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Barrel Aging



Scotch is truly the world's whisky. While American whiskey may be the go-to whiskey for cocktails, if you order a whiskey just about anywhere outside of North America, you'll get a scotch (and the whisky won't be spelled with an "e"). 


Scotch whisky can only be made in Scotland.  Its primary flavoring grain is barley, though wheat and corn play important roles as well.  Scotch is commonly known for having a smoky, campfire-like flavor.  This comes from burning peat, which is essentially young coal found in wetlands, (more on this below) to dry the barley.   This why you sometimes hear scotch described as “peaty”.   But not every scotch tastes smoky.  In fact, most don't.  Some are very subtle with notes of honey, flowers, and dried fruit.  And those that are smoky, vary greatly in intensity.


Single Malt vs. Blended Scotch

There are two main types of Scotch: single malt scotch and blended scotch, though there are others.  Single malts are made from 100% malted barley and all the whiskey must comes from one distillery.  They are generally more intense tasting, aged for longer, and more expensive.  Blended scotches are a blend of 15-40% single malt scotch and 60-85% grain whiskey (technically called single grain scotch) which is typically made mostly from wheat and much lighter and cheaper to produce.  Thus, blended scotch tends to have a milder flavor profile that's easier to drink and is less expensive, making them great for mixing into cocktails.  


Given these definitions, it’s tempting to view single malts as superior, purer expressions of scotch and blends as second-tier, budget options only fit for mixing, but that’s a misguided assumption (and one I, admittedly, once held).  They are apples and oranges. Single malt scotch represents the singular expression of a distillery.  It’s meant to be more pointed and fuller flavored.  A blend tries to bring multiple scotches and flavors into balance.  That's why it can work better for mixing; it plays well with others.



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Other Scotch Categories


  • Blended Malt Scotch - A blend of single malts, but from different distilleries.  This was once also referred to as a vatted malt or pure malt.

  • Single Grain Scotch - Made from primarily wheat and sometimes corn, all in one distillery, like single malt scotch.  Mostly used for blended scotch.

  • Blended Grain Scotch - A blend of single grain scotches, but from different distilleries, like a blended malt.










Scotch in Cocktails

Scotch is a challenging spirit to mix with.  It can easily clash with or downright dominate other ingredients.  But when you get it right, Scotch cocktails don't just succeed, they thrive.  As is usually the case, it's best to start with the classics.  My favorites are the Rob Roy (a scotch Manhattan), the Bobby Burns (a Rob Roy with a little Benedictine) and Sammy Ross’ gingery modern classic, the Penicillin.

How Scotch is Made

In addition to the divide between single malts and blends, scotch offers a diverse range of flavors and styles, far wider than other whiskey categories.  Between different regions, different aging casks, the use of peat - or lack thereof, and a host of other variables, scotch production can take many twists and turns.  Here we’ll dig deeper into how these all impact what's inside the bottle.


Base Ingredient: Malted Barely

Malted barley is the bedrock grain of scotch. It is used exclusively in single malts and is present in smaller quantities in grain whiskeys. Barely creates a distillate that is a bit drier than other grains. Malting is a technique that means the barley is steeped in water to convince it that it is time germinate.  This causes the starches to break down into simple sugars so the yeast can consume and convert it into alcohol. More on this here.  In addition to scotch, malted barely is used in beer, where it is also known as malt, as well as in whiskeys all over the world.  


Wheat and Corn

Wheat is the primary grain used to make grain whiskey for blends.  It supplanted corn in the 1980s, which is still sometimes used. Both grains create distillates that are lighter and a bit sweeter which is perfect for blended scotch.


The smoky campfire flavor that accompanies some scotches comes from peat, which is partially decomposed plant matter found in bogs and wetlands that is basically young coal.  But coal takes 300 million years to form, peat only takes a few thousand.  


Peat makes its way into scotch in the final step in the malting process when the barley is dried in a kiln to halt germination.  Sometimes this kiln is fueled with peat.  Burning peat essentially seasons the smoke which penetrates the barely, imbuing a flavor profile that it will carry all the way to the bottle.  Scotches that incorporate peat, are referred to as "peated" or "peaty".


Peated whiskey originated, and is most common, in Scotland where for hundreds of years it was a vital source of fuel for rural areas.  Originally, it was used to stoke the barley kilns purely out of convenience.  But even after modern transportation made coal widely available, peat continued to be used in whisky production because many had come to love its distinctive flavors and associate them directly with scotch.  Peat had become a proprietary characteristic. 

Though Scotch is best known for peated whiskys, they do exist elsewhere, including Japan, Ireland, a few in the U.S. and beyond.

Phenols and PPM

Peat is cut from the earth in bricks, which are then dried. The flavors it will impart will vary depending on how deeply the peat was cut - some areas it goes down over 20 feet - and where it formed.  Different vegetation will develop different peat with different flavors.  The type of compounds that carry peat's flavor are called phenolic compounds. The concentration of phenolic compounds in a scotch are measured in ppm (phenolic parts per million) which will give you a gauge on how smoky it will be.  While ppm is not generally listed on the bottle, some brands will promote their higher levels of ppm.  A very peaty scotch with have a ppm in the 30-50 range. Bruichladdich's Octomore has a bottling that proudly owns the highest level at 309.  But I have no doubt that another brand will eventually top that, in the spirit of one-upmanship.


Scotch fermentation typically takes 2-3 days, though some go for longer to create more esters and denser fresh fruit flavors.  After fermentation, it's time for the still.


Single malt scotch must be distilled at least twice in copper pot stills, a few do three runs. Grain whisky is almost always made in a column still to maximize volume and create a lighter spirit that will serve as the backbone of a blend.  Each still accomplishes what is required of each style, but most of the romance in scotch distillation occurs in the pot still.

The Romance of the Pot Still 

There are a slew of variations on how a pot still can be constructed all of which are meticulously take into account by Single malt scotch distillers, as well as fanatics. The size of the still, how it’s heated, how long the neck is, the angle the lyne arm, which type of condenser is used, and a host of other factors are considered (more details on how a Pot Still works can be found here).  There are famous accounts of distillers going so far as to recreate a particular dent in a new still to mimic the old one because they don’t want to lose a certain characteristic the old dent might have been contributing.


By law scotch only can be distilled up to 94.8% ABV, which is about the maximum proof possible.  Some column-distilled grain whisky may go that high, but most single malts will be distilled between 65-75% ABV.


All these variables are aimed at cultivating the desired congeners to produce the type of scotch distillate a distiller is after, be it heavier, lighter, richer, oilier, more delicate, or what have you (more about congers and how they impact a spirit's flavor can be found here).  The distiller will then make a cut (how much and the head, heart, and tails of the run to use). From there, the spirit will go into a barrel and undergo another transformation.

A row of copper pot stills ready to pump out some scotch.

Courtesy of Wikimedia Commons

Maturation & Barrel Aging

By many estimations, the barrel is where a scotch receives 70% of its flavors.  The rules surrounding scotch barrel aging are pretty much open season, which is curious considering many most other areas of production are tightly regulated.  The wide variety of aging options is a primary reason for why scotch is the most diverse whisky category in the world. 

In addition to scotch, the information below also mostly applies to Irish and other international malt whiskeys.  For a deeper look into the scientific inner workings of barrel aging visit the maturation page.

Used Barrels

The only official aging requirements for scotch are that it must be aged for at least 3 years in oak barrels. Any kind of barrel, or combination of barrels, can be used.  Over 90% of scotches are aged in are ex-bourbon casks.  The other 10% are largely ex-sherry, which used to be much more common and is still utilized by many high profile single malts, including Macallan.  

When I first heard about scotch being aged in recycled bourbon barrels I assumed those casks would be jam-packed with extra flavor because of the bourbon-soaked wood.  But the reality is used barrels actually impart less flavor than a new barrel, more about why this is here. But this isn't a bad thing.  The less aggressive flavors in used barrels is one reason why scotch can be aged for longer periods without becoming dominated by oakiness, allowing other subtitles to come through.


Ex-Bourbon Casks

Ex-bourbon barrels are so common because bourbon by law can only be aged in brand new oak barrels, so there are plenty of barrels leftover.   A new American oak barrels impart flavors like coconut, vanilla, and toffee, aka the primary flavors found in bourbon.  A used American oak barrel will still impart these characteristics, but less dramatically and they will be a bit drier overall. 


Ex-Sherry Casks

A scotch aged in a sherry cask - which used to be predominantly made from European oak, but today mostly American oak - is very distinctive. You can expect rich flavors of dried fruit and a hint of sherry’s signature nuttiness which becomes increasingly apparent the older the scotch gets.  Some would say these scotches are sweeter, but I think it's the suggestive sweetness of the fruit flavors rather than actual sugar content.   Macallan is a quintessential example of sherry cask scotch.  

Other Barrel Options

The options don’t stop there.  Some scotches are aged in multiple barrels. Balvenie Doublewood is a marriage of bourbon and sherry casks.  Others are “finished” in a barrel for a few months up to a couple of years to add a final layer of complexity.  Rum, port, Madeira, and sherry casks, among others, are commonly used for finishing.

Time and Price-point

Scotch has the capacity to age longer than most whiskeys, which is one of the primary reasons it is so remarkable.  Though the legal minimum age is 3 years, most single are at least 8-10 years old and plenty stretch into the upper teens and beyond.


One reason for this is the used barrels, as mentioned above. Another factor is Scotland's climate, which ranges from cold and wet to temperate and humid over the course of a year.  This causes the whisky to mature slowly.  More about how climate affects barrel aging here.


This longer aging is a big reason why scotch is so expensive, time is money of course.  But as is the case across all spirit categories, older scotches aren't necessarily better.   Of course, they can be incredible.  The best Scotches possess remarkably deep and subtle complexities that can only be achieved through prolonged maturation.  But I’ve known plenty of cases where I've preferred 12-year-old scotch to its 18 or 20-year-old counterpart.  These “younger” scotches often have more vibrant flavors that speak to the spirit's true quality.  A Scotch with a lot of wood may be smooth and rich, but it may also ultimately be less interesting. 


So yes, age is important.  But the number on the bottle is not the be all and end all of quality markers.  


Blending scotches, whether it's malt and grain whiskeys, all malt, or all grain whiskey, is a form of artistry all unto itself.  A popular metaphor likens it to a composer constructing a symphony, with each scotch representing a different section in the orchestra, trying to bring them together in perfect harmony.  Sometimes a blend will contain over 30 whiskeys of varying regions, distilleries and age statements. 

You can read more about blended scotches, their different styles, including brand recommendations here.


Scotch may have coloring added, and many do. Distiller’s caramel, aka E150A, is typically used.  This is surprising to many, it was to me.  But it does affect flavor or add any sweetness.  It's just to give the whisky the darker appearance that one would expect from a spirit that spends as long in a barrel as scotch does.   You see, recycled barrels impart less color.  Bourbon with their new barrels is the color of rich mahogany after about a year.   Scotch takes longer to get there.



Most scotch is bottled at or around 40-43% ABV, a few degrees lower than American whiskey’s average of about 45%.   Though some scotches are sold at cask strength which goes as high as 60% ABV.

Who Makes and/or Owns the Brand?

As consumers, all we have to identify a scotch by the name on the label, which is simple enough. But if you want to know who is actually making the product, where they are making it and who owns it, it’s not always so clear.  


Brands: Distilleries vs Blenders

One way to begin to parse it out is with single malts.  In those cases, the name on the label and the distillery are the same.  The distillery literally is the brand.  If you look at a map of scotch distilleries, you’ll see the names of your favorite single malts dotting the landscape.  With blends, the name on the label just a brand, there’s no, say, Johnny Walker distillery.   Though you could visit the distilleries where parts of Johhny Walker are made, like the Cardhu distillery in Speyside.  There are also plenty of distilleries that don't make single malts, but rather grain whisky to be used in blends.


I have to say, I find it very comforting that each single malt belongs to one specific distillery. It's probably my favorite thing about the scotch industry.  American whiskey, by contrast, is highly conglomerated.  Most of the brands you know come from one of only eight distilleries, as shown in this diagram here.


The Corporate Owners

That all being said, all these single malt distilleries are not operating independently.  Most of them, as well as blended brands, are usually owned by a larger company.  For example, Diageo owns the Johnnie Walker brand and the Lagavulin, Talisker and Oban distilleries.  A list of corporate owners of distilleries can be found here.



An look at the single malt distilleries on Isaly.

Independent Bottlers

Finally, there are the independent bottlers. These producers purchase scotch from a distillery and age (sometimes), blend (sometimes), and bottle it under their own label, usually listing the distillery where it originated.  These are a source of controversy within scotch aficionado corners because they don’t always play by the rules.  Though I think a little boundary-pushing can be healthy, particularly in categories as old and staunchly tradition-steeped as this scotch. 


One example of an independent bottler that’s I think is a force good in the scotch industry is Compass Box. It was started by John Glaser, who used to be the International Marketing Director at Johnnie Walker.  He uses a lot of unconventional aging and blending techniques and offers a dizzying array of bottlings which are all interesting and delicious. His company is also completely transparent about its operation.  So much so that they’ve technically broken the law with their honesty. But Compass Box is pushing back, advocating for more freedom for transparency within the scotch industry, which I find very inspiring.  Knowing more about what’s in our glass of scotch and how it got there would be better for everyone, especially us nerds. 

Independent Bottlers
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