produce a neutral base spirit that can be flavoured with other ingredients.

Once you have produced a wort—either with “simple” sugars or with ingredients that require conversion—it is time to ferment. In the case of mashing, the cloudiness of the wort impacts the final flavour quite significantly, with more cereal character coming from cloudy worts. There are many options that impact fermentation, but two significant ones are:

• Yeast:
  • Yeasts can contribute a huge amount of character to the final spirit, or they can contribute nothing apart from their metabolic activity turning sugar into alcohol and CO2. Depending on what the distiller wants, any number of yeasts can be selected. These can be generalised in two ways:
    • Wild Yeast: can contribute a huge amount of character but come at the price of unpredictability. Wild yeasts are utilised by leaving the wort open to the elements and allowing the yeast present in the distillery environment to do their work.
    • Commercial Yeast: produced in a lab, usually harvested from a reliable yeast found in any number of commercially successful distilleries. Certain strains are bread for particular qualities, such as ability to produce certain groups of esters, efficiency in fermenting all sugars, or working at a specific temperature range. They can be made to exacting specification, therefore allowing the distillery to be in almost complete control of the fermentation process.
• Vessels:
  • The main choice with vessels is: open or closed. By fermenting in a closed vessel (with vents for the CO2 produced) then risk of any undesirable bacteria or wild yeasts entering the fermentation are diminished. Open top fermenters, on the other hand, allow wild yeasts to get in and get busy.
  • Other important considerations are the shape and material from which the fermentor is made. Certain materials can be more or less porous and retain to varying degrees yeasts and other flavour compounds from previous fermentations. Wood, stainless steel, concrete or even clay amphora (although not too common in pre-distillation fermentation) all produce difference in character.

Once yeast and vessel have been chosen, the temperature at which fermentation occurs dictates to an extent how long fermentation lasts. The ideal temperature depends on the yeast strain, with many operating in the mid to late twenties. Fermentation can take anywhere between 40 to 120 hours—or even longer—depending on how much cereal and autolytic flavour the distiller wants in the final spirit. Once fermentation is complete, you are left with a low alcohol liquid of anywhere from 6 to 16% ABV. In the nomenclature of whisky, this is known as “wash” and is now ready to be put into the still.

The fundamental chemistry of distillation is that by heating up a low ABV liquid, the alcohol is separated from the water and other compounds as it has a lower boiling point, thereby concentrating the ABV in the final liquid. Depending on the type of still, wash and intended final product, this process can take place over successive distillations, each time concentrating the alcohol, or in one distillation comprising many micro-distillations. Rather than talk about the process in the abstract, we will look at the two primary types of stills and how their design effects the process of distillation.

Types of Still

There are two main types of stills: pot still and column still. They are pretty much as they sound: the former a bulbous pot into which the wash runs and the vaporised alcohol channelled out via a long slowly bending pipe at the top; the latter composed of two tall columns divided up internally by a series of perforated plates that allow the alcohol vapour to condense and re-evaporate. A helpful way of understanding how these two types of stills differ in their operation is that a pot still produces spirit in batches, whereas the column still system can be run constantly. In other words, pot still distillation runs discontinuously, and a column still runs continuously.

While the type of still will understandably make a huge difference to the type of spirit the distillery want to produce, there are many minor alterations to the design of the still that can affect the character of the spirit to a greater or lesser extent. As we will see in the anatomy of the pot still, the pitch, length and diameter of the lyne arm (the elegant pipe attached to the top of the main part of the still) influences whether the spirit is light and fruity or dense and rich.

Pot and column stills produce wildly different kinds of spirits, yet share the same fundamental components. Each require in inlet for pumping wash into the main distillation chamber, a condenser to cool the alcohol vapour thereby turning it into a liquid, and a chamber for collecting the liquid spirit. Aside from these shared anatomical features, there is a world of difference between the two.

Pot Still

The pot still is the classic, ancient still used primarily in the production of whisky and cognac – although there are many other spirits produced using this method. It is a discontinuous distillation, meaning that a spirit is produced in batches, rather than continuously, as is the case with a column still. It works by heating up the wash in a big copper pot; the vapours rise up and are channelled through the swan’s neck into a condenser that turns them back into a liquid. Although this seems kind of like boiling a kettle inside a balloon, it is a slow, carefully controlled process.

As alcohol has a lower boiling point to water, when the wash is gently heated up, the alcohol vapours are the first to rise up, leaving the water vapours behind. When these vapours hit the swan’s neck, some will pass through and some will fall back down – this is called “reflux” and contributed to the characteristic of the final spirit. The more reflux, the lighter the spirit will be. The height, bend of the swan’s neck and the angle of the arm leading to the condenser – called the “lyne arm” – all affect the final spirit. We’ll discuss this more when we talk about whisky.

After passing through the lyne arm, the spirit passes through a condenser – a spiral of pipes in which cool water is circulated– where it is returned to a liquid state. The spirit is finally collected in the collector. It will only be around 25-30% ABV at this point, so is distilled again. In the case of some Irish whiskeys and vodkas, this is distilled a further time. However, not all the distillate is used: the first and final parts of the distillation – the “heads” and “tails” – are discarded as they contain a higher portion of methane, which is toxic.

Column Still

A column still is effectively a number of pot stills arrange vertically over two columns. In other words, you take the process of heating and condensing – then re-distilling – the wash that takes place in a pot still and incorporate it into a still that allows it to happen numerous times within the same machine. Moreover, it is designed in such a way that you can continuously feed it with wash and continuously harvest the spirit. This is why we have so much Smirnoff vodka in the world!

There are two columns to this still: the analyser (left hand side) and the rectifier (right hand side). The wash is fed into the analyser, where it is heated from the bottom by steam. As the liquid is heated, it rises through a series of vertically arranged perforated plates; some passes through, while some condenses and is re-heated by the plates. This process enriches the vapour with more alcohol. This is happening at a higher temperature than in a pot still, so the vapour that passes through the analyser still is made up of both alcohol and water. It descends the lyne arm into column 2 – the rectifier – which is at a lower temperature. The process continues – alcohol vapour rises to the top, while water falls to the bottom to be drained off. After the spirit vapours reach the head of the second column, it runs through a condenser, and is collected.

The Coffey still – named after its inventor, Aeneas Coffey in the late 19th century – is the name given to the modern column still. As it recycles more alcohol vapours than the pot still, it produces a higher ABV spirit at the end – upwards of 90% ABV – and lighter character. This still is commonly used in the production of vodka, bourbon and Armagnac.

The freshly distilled spirit—called “new make”—is not quite ready to be bottled and sent to market just yet as there are a number of finishing touches that need to be made—some more substantial and time-consuming than others. We can break these down into a few simple categories:

Cutting

The most prevalent step for finishing off a spirit is reducing the ABV from still strength down to a more acceptable ABV depending on the style and characteristics the distiller wants to achieve with the spirit. With spirits such as vodka, gin and cognac, for example, the spirit will be cut down with purified water to around 40% ABV, for a range of reasons. Not only is a high ABV much more difficult to consume raw, it also incurs a significant duty charge: indeed, even at 40% ABV, the duty incurred per 700ml bottle in the UK is around £8, before VAT of 20%. Therefore, it is more sensible to release a product with a lower ABV when your target is to move significant volume of a spirit where the flavour is not altered by sitting at a lower ABV. A further reason is that certain flavour profiles—as in London Dry Gins—work better at around 40-46% ABV, while a juniper-heavy gin works better at a higher ABV such as 50%.

Of course, the economic argument for cutting the still strength alcohol with water is to stretch out the quantity of finished bottle per distillation, thereby reducing the overall cost to both the distillery and the consumer.

Many more specialist products will be bottled at a higher ABV either because their characteristics suit a strong influence of alcohol, or to minimise the processing that goes on. As concerns the latter point, cask strength whiskeys are an excellent example. The final point in this section will further expound on the maturation process, but a significant category of whisky loved by purists and connoisseurs alike is single cask or small batch spirits where the liquid is drawn straight from the cask into bottle with minimal filtration. The purported effect is that the whisky is more true to character—or natural—and therefore expresses the entire flavour profile developed while in cask. It is typical to find these spirits at around 50% ABV, although several can end up in the late 60s, depending on how much pure alcohol is lost to evaporation (known as “angel’s share).

Filtration

Filtration is an interesting topic as, when it comes to unnamed spirits like vodka, it is the norm and even highly desirable for the spirit to undergo quite an intense process of filtration. However, when it comes to whiskies or rums, significant filtration is frowned upon as it removes more of the flavour found in micro particle of sediment and colour. As a very general rule of thumb, it is mostly preferable for white spirits to be filtered, and undesirable for aged spirits to undergo intensive filtration. In the case of the latter, there will always be a filtration to remove large particle of sediment, while some mass-produced spirits will receive chill filtration which involves chilling down the spirit so that any impurities will coagulate and be skimmed off.

When it comes to white spirits, there are many types of filtration that range from being very functional to being luxurious and questionable. For instance, charcoal is a common medium through which to pass a white spirit as it removes a number of minute impurities that naturally accrue in the distillation process. Often, other natural mediums are used to filter, such as certain types of stones. Certain distilleries use elaborate materials to filter their products. Crystal Skull Head vodka, for example, use quartz crystals, known as Herkimer diamonds. The extent to which that makes any kind of perceptible difference is debatable.

Maturation

Alongside cutting, maturation is the most significant process involved in producing spirits. Barrels of varying sized are the most common vessel in which to age spirits, although other choose containers that impart their characters to a greater or lesser extent depending on the type of spirit being produced. Sometimes a producer will want to hold their spirit in a relatively inert vessel in order to let the product settle out post-distillation so that when it is bottled, it is a bit more balanced. Typically, though, maturation will take place over a much longer period of time. Depending on the type of spirit, it can be anywhere from two months (such as Joven tequila) to three years (the legal minimum for Scotch whisky) to decades. Once a spirit has been aged for a significant period of time, it will sometimes be transferred under very stringent conditions to an inert vessel where it can sit without undergoing any further significant maturation (such as old and ancient cognacs stored in glass demi-johns).

The main material used for maturing spirits is oak, much as it the common material for aging wine. American and European oak (however vague those designation may be) are the two most common types when it comes to whisk(e)y in particular. To generalise further, it is more common in the USA to use fresh barrels or “virgin” oak than it is in the Scotch whisky industry (although certain distilleries do use them) as the intense char and caramelised oak character better suits an intense, full bodied spirit such as Bourbon than it does a more subtle, gentle Scotch single malt. Indeed, barrels that have been used to mature Bourbon for a year or two find their way to different countries to impart are gentler, but still assertive, character on different spirits. Many Scotch whisky distilleries use ex-Bourbon casks as the first vessel for maturation as its soft, oaky sweetness adds a depth of flavours that compliments the cereal sweet of a Highland whisky, for example, or fills out an often-drying peat smokiness with Islays whiskies. Distilleries in Mexico use these same casks for resting tequilas.

The main options for finishing spirits are as follows:

1. Ex-bourbon. These casks are made from American oak that has previously held bourbon whiskey. They are highly charred, and have flavours of caramel, vanilla, pine and eucalyptus.

2. Ex-sherry. Made from European oak, the sherry imparts flavours of dried fruits, walnuts, smoked almonds, and clove. They typically have more mouth-drying tannins.

3. Refill. Casks can be used multiple times and will typically be less effective at imparting their own flavours the more often they are used. Refill casks allow the distillery character, rather than the character of the barrel, to shine through.

4. Cask finishing. Certain casks will be used to finish off a whisky by giving it a second cask maturation for a short period of time. These casks are typically intensely flavoured, so are used to add a bit of flair to the whisky. Wine, port and Madeira are common casks used for finishing.

While these are the main types and uses of casks, there are also a plethora of other materials and other vessels. Walnut, cherry and maple are sometimes used in whisky production, while giant vats constructed from old casks can be used to blend together differently aged whiskies. Stainless steel is of course ideal for storing spirits when no further flavouring or maturation is required, as are glass demi-johns. There have been trials with clay and concrete—even amphorae—although they are much less common.

Additions

It is sometimes required to add a little something else to the liquid before bottling. This can be anything from the addition of bison grass in vodka, to fruits, vegetables, flours and roots, or even insects (scorpions and grubs, for instance). However, it is much more common to find a flavourless caramel added to darken the colour of whiskies and rums. This is a contested practice that owes much to the consumer’s perception that a particular spirits should be a certain colour than it is for any technical reason—although it can be used to cover up defects in appearance. As a very general rule of thumb, this is a practice common in cheaper whiskies, but it surprisingly prevalent in rum production at nearly all price points. Even certain cognacs will darken their spirits to give the impression of richness and luxury. While it is not a practice to be derided tout court, the trend of natural products that have been touched as little as possible by industrial processes means that brands think twice before adding now.