The large variety of whiskies produced around the world contributes to the fact that this “eau de vie” is recognized to have the widest olfactory palette among all spirits. This is explained by the multitude of factors involved and allowed in the whisky creation process depending on its geographical origin.

Factors that shape the aromatic palette of a whisky:


1) Cereal / Grain

Whiskies can be made with wheat, rye, corn and barley (also known as “malt” once germinated). There are whiskies made with only one type of grain, such as Single Malt Whiskies and others mixing different grains together such as Bourbon Whiskies. Grains can be sourced locally or be imported, so the concept of “terroir” often used when talking about grapes and wine doesn’t apply to whisky. The quality of grain depends on its level of starch and its aromatic potential.

Whiskies made of barley (malt) have a roasty, toffee-flavored cereal quality. Whiskies made of corn show sweeter flavors of vanilla and maple syrup, while whiskies made of rye show sharp peppery spice and whiskies made of wheat tend to have flavors more towards whole bread with honey.


2) Water

Water is vital to the making of whisky. Distilleries are using large quantity of water to produce whisky and need to secure their source through the year.

The source of this water varies widely from streams, rivers, lakes, and boreholes to public supply. Water is used in malting, in mashing, in diluting the spirit to reduce its alcoholic strength to the desired bottling level and also used for heating stills and cooling condensers that turns vapor into a new make spirit. Of course, the quality of the water is very important, it has to be clean and wholesome in order to avoid any nefarious effect on the finished product. While the mineral content and the pH level in water plays an important part in the concentration of aromas during the fermentation process, there has been a misperception from consumers that water has a dramatic impact on the whisky final flavor. The truth is that most distillers agree that the type of water used only contribute to less than 2% of the final whisky flavor. For instance, peaty character in whisky from the Isle of Islay is not sourced from the local water, but exclusively from the burning of peat during kilning.



3) Malting

Malting is the process of partially germinating the grain to convert unfermentable starch to fermentable sugars, so alcohol can be created with the addition of yeast afterwards. The malting is done in two phases with the “steeping”, where the grain is soaked in water for few days to allow it to germinate. Then the “kilning”, where heat is applied to dry the grain and halt its germination when the optimum amount of starch has been converted. During the drying process amino acids and sugar are reacting together to produce roasted, biscuity flavors (known as the Maillard reaction).

In Scotland, particularly on the islands where there are few trees, Scots traditionally uses heat from peat fire. Peat forms in poorly drained wetland conditions from a build-up of decomposed vegetation such as mosses, grasses, reeds, heather and shrubs. In some cases, some level of smoke from a peat-heated fire is introduced to the kiln to add phenols, a smoky aroma and flavors to the whisky.

The seaweed, tar and iodine aromas typically found in malt whiskies from the Isle of Islay are partly due to decomposed marine vegetation and sea salt in the island’s peat. The peat on the Orkney Islands is composed almost entirely of heather so imparts to their whisky with a floral-like smokiness. Highland peat varies according to location but contains more decomposed trees so tend to impart a more wood fire-like smokiness.


4) Fermentation

The malted grain is milled into flour then mixed with hot water in order to convert the remaining starch into sugar. The result is a sweet, sugary liquid called the wort. Then yeasts are added to the wort to start the fermentation. Yeasts are micro-organisms from the mushroom family that feed on sugar, producing not only alcohol and carbon dioxide, but also a variety of odorous molecules that gives rise to fruity aromas (citrus, apple, pear, pineapple, banana…), floral aromas (rose, lavender, violet…), grainy aromas (malt, biscuit…) and vegetal aromas (cut grass…).

Not all yeasts are the same. Each type of yeasts produces different aromatic substances. Some yeast strains produce fresh fruity aromas such as citrus, apple, pear; others produce riper aromas such as plums, apricots and berries.

The varieties of yeast used by distilleries are widely known, but each distillery jealously protects the nature of the strains it uses, as well as their proportions and blends. This constitutes a top-secret recipe.

Much of a whisky’s flavor is created during fermentation. The amount and strain of yeast used, the length of time and temperature of the fermentation play an important part in the final taste of a whisky. After being fermented the wort is now called the “wash”, which tastes like a beer.



5) Distillation

The distillation is a process of heating up the “wash” in a still in order to eliminate most of its water and concentrate its ethanol alcohol and aromas by evaporation and condensation. This process is complicated by the fact that in the “wash” there are different types of alcohol and other chemical compounds present beside ethanol, all with different boiling points. Some of these compounds are desirable, because they give character and flavor to the spirit, others can be harmful or have unpleasant flavors and should be removed as completely as possible during distillation.

With pot stills, the distillation is fractioned in three parts, the “foreshots”, the “middle cut” and the “feints”. The “foreshots”, also called “heads” are composed of the most volatile compounds, those with a low boiling point. Foreshots include acetaldehyde, which is believed to be a major contributor to the severity of hangovers; some acetone, which is commonly used as a cleaning solvent or nail polish remover and some methanol, which is very bad for the liver and can lead to blindness. This is why it is highly recommended not to consume garage made spirits or those from questionable source.

Next comes the “middle cut”, also called the “heart” that contains ethanol alcohol, which is the potable alcohol that the distiller wants to capture together with esters, which are the chemical compounds responsible for the aroma of many fruits (green apple, pear, pineapple, banana, strawberry, melon, peach and dried fruits). Follows grassy and cereal notes with tones turning towards citrus. It’s the heart that is collected and aged to become whisky. By diverting the flow of spirit emerging from the condenser, the “heart” can be separated and saved.

As the alcohol with lower boiling points have now evaporated, this leaves water proteins, carbohydrates and less volatile alcohols with higher boiling points, better known as “feints” or the “tails”. The “feints” are the last volatile fraction of spirit distillation. Feinty aromas increase slowly towards the end of distillation, developing from quite pleasant leathery and tobacco aromas to waxy and fishy notes not usually approved in whisky. The feints are usually rich in phenols and smoky aromas important for peaty whiskies. Therefore the second cut point must be determined with care to produce peated but not feinty spirit with off-notes.

The aim of the distiller is to separate the “heart” from the “foreshots’ and “feints’, but a small proportion of some of the compounds in the foreshots and feints can contribute desirable flavors to the finished whisky. So the distiller’s art is to make cuts in the run of distillate at points that will allow these desirable compounds from the foreshots and feints to be incorporated in the heart, rather than being discarded.

Most stills are made of copper, since it removes sulfur-based compounds from the alcohol that would make it unpleasant to drink. The simplest standard distillation apparatus is commonly known as a pot still, consisting of a single heated chamber and a vessel to collect purified alcohol. All scotch malt whiskies are distilled in pot stills. Column stills, also called continuous stills are frequently used in the production of scotch grain whisky and are the most commonly used type of still in the production of bourbon and other American whiskeys. Column stills behave like a series of single pot stills, formed in a long vertical tube.

Pot stills are very inefficient compared to more modern continuous stills, but they have the advantage of retaining much of the flavor from the malt. The new make spirit from pot still leaves the still at an average of around 70% abv compared to new make spirit made in continuous stills which leaves the stills typically at 95% abv.

The more contact the alcohol vapor has with copper so the lighter the spirit will be. Hence, the size, shape and design of the pot still will have a considerable effect on the character of the spirit produced. The height of the still will also dramatically affect the character of the spirit produced. Tall stills have a larger copper surface area and also make it harder for the vapor to rise so promoting reflux. Tall stills tend to produce light spirits, and conversely, shorter stills typically produce heavier and richer spirits.

Other factors, such as the duration of the distillation also affect the spirit. Depending on how the distiller regulates the heat, the distillation can be slowed or speeded up. The slower the still is run the longer the distillation takes, which prolongs exposure to copper and produce a lighter spirit.


6) Maturation

The New Make Spirit is placed in oak barrel for maturation purpose. The maturation in oak casks is the process that has largest effect on the final taste of a whisky. The influence of the wood can represent up to 75% of the final taste of a whisky.

The maturation in oak barrels is the result of complex chemical reactions that occur between the distillate and the wood’s various compounds. The latter include tannins, lignin, lactones, glycerin, polysaccharides, fatty acids and aromatic aldehydes. By breaking down the lignin, the alcohol releases aldehydes, the most well known of which, vanillin (vanilla flavor), appears from the very beginning of the ageing process. Fruity and floral esters emerge over more time. They are most noticeable in whiskies aged from ten to fifteen years. Younger whiskies display aromas closer to those of grains, and reveal a subtle fruitiness like pear aroma. Whiskies aged over twenty-five years reveal refined coconut notes, due to the lactones that take some time to infuse the whisky.

The length of time the whisky stays in the barrel defines the whisky’s age. Note that whiskies do not mature in bottles, but only in wooden barrels, so the “age” of a whisky is only the time between distillation and bottling.

This reflects how much the cask has interacted with the whisky, changing its chemical makeup and taste. Whiskies that have been bottled for many years may have a rarity value, but are not “older” and not necessarily “better” than a more recent whisky that matured in wood for a similar time. After a decade or two, additional ageing in a barrel does not necessarily improve a whisky. The ideal point for bottling is when the whisky has reached a harmonic balance between the original character of the spirit and the character of the oak.

Time is an important factor in maturation, but other factors such as the surrounding environment where barrels are patiently stored and the barrel itself are also very important. The surrounding air that seeps through the wood’s pores also plays a role. Whiskies that are aged close to the sea are infused with maritime aromas, often with very pronounced salty flavors. In warm weather the wood expands, as does the volume of liquid, meaning that there is more interaction between the spirit and the wood. There is also higher level of evaporation taking place in warmer conditions, referred to as the angel’s share. The air outside the cask is also absorbed by the wood and oxidizes the liquid, creating esters, which give off a fruity flavor. Conversely, cold weather makes the wood pores contract and slows down the maturation process. Whiskies matured in a warm climate will usually be bottled at a relatively young age, while whiskies matured in a cold climate will need more years to reach the desired balance and will be bottled at a more advanced age.

The concentration of the spirit also effects how the liquid interacts with the wood, with a lower concentration being optimal. However, this requires more casks and more warehouse space. As such 63.5% ABV has become the standard concentration for maturing Whisky.

Since, wooden barrels play such a big role in the final taste of the whisky, let’s review how wooden barrels can be different from one another:


  1. A) Type of wood

Oak is the only type of wood used for two reasons. Firstly oak is dense and so the barrels don’t leak and secondly when “charred” or “toasted”, the wood imparts color and lots of sweet, toasty characters to the liquid. There are two primary types of oak used, American or white oak (Quercus Alba) and European oak (Quercus Robur and Quercus Petraea).

American oak is a fast growing type of oak found in North America and is very popular for Bourbon maturation. The wood is very dense and has a high vanillin content, which typically brings vanilla and caramel flavors to the whisky.

The European oak grows all over the European continent far into Hungaria and Russia. It grows slower than the American counterpart and is a bit less dense. It contains more tannins that gives to the whisky a slightly bitter note and spiciness. European oak typically brings bitter, spicy and strong woody flavor to whisky.


  1. B) Second-hand casks

Most of Scotch whiskies are matured in second-hand casks, which means that these casks used to mature another wine or spirit before being used for whisky. The taste of fresh European oak casks is too intense to age whiskies, instead, milder second-hand casks, which have been seasoned by another liquid are used. The wine or the spirit, which matures first in the cask removes strong woody flavors and bitter tannins, but also leaves traces of its own flavor and color in the wood for the whisky to mingle with. For instance, casks of a fortified red wine like Sherry impart distinct traces of their own heritage on the whisky such as raisins, plums, nuts and wine tannins, along with a typically darker coloration as well. To see whether it was oak or Sherry that gave a ‘Sherried’ character to a Scotch, scientists simply added Sherry to Scotch to see whether the combination was the same as maturing in a Sherry butt. It wasn’t. Equally, ageing in an untreated European oak butt gave a different result. The ‘Sherried’ character, therefore, is down to the Sherry itself, but also to the oxidation and the way in which the Sherry has interacted with and changed compounds in the oak.

Historically, European oak used to mature whisky was most often used previously for fortified wine, either sherry or port.  The United Kingdom is the biggest consumer of sherry in the world and for years sherry was shipped to the UK in casks. These casks then found their way to Scotland where they were put to very good use maturing whisky. Nowadays there is such a huge demand for sherry casks that some Spanish wine makers make sherry purely for conditioning whisky casks. Nevertheless, due to the lesser consumption of Sherry wines and the increasing demand for Sherry casks for the whisky industry, distillers have been looking for other alternatives.

For Bourbon whiskeys, it is a different story, because their regulation obliged them to be matured in virgin oak barrels, which means that barrels used for maturing Bourbon whiskies cannot have previously held another alcoholic beverage.

Nowadays, a huge amount of ex-bourbon American oak barrels are being shipped to Scotland to be used as second-hand casks to mature Scotch whiskies as an alternative to fortified wine casks.

In the late 1990’s in Scotland, a new trend called “finishing” or “double maturation” in casks emerged. This is a concept where the whisky is transferred in a different second-hand cask to finish its maturation and gets more complexity from a cask, which used to mature another wine or spirit. Whiskies aged in this way are often marketed with the type of “wood finish” indicated on their packaging.


  1. C) Level of charring and toasting

During construction at the cooperage, the inside of the cask is exposed to fire. The heat helps shaping the staves while also converting some of the wood’s starches into sugars, which caramelize and break up to form volatile aroma compounds like furfural (almonds), maltol (toast), cyclotene (caramel). The same occurs with lignin that breaks into aldehydes and phenyl ketones (vanilla), guaiacol (smoky), eugenol (cloves) and other aromatic compounds.

Charring burns the wood to a point where the surface breaks and leaves the wood with a surface that looks like an alligator skin. The interior of charred casks is black and has a lot of ash residue, resulting in a dark color for the whisky. As far as flavor goes, charred wood imparts sweeter flavors like caramel and honey. The reason for this is that the wood sugars are caramelized when heavily burned. Charring also breaks down the oak cracking its surface so better allowing the spirit to seep deep into the wood.

Toasting just darkens the top of the wood and acts in the depth of the staves. It leaves the wood with a black flat layer on top. Unlike their charred brethren, toasted barrels are heated much more gently, resulting in a dark brown toast rather than a blackened char. Toasted barrels add a bit more vanilla flavor to the liquor, as well as spicy accents. Because the wood hasn’t been burned to a crisp, the sugars haven’t had time to caramelize, making the whiskey a bit sharper on the tongue.

Toasted barrels also do not impart as much color into the spirit, resulting in a lighter shade. European casks tend to be only lightly toasted while American casks can be very heavily toasted.


  1. D) Size of the barrel

The size of a barrel also plays a role in defining the whisky. Generally, smaller barrels mature whisky a lot faster than large barrels. This is because there is a greater wood to liquid ratio in small barrels, meaning that the liquid is in contact with a lot more wood than in a larger cask. The cost of production is higher when using smaller barrels, but many argue whisky matures more gracefully in larger casks.


  1. E) Reuse of the casks

The last point about second-hand casks is the number of times they are being used. The more you use a cask the less flavors it will release into the whisky.

Most often, casks are rejuvenated before being refilled. To do this distillers scratch the inside of a cask back to fresher wood before re-charring or toasting it. This restores vanilla and caramel flavors, but also result in a spicier flavor. Casks that are maintained in this way can last for up to 100 years.

A new barrel, that is to say a barrel that is holding single malt for the first time (but may have held Sherry, Port, Bourbon…previously), is called a first fill barrel and has the most profound effect on the spirit because there is still a lot of the original liquid remnant in the barrel. A barrel used for the second time is called a second fill barrel and has less of an effect on the spirit. Casks can be used three to four times, but afterwards the wood will have lost most of its flavors.

The production costs of casks account for a considerable part of the overall costs of whisky production (about 10% to 20%). Increased demand for casks in the whisky industry has also contributed to the rise of cask prices. That’s why it makes sense for distillers to use casks several times. But saving money is not the only factor. It’s also the spicier flavor characteristics that make rejuvenated casks popular in the whisky industry.