How is whisky made and where does its flavour come from? Distilling and Maturing whisky


Whisky is defined in the Oxford Dictionary as ‘[a] spirit distilled from malted grain, especially barley or rye. There is more to it than that, of course. Making whisky is complex. Whisky is not “made” into a finished product, it takes time. The spirit that becomes “whisky” is clear as water when it is distilled. To become whisky, this clear liquid (which is called “new make”) needs to mature in oak. This post will show you how this “new make” is distilled, and then matured into whisky. It will explain how different types of whisky get their distinct flavours, and from where. 

It should be noted here that this post is about how whisky (without the “e”) is made. It will therefore focus on the typical production of Scotch style malt whisky, and not Canadian whisky, American whiskey (such as bourbon or Tennessee whiskey which is made from a mash of at least 51% corn, and which needs to mature by law in new charred oak barrels) or Irish whiskey (which tends to be made from malted and unmalted barley, and is usually distilled three times; though this is a generalization as I am advised that whiskey distilled at the Cooley distillery in Ireland is distilled twice and some Irish whiskies are made purely from malted barley). Now, let’s begin the story by introducing you to the grain that tends to be used to make whisky: barley. 

1. In the beginning, there is barley…


In the beginning, there is barley. This barley is malted (for malt whisky), and then dried. It is dried either with air, or, smoke from burning peat (“peat smoke“). Peat is a spongy material which, put simply, is an organic fuel formed by the partial decomposition of organic matter – usually plant matter – in wetlands.  The creation of peat is the first step in the formation of coal.

Many distilleries do not use peat when drying the barley that is used to make their whisky; this means the natural flavours and aromas of the barley are likely to be pronounced in the “distillery character” of these distilleries. Examples include Glengoyne (Highland single malt Scotch whisky) and The Glenlivet (Speyside single malt Scotch whisky), among others. 

In contrast, many distilleries on the Isle of Islay – Ardbeg, Laphroaig, Caol Ila, Lagavulin, Bruichladdich, Bowmore etc – and the islands – Talisker, Highland Park – use “peat” to dry the barley which is used to make many of their expressions. As the barley is dried with peat smoke, it absorbs “phenols” from the smoke. This means that natural flavour of the barley is paired with the vegetal and earthy peat smoke (resulting whisky is sometimes described as medicinal, tarry, earthy, carbolic etc). Some of these expressions are also coastal or maritime in flavour, with sea spray and salt (probably because sea spray saturates some peat used). More lightly peated whiskies tend to have a mossy character. 

smoke dry barley.jpg

The level of peat in a whisky depends on the whisky’s  “Phenol Parts Per Million” or “PPM“. A number of factors can determine a barley’s PPM, including how long it is exposed to peat smoke, the type of peat used, the density of the smoke etc. Different distilleries produce whisky with different PPMs; the higher the PPM the more “peaty” the whisky is likely to be. 


2. The barley is used to make Wort (barley/grain infused water)

The dried barley is then cracked or ground into a grist (“milling“). It is then steeped in hot water (usually from 63°C) for a while in a process called “mashing“, so that the flavour and sugar in the barley (or other grain, for grain whisky) can be soaked up by the water. The mashing process involves putting the barley and water in a machine called a “mash tun“, which stirs the mixture – this process develops enzymes which converts the starch in the grain to sugar. Now, the resulting sugary water, called “wort“, is ready to be converted to an alcoholic “wash” (what is essentially a simple, and pretty gross, beer). 


3. Wort is fermented and made into a Wash (a simple beer)

To make the “wash“, yeast is added to the wort when it reaches somewhere between 18°C to 28°C (depending on the kind of yeast that is used). The yeast converts the sugars in the “wort” to alcohol while adding flavoursome impurities called “congeners“. This process is called “fermentation” and occurs in vessels called “washbacks“. It usually takes a few days. The wash tends to be around 8-9% alcohol by volume. 

Many different types of yeast are used to create alcohol of different flavours, from bakers yeast used by many moonshiners to the more sophisticated ale yeasts that tend to be used by artisan distillers. You will find that every distillery has a particular strain of yeast they prefer to use, and this yeast contributes to their distillery character. Sometimes the batch is spoiled by wild yeast hanging around, eager to ferment any sugary liquids into a fowl smelling (and no doubt tasting) wash.

The race to beat that pesky wild yeast from spoiling the show requires a clean work area and getting the wort down in temperature as quickly as possible to add the nice, friendly, yeast – the longer it takes, the more time the wild yeast has to ruin the batch, but if the wort is too hot the nice cultured yeast will die. Once the yeast is placed into the wort the wort begins to slowly ferment into a wash – a form of, as noted above, pretty disgusting beer. The wash neither looks nor smells palatable, having a layer of foam and scum on the top. 


4. The Wash is distilled 

The wash, now containing alcohol, is placed in a copper still. Copper stills are used because the copper helps remove sulfides in the wash and it can also accentuate esters and aldehydes. A still allows alcohol vapour to travel up its neck and then be converted to liquid at the top when it reaches a condenser (which is usually a coil with cool water running through it).


Once in the still, the wash is heated. Because alcohol evapourates at a lower temperature than water, it can be separated from the water in the wash by heating the wash to a temperature above 78.4°C but below 100°C.

As the wash reaches around 65°C a liquid clear as water begins to travel up the still as vapour. The alcohol that boils at the lowest temperatures is acetone and methanol – real nasty smelling and poisonous – and this, fortunately, comes out of the still first. The small portion of alcohol that comes out of the still first is called “foreshots“. It is collected and discarded by a distiller. Then, when the wash reaches 78.4°C, that is when the good drinkable alcohol – ethanol – boils and travels up the still as vapour to be condensed into liquid and collected by the distiller. As the wash heats up, though, higher alcohols such as  fusel oils/alcohols (2-Propanol/rubbing alcohol at 82.4 °C and 3-Methyl-1-Butanol at 99.5 °C) also begin to boil and then follow the ethanol up the still. 

A distiller using a “pot still” needs to typically undertake two distillation runs – first distilling the wash to produce “low wines” (usually 18-25% alcohol by volume) and then re-distilling the “low wines” to get a stronger alcohol usually between 60-70% alcohol by volume. Sometimes, to get a lighter spirit, a distiller might want to run it through the still again (as is typical with three times distilled Irish whiskey or Auchentoshan, which defies the Scotch industry standard of distilling twice).

Other types of stills can include “reflux” or “column stills” which can distill a wash multiple times in one cycle, by using packing or bubble-plates (these kind of stills are mostly used to distill vodka because with each distillation the alcohol is stripped of some flavour and it becomes purer and more concentrated with ethanol – this is why whisky is usually distilled only two or three times, to keep the flavour of the barley and fermentation). These reflux or column stills sometimes have a condenser at the top of the still neck with cool water running through it, and this cool water blocks heavier vapours so that lighter vapours can pass through to be collected thereby producing a very fine spirit. 


5. A distiller needs to make “cuts”, and choose what distillate to turn into whisky

During distillation a distiller needs to make what is known as “cuts” after discarding the foreshots. As just noted, foreshots come out of the still first at the start of a distillation run because it comprises mostly of volatile alcohols that boil at low temperatures – acetone and methanol. It is important to emphasise that the alcohols in foreshots are poisonous and must be discarded.

The first part of the run is the “heads”. Heads comprise of acetone, acetaldehyde and abetate; these alcohols smell bad and taste very harsh on the palate. Consuming them will likely give you a killer hangover. They smell like nail polish and nail polish remover. 

As the amount of ethanol flowing out of the still increases, the drinkable spirit emerges in the form of “hearts”. Hearts comprise predominately of ethanol and the most desirable congeners; they have a rich pleasant aroma and flavour, with sweetness and much more smoothness than heads. Hearts are more expressive of the raw ingredients used to make the wash. They smell and taste like something you would want to drink – often quite fruity, sweet and malty.  This alcohol can also smell yeasty from the fermentation process, much like sake, Chinese rice wine or Shōchū made from grain. 

After a while, the hearts begin to fade into the “tails”, which smells very much like wet cardboard, baby sick, wet dog etc. It not poisonous, it just smells and tastes strange and undesirable. Tails may sometimes add some congeners and flavour compounds not found in hearts, and they are sometimes very important ingredients to a distillery’s character.

The unused heads and the tails tend to be recycled, and used in future distillation runs to squeeze out any ethanol out of them.   

Once the distiller makes all the desired “cuts”, you have whisky in its infancy – “new make”. The important thing to remember is that different distilleries tend to produce “new make” that taste different to one another. The below tasting notes will take you through “new make” from different distilleries, and “new make” that is made from different types of grain and barley. The aim of these below tasting notes are to demonstrate how different distilleries and different types of grain produce “new make” of different character. 

Bruichladdich is a whisky distillery that is located on the isle of Islay in Scotland. While Islay is famous for producing peaty whisky – ie: whisky that is made from peat dried barley – the distillery is known for pushing the boundaries and it also releases non-peated whisky. Perhaps more interesting, Bruichladdich are now releasing whisky that is made from different types of barley.

Malt Mileage is very fortunate to have had the opportunity to try three samples of Bruichladdich new make, each of which are made from different barley – Islay grown barley, bere barley and organic barley. The below tasting notes show that the type of barley used in making whisky has a big impact on its flavour profile.    

Tasting Bruichladdich Islay Barley new make 

Bruichladdich are opting to make some of its whisky from barley that is grown close to home on the Isle of Islay in Scotland. Just as the soil matters to making grapes and the eventual flavour of cognac made from such grapes, differences in soil would also seem to matter to growing barley and the eventual flavour of whisky made from such barley.

My immediate impression of this new make is that is packed with nuances of flavour and sweet ethanol.  The spirit itself is rich in raw materials that the oak can work with and develop into more tasty flavours. Wet dog fur, a smell often associated with the tails of a distillation run, is noticeable but it is softened by layers of complexity that cut through the center – chocolaty and nutty malt, shades of golden and dark honey with earthy and herbal hues, black olive tapenade, anchovies, mild pepper in a blanco tequila kind of way and pronounced sweet ethanol. The taste of capricciosa pizza, as strange as that sounds, seems to be a noticeable theme in this new make. Bravo. 

Tasting Bruichladdich Bere Barley new make

In stark contrast to the Bruichladdich Islay Barley new make, the Bruichladdich Bere Barley new make is fiery, spicy and explosive with lots of depth, complexity and rich flavour. I found lots of caramels and vanillas – especially under-cooked doughy vanilla slice with some flaky pastry – crème caramel, powdered mushroom soup, toffee apple, brown pear, celery, peeled raw potato, fried black olives and mint sitting beneath overtones of husky gristy barley and malt. 

Tasting Bruichladdich Organic Barley new make

The Bruichladdich Organic Barley is sweet, caramel rich and fruity with the most curious note of strawberry cheesecake, red toffee apple, raspberry seeds and hints of baby sick (which are tasty congeners the oak will work with). It is big, bold, raw and full of the flavour of that organically grown barley. 

Tasting Glengoyne new make

Glengoyne air dry their barley, never using peat, which means that from the start this little baby whisky is designed to have no peat flavours – just clean, fresh malty notes from distilling the “wash”.  Glengoyne new make is very clean spirit that is strikingly rich with hearts; through there are some rough edges and heavy congeners which will mature nicely in oak to amplify the finish in an eventual whisky. 

Find in Glengoyne new make caramel (werthers original), brown pear, peaches, nectarines, crushed apple and orchard fruit with a lingering, flavoursome finish that offers disgorged digestive biscuits with mild creaminess. This is superbly crafted new make.

Tasting Lark New Make (run 1033, 50% peated, 63.4%ABV)

Lark would be forgiven in my book if they just skipped barrel maturation, and put this peaty new make right in bottles for consumption for the peat crazed among us to dabble in the taste of untouched “virgin” peat (because, as you may know, peat fades as it ages in oak).

Being made from 50% peated malt, the flavour profile is not dominated by the peat. The flavours are powerful, lasting and delicious – find licorice, plum, caramel and a thump of peat, all of which slowly taper away over a few minutes. The finish is spicy and warming, full of ground fresh red chillies, cracked pepper and hints salt. The peat is not what you would expect of a whisky from Islay, but it is both grassy and woody, and the barley notes provide a lovely sound foundation.

Mature this for a few years in oak to iron out the kinks invariably found in new make and infuse it with some more vanilla, fruit and chocolate, and I think the product will be pretty darn spectacular. There are some notes typically associated with the tails end of a run, especially on the nose – damp cardboard, wet dog fur and a creamy baby sick note emerges with a dash of water. Now that can be a good thing, because the oak has plenty of raw materials to work with.  

Tasting Glenrothes New Make 

The Glenrothes new make seems to have a nice spread of flavour – some light bitey alcohol, sweetness and heavier congeners. Tasted neat, it explodes with blackcurrant and a superb peppery finish. With water, the style of Glenrothes distillery character emerges – find spice, pepper, brown pear, and a solid dose of malty goodness. The spice and pepper is a dominant theme which lingers on the finish, not dissimilar to a fine blanco tequila or a non-polished grain based vodka. It is a rare pleasure to be able to taste Glenrothes new make, and discover that aspects of its rich fruity-spicy signature style seem to come right from the new make. The rest is the oak barrel’s job, perhaps some more vanilla, sherry and wood spice will do the trick? My mouth is watering just thinking about it.

This, as you have probably noticed, certainly does not sound like whisky! I can visualize your cringe at the idea of tasting disgorged digestive biscuits, baby sick and wet dog fur!! To get the aroma, flavour and colour of whisky the new make needs to spend at least three years (usually much much longer) in oak casks. That is known as the “maturation”. This process gives whisky most of its flavour, and it also irons out ripples in the new make.  

6. The distilled “new make” is placed into oak barrels to mature


People figured out a long time ago that even the best “new make” can be improved. In days now long passed, oak barrels were used to store and transport almost everything – fish, meat and even “new make” spirit. Of course, a trader – if they wanted to keep their clients – could not transport new make in barrels that had for example contained fish; or the new make would, excuse the pun, smell and taste fishy. So, before placing the new make in the barrel, they would burn and char the insides of the barrel. What they probably didn’t realise, but what they found out later, is that this charring actually unlocked a lot of the sugars and flavours in the oak which could then be infused into the new make. People found that after spending time in these charred oak barrels the “new make” began to not only change colour but also smell and taste better. People noticed that the kind of colour, aroma and flavours that developed relied very heavily on what type of oak the “new make” was stored in. Distillers now use oak casks to add flavour to their “new make” over the course of many years, and rightly so because it is estimated that a whisky gets upwards of 60% of its flavour from oak.  

When new make sits in oak barrels, the pores in the oak expand when it is warmer, soaking up the new make, and close when it is colder, squeezing the new make back out into the cask. Over time, from being soaked up and squeezed out over and over from the oak, the “new make” slowly begins to take on colour, aroma and flavours from the oak casks in which it matures.

Because the casks are often charred (toasted or burnt on the inside) the “new make” also gets filtered by the oak, in a similar process to carbon filtration which removes some of the more undesirable compounds in the “new make”. The alcohols that boil at at low temperature are even sometimes burnt away as the new make rests in the casks, especially in hot summer months, escaping through the pores in the oak or the bunghole as it is opened. By far, however, the “new make” gets most of its flavour from the oak itself.

Glengoyne Distillery Warehouse 8

American oak

The most popular cask in which to mature whisky, at least in Scotland, is American oakAmerican oak (or Quercus alba) is an oak tree that grows in North America. As “new make” rests in American oak, the “new make” tends to draw out oils from the wood called “vanillins” and in American oak these typically resemble the aroma and taste of vanilla. American oak also offers compounds which smell or taste like caramels, coconut, butterscotch, fudge, and, particularly with older whisky, ginger.  When a whisky is matured in American oak that has never been used for anything else before, it is called “virgin wood” or “virgin oak”.

This typically gives a whisky a “bounbony” character (think pancakes, brown sugar and syrup), because bourbon is matured in new oak barrels that have been charred (although bourbon must be made from at least 51% corn so it is different from malt whisky, which is made from barley). Using “virgin wood” to mature malt whisky is not very common, though it does increasingly now occur, and this practice is seeing what many call the “bourbonisation” of malt whisky.

French oak

Whisky can also be matured in French oak, which, broadly speaking, tends to give a whisky subtle spicy notes, clove and a satiny texture. The main French oak forests are, however,  Allier, Limousin, Nevers, Tronçais and Vosges. Oak from each oak forest is thought to be distinctive.

Japanese oak

Whisky can also be matured or finished in Spanish oak, Hungarian oak, or Japanese oak (also known as Mizunara oak). Japanese oak tends to have strong notes associated with cinnamon and a distinct spiciness. 


Ex-bourbon, ex-sherry, ex-wine, ex-brandy, ex-wine casks 

Whisky not only derives its flavour from the oak, but also whatever the oak barrels held before it stored the whisky. Malt whisky distilleries tend to mature “new make” in casks that have already matured something else – bourbon, sherry and wine are the most common examples. This means that not only does the new make draw out flavour from the American oak or French oak, but it also becomes infused with the flavour of the previous contents of the oak whether than was bourbon, sherry or wine. Increasingly, distilleries are also using barrels that have held rum and brandy or Cognac. Jameson has even finished one of its expressions in barrels that have been seasoned with craft stout beer.

Whisky matured in American oak ex-bourbon barrels


It is estimated that about 90% of Scotch malt whisky is matured in American oak casks that have previously held bourbon. By spending time in American oak that has previously held bourbon, a malt whisky tends to draw out some distinct “bourbony” notes alongside the American oak influence – common notes include raisins, honey, sultana, vanilla, cereal notes (such as rye, depending on the type of bourbon barrel used) and raw sugar. Bourbon matured whiskies also tend to have a golden colour, noting of course caramel may be added to enhance colour. 

Whisky matured in ex-sherry barrels 


Contrary to what you might think, not all sherry is stored in Spanish oak. Some sherry makers store their sherry in American oak, and then pass on the used sherry casks to whisky makers who put their whisky in those used sherry casks. Apart from flavours from the American oak, ex-sherry American oak casks can give a whisky a more fruit cake and chocolaty flavour profile with peel and spice, though the flavours do seem to vary depending on the type of sherry that was used, whether that is Oloroso, Pedro Ximiniez, Fino etc, the interaction between the sherry used and the distillery’s malt character. It might offer nuts, prune, cherries, Christmas cake, raisins, sultana or dried fruit. It might be dry (Fino), medium-dry (Oloroso) or sweet (PX).  Sherry matured whiskies also tend to have a reddish or brown hued colour, noting of course caramel may be added to enhance colour. This is not always the case however.

=glenlovert oloroso

Whisky can also be matured in ex-wine, ex-port, ex-brandy, ex-rum barrels, basically any oak barrel that has previously held something delicious. The two most popular ones are however ex-bourbon and ex-sherry barrels.

7  Bringing it all together: distillery character + oak influence over time = whisky

As discussed above, it is clear that a whisky’s flavour profile is made up of two different main elements: (1) distillery character; and (2) oak influence. Let’s further explore these two concepts. 

Distillery character

Distillery character from the distillate makes up the first main element in a whisky’s flavour profile. That flavour comes from the kind of barley that is used to make the wort, whether or not the barley is dried using peat smoke, the kind of yeast that is used to ferment the wort into a wash, and, the cuts that are made when distilling the wash into “new make” spirit (all explained above). 

Typical distillery characters would mainly be made up of the sweet fruity esters that are produced when fermenting wort into wash, and, then concentrated by distilling the wash into “new make” spirit. Let me give some examples, by referring to the distillery character which I tend to perceive in some expressions of three Scotch whisky distilleries: Glenfiddich, Glengoyne, and, Bruichladdich. 

Glenfiddich tends to have (in my opinion) a distillery character that is dominated by apple and pear flavours. Glengoyne, however, tends to (in my opinion) tailor its new make to have more toffee and honey characters alongside the fruity esters and resinous cereals. The distillery character of Bruichladdich is very hard to pin down because the distillery uses different types of barley (such as barley from Islay as opposed to barley from the mainland of Scotland, for example). However, when we look at the flavour profile of the Bruichladdich Islay Barley (tasting notes are above) we can see that the “new make” seems to have drawn out not only sweet honeys but also earthiness and coastal salinity (or, something that tastes like salt). So, what we have here is the “bare bones” of the whisky – the distillery character – before the “new make” is placed into oak. 

Oak influence from time in oak

Oak influence makes up the second main element in a whisky’s flavour profile. As explained above, a whisky gets most of its flavour from the oak in which it matures (conservative estimates would be that whisky gets upwards of 60% of its flavour from oak). So, when “new make” rich in certain flavour compounds that tend to make up a distillery character are put in oak that “new make” begins to draw out even more flavour from the oak. The main oak types used in whisky production, and the flavours that tend to be drawn out of these oak types by “new make”, are discussed above. 

As you can imagine, the more time a whisky spends in oak the more likely it is that its distillery character will gradually fade to the flavour of the oak (and whatever was stored in the oak before the whisky was put into the oak, whether sherry, port, bourbon etc). 

Bringing it all together: distillery character + oak influence over time = whisky

Let’s explore three magnificent expressions – one from Glenfiddich, one from Glengoyne and one from Bruichladdich – to make some more sense of how oak infuses new make with oak flavours.

1. Glenfiddich 12 year old

gf12In my review of Glenfiddich 12 year old it is evident that my nose and taste buds could detect lots of apple. When pouring Glenfiddich 12 year old into a glass, I could smell the lovely apple from arms length away. That apple, clearly, is from the distillate. There was also spice, vanilla and toffee alongside faint sherry in the background, suggesting American oak and sherry wood was used to mature the whisky and add more layers to Glenfiddich’s apple rich core. Vanilla, for example, is a classic sign of American oak. Some of that honeyed character may also be from the malt, too. It is hard to know for certain, but we can roughly break down the flavours and take a good guess where they have come from. 

2. Glengoyne Cask Strength 

Glengoyne CSIn my review of Glengoyne Cask Strength it is clear that I detected in the whisky creamy crushed biscuits and wholemeal notes with cooked apple/pear and sweet ethanol, hints of spice, English style overproof rum, sweet mint, raisin/fruit cake, chocolate and cocoa, and, sherry. It seems likely that the cereal, and obviously the sweet honeyed ethanol as well as the fruity apple and pear, would probably be remnants of the distillate after maturing in the oak. Those sugary notes, and that lovely spice along with the raisin/fruit cake, are typical of sherry wood. There may be a fusion of character from the distillate and the oak that gave off the rummy notes; as opposed to stronger sugary sherry notes mainly from the cask.

3. Bruichladdich Octomore 7.3 Islay Barley 

Octomore-07.3In my review of Bruichladdich Octomore 7.3 Islay Barley, I describe this whisky as a “peaty sea monster”, and give it descriptors such as toffee apple, fudge, vanilla, tobacco etc. We know that the whisky is made from Islay barley that has been dried using peat smoke. That is where it gets its smoky peat flavours, and, probably, also its sea spray and salt notes. The Glenfiddich and Glengoyne expressions just mentioned, in contrast, offer no peat smoke. There is also that apple, which probably comes from the distillate. Then we have flavours that tend to be associated more with American oak, so on top of those peaty maritime flavours we get vanillas, tobacco, toffee etc. Obviously, it is hard to know with absolute certainty what flavours would come from where, but this may be a decent guess.  

To summarise, we can see that whisky is a muddled mix of flavours from the distillate (i.e: apple in the Glenfiddich, peat and sea spray in the Octomore), the oak (i.e: vanilla from American oak, raisin/fruit cake from sherry wood), and the marriage of distillate and oak. 

Slainte / Cheers / Salute

11 thoughts on “How is whisky made and where does its flavour come from? Distilling and Maturing whisky

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