Distillation & Maturation
Distillation
Distillation involves separating water from the alcoholic part using heat. The wash obtained during the fermentation phase is boiled in large copper stills, called pot stills, or in more modern continuous stills. In both cases, the alcohol, which evaporates at a lower temperature than water, rises as vapor through the neck of the still and passes through a water-cooled condenser. The temperature difference causes the vapor to condense back into its liquid form. The use of copper in this phase is crucial because this material removes unwanted sulfur compounds and retains oils and fats that can alter the taste of the alcohol.
Continuous Distillation
Almost all bourbon, rye, Tennessee, and Canadian whiskies, as well as the grain spirit used for blending Scotch whiskies, are produced using the continuous distillation method. The most common type of continuous still is the Coffey still or "column" still (see the image of the Kilbeggan distillery from Wikipedia), patented in the 1830s by the Irishman Aeneas Coffey. Compared to malt production with pot stills, continuous distillation in Coffey stills is more akin to an industrial process: the stills are large, versatile, and efficient, and can operate continuously, whereas malt distillation in pot stills occurs only in "batches" and requires cleaning the equipment between production phases.
The continuous still consists of two large parallel stainless steel columns, connected to each other, called the rectifier and the analyzer. In the rectifier, the wash is heated by hot steam and is made to pass through a series of perforated plates (made of copper) that retain the heavier compounds. The lighter (volatile) compounds are vaporized and, with an alcohol content between 10% and 20% by volume, pass into the second column. The analyzer cools the vapor during its ascent and condenses it back into liquid form (distillate). In a Coffey still, it is possible to distill distillate up to nearly 95% alcohol by volume.
In America, the wash is referred to as "beer," and the first column of the continuous still is called the beer still. The second distillation takes place in a still known as a "doubler" or "thumper," which is not very different from a pot still.
In Ireland, both methods of distillation are used: the grain spirit, generally derived from corn, is distilled using continuous stills, while "pure pot still whiskey" is produced using a mixture of malted and unmalted barley distilled through a pot still.
Double Distillation (Discontinuous Distillation)
Discontinuous distillation is the traditional method for making whisky and is used by distilleries to produce single malt. This method also involves two copper stills (known as pot stills, see side) and the process begins when the wash is placed in the first still, the wash still, and brought to a boil. Controlling the boiling of the wash is crucial because the heated liquid creates a foam that rises upwards; the distiller must adjust the boiling temperature to prevent the foam from reaching the top of the wash still and consequently passing into the condenser.
The boiling process of the mash continues until the alcohol content of the liquid reaches about 20 or 21% ABV (alcohol by volume). After boiling and alcohol evaporation, the residue called pot ale is removed (pot ale is usually used as animal feed). The alcohol in vapor form passes through condensers to return to its liquid state. The condensation of the liquid occurs through tall cylindrical copper condensers with thin tubes through which cold water flows, and where the alcoholic vapor condenses. Some distilleries still use the traditional condensation method, which involves a copper worm immersed in a cold water tank, known as a worm tub, located outside the distillery. As the vapor passes through the copper tubes, it condenses and becomes liquid.
The liquid produced in the wash still is known as low wine and is mixed with the residue from the previous distillation cycle, bringing the alcohol content to about 30% ABV. At this point, the mixture is introduced into the second copper still, the spirit still, and the liquid is brought to a boil again. Unlike the first distillation in the wash still, where almost all the liquid is recovered, much of it is discarded in the spirit still. As the mixture is heated, the first to evaporate are the more volatile alcohols, which have a higher alcohol content (about 75-80% ABV) and tend to be either toxic or, at best, unpleasant in flavor. This liquid is known as the foreshots or heads and is collected in a separate container and set aside. As the process continues, the alcohol vapors have a lower alcohol content, and the liquid that emerges, characterized by desirable aroma and flavor, is set aside to be matured in casks; this liquid is the "heart" of the distillate and is collected in a container called the intermediate spirit receiver. The final portion, called the aftershots or feints (tails), has an unpleasant aroma and flavor and is collected in the same container as the heads. Heads and tails are added to the next batch of low wines to be distilled again.
The method of separating the three distillates varies depending on the distillery, and in some cases, only a very small portion is obtained. The skill of the distiller is crucial here, as they decide which parts, or cuts, to keep and which to discard. The distiller uses the spirit safe, a brass container with hydrometers for measuring alcohol content and transparent walls for assessing the clarity of the distillate, for this selection.
The distillate from the intermediate spirit receiver is a clear and pure liquid with an alcohol content of around 70% ABV. Before it can be put into casks, its strength is diluted with water to reach 63 or 64% ABV, a value considered optimal for whisky maturation.
Triple Distillation
While in Scotland single malt whisky is produced exclusively using the method of discontinuous or double distillation (with the exception of Auchentoshan and Springbank, which produce their Hazelburn using triple distillation), in Ireland the process is slightly different. Most Irish whiskey is produced by distilling it three times (with some exceptions and distilleries that prefer double distillation).
The process is very similar to the Scottish method: the wash is brought to a boil in a still (wash still), and the resulting low wine is transferred to a second still (known in Ireland as the feints still or intermediate still) for the second distillation. As in Scotland, the heads and tails of the distillate are discarded, and the heart, known as feints, along with the heads and tails from the previous distillation, is introduced into the third still, the spirit still. The third distillation follows the same principle as the second: the heads and tails, referred to as strong feints, are discarded (they will be reused for the next distillation), and the heart, a liquid with 82-85% ABV, is set aside for maturation in casks. This method produces a whiskey that is purer and smoother compared to Scottish whisky, but the third distillation leaves behind some aromatic and flavor characteristics in the spirit still.
Maturation
This is the crucial part, the mystery of malt whisky: maturation. Two different casks never yield the same result. Casks can vary in size and may have held different liquids, but even two identical casks that contained the same liquid and were placed in the same environment, side by side for the same period, can still produce different outcomes. Some believe that a whisky acquires about 80% of its final character from the cask in which it rests; certainly, a cask cannot improve a poor whisky, but it can elevate a good one.
Oak Wood
As mentioned in the previous paragraph, casks are fundamental to good whisky, so the type of wood is a key factor in its success. Oak has always been the wood used for whisky maturation. An oak cask is capable of imparting excellent flavors and aromas to the whisky during aging; the tight grain of the wood also prevents any leakage, while its pores allow the spirit to breathe. Additionally, oak is durable and can be bent hot without cracking.
Whisky is matured in either European oak casks or American white oak casks. European oak casks primarily come from Spain or Portugal due to the historical practice of shipping sherry in casks to be bottled in the UK. In 1983, the European Commission banned the shipment of sherry in casks, so today distilleries purchase casks directly "on the tree" and commission Spaniards to make the casks, then arrange with the bodegas to season the casks with sherry before they are exported to Great Britain.
American oak casks serve a dual purpose in the whisky world. In the mid-1930s, American distilleries agreed that bourbon or rye whiskey should be aged only in new casks. This agreement led, in the years following World War II, to a large number of used white oak casks becoming available at very low prices. Bourbon matures exclusively in barrels containing about 200 liters of liquid. These barrels are exported from the United States either intact (American Standard Barrels) or disassembled into staves that are later reassembled in Scottish distilleries into a slightly larger format (250 liters). These reassembled ex-bourbon casks are known as hogsheads.
Use of Old Casks
Except for whisky produced in the United States, it is rare for whisky to be aged in new casks. This is because the wine or spirit previously held in the cask "seasons" the wood, eliminating some of its stronger flavors and adding pleasant traces of its own character. In Scotland, a cask used for the first time is called a first-fill, while a cask that has already been used to age whisky is referred to as a refill. Naturally, the more times a cask is used to age whisky, the less influence the wood has on the distillate, and the fewer unwanted odors and flavors remain. After three or four uses (the duration depends on the number of years the whisky has aged in it), the cask is considered exhausted and is either discarded or "rejuvenated." To rejuvenate a cask, the internal walls are scraped and the wood is burned again, which reactivates the underlying layers of wood that can still release flavors and aromas, although not as strongly as a first-fill cask.
Storage
Here is the final crucial process of maturation: the wood is affected by the microclimate present in the maturation warehouse where the casks are stored. Consequently, the warehouse and its location are of considerable importance. During maturation, ethanol and water evaporate (this is referred to as the angel's share) while oxygen simultaneously penetrates the casks. In Scotland, the tradition of island whisky maturation involves aging the distillate in warehouses close to the ocean, allowing the whisky to acquire a distinctive marine aroma. The volumetric loss and maturation rate depend on two factors: temperature and humidity. At higher temperatures, the distillate expands, extracting flavors from the wood relatively quickly, while in humid conditions, the liquid tends to evaporate less and maintain its volume but loses alcohol content; the opposite occurs in dry environments.
In traditional Scottish warehouses, usually built of stone with a dirt or peat floor, the temperature remains constant throughout the year, thanks to the thicker walls and more insulating building materials. The cooler climate slows down evaporation and consequently the maturation of the whisky.
Americans, on the other hand, tend to use brick or metal warehouses, often on open grounds to take advantage of seasonal temperature and humidity variations. This results in higher distillate evaporation and, therefore, faster maturation.
Finishing
Known primarily as "cask finishing" or "double maturation," this process involves transferring whisky that has matured in one type of cask to another (typically a first-fill cask or one previously containing a different liquid such as wine, rum, champagne, etc.) for a final period of maturation. The whisky acquires additional "layers" of aromas and flavors, thereby enhancing its complexity.