Source of Whisky’s Flavor

barrelsOne of the first questions newcomers to whisky tend to ask is “why does this particular brand/expression taste this way”?

It’s an understandable question. But a much better one is “why does this particular expression of whisky continue to taste the same way over time”?

The first question can lead you astray (and vulnerable to all sorts of unwarranted “answers” out there), whereas the second gets you to closer to understanding the true process.

Terroir (or, The Great Misunderstanding)

One my biggest beefs with the whisky industry (writ large) is how they like to over-emphasize the idea of physical “terroir.” Now, this concept is quite valuable when considering how another agriculturally-sourced alcoholic beverage is produced: wine. Wine is definitely about which grape variety is grown in which climatic area, with what available nutrients (sun, water, soil quality, etc.). Certainly, wine-making process matters intensely as well. But ultimately, the grape source and local environment present during its growth has a large influence on the final product. This is why there are good years and bad years in vintage-defined wine, thanks largely to how much rain and sun that vineyard experience in any given growing season.

But carefully thinking about the process with whisky reveals why that is not likely the case here. Whisky is a distilled product. Indeed, it is generally double or triple distilled (or somewhere in-between – it’s a long story). With the exception of American straight bourbon, which places a limit by law of distilling to no more than 160 proof (i.e., 80% alcohol), whisky distillers in other jurisdictions don’t typically have limits. They could distill the grain up to near-vodka levels of purity if they wanted (though they usually don’t). If they did, it wouldn’t really matter very much at all as to what the source material was.

This is the dreaded boogeyman of “neutral grain spirits” (i.e., neutral as to the source), for which some countries are unfairly accused (Canada being one of them at times). In practice, whisky distillers do like to keep some character of the source grain, and so do not distill all the way to true neutral spirit (the exception being the really cheap budget brands). But really, the underlying grain itself can be a relatively minor contributor to the flavour of the final product.

Just to be clear: I am NOT saying that the starting materials of whisky production don’t matter.  It is just that they are a far less important important source of overall flavour, relative to everything that comes next.

Major Sources of Whisky Flavour

Starting points important for whisky flavour are the fermentation and distillation processes.

Distillation shouldn’t be surprising, given the discussion of the differences between small batch pot distilling (“single malt whisky”) and continuous distilling (“grain whisky”) described here. The copper pot stills and the production methods used for malt whisky are critical for explaining many of the differences between the malt and grain whiskies.

Fermentation is one of the key sources of flavour compounds that persist into the final spirit. I won’t go into all the details here, but the chemistry behind whisky flavours (especially many of those “fruity” notes) is well described on the Whisky Science website.

That said, probably the greatest overall contributor to whisky flavour more generally is wood barrel aging. Indeed, without barrel aging, all you have is moonshine – not whisky. I’ve seen it estimated that 50-80% of a whisky’s overall flavour comes from the barrel aging process.

Those estimates for the role of barrel aging seem a little high, but that’s likely because  because they are referring to the predominant flavour components of the final whisky. The greatest diversity of individual flavour molecules likely still comes from the fermentation step (refined during distillation) – but the wood imparts a lot of bulk flavours.

I will explain the role of wood in more detail below. But for now, you can think of it as wood barrel aging sets the overall flavour profile of a whisky, while the fermentation and distillation methods explain a lot of the subtleties that differentiate similarly barrel-aged whiskies from one another.

So, is there a role of the underlying “terroir” of the source product more generally (i.e., type of barley and where it was grown, or geographical location of the distillery) in setting the flavour? Not so much.

peatThere are exceptions to that comment – the most obvious being the use of peat in the malting/kilning process. This is one “contaminant” of the source material that definitely introduces its own distinctive flavour profile into a whisky (thanks to the production of new compounds, especially phenols). And peat from different regions can even impart different signature characteristics to the final spirit.

As an aside, the “smokiness” of peated whiskies tends to decrease with extensive barrel aging, which gets back to one of the core functions of wood aging, as I’ll explain in a moment. Also, it seems the emphasis on heavy peat-smoke flavours in the final whiskies is also a relatively new preference among whisky drinkers – it wasn’t always so highly prized and emphasized historically. Of course, these flavours would always have been there for distilleries using peat, but it seems to have had a resurgence as something people like detecting in their whiskies now. As an interesting aside, the story of the recovery, analysis and recreation of the century-old samples of Shackleton’s Mackinlay Rare Highland Malt whisky from the Antarctic is a fascinating read (and the observation that it is surprisingly not very smokey). Here is a good scientific analysis of the actual samples.

Anyway, aside from that aspect of local peat, what really matters in terms of “terroir”? It’s not like the distilleries are growing barley outside their front doors – they buy in bulk from suppliers from all over. Yes, there are different varieties of barley that can be used – and whisky makers are careful in their selection (mainly to ensure they don’t introduce unwelcome components into the spirit). But it should be evident that this is not a major factor in whisky taste – and yet, believe it or not, I have even seen serious attempts at analysis talk about soil composition across Scotland! Again, geographic location is largely a red herring in our search for the underlying main contributors of whisky flavour.

Water, Water, Everywhere …

The same goes for water. The PR firms of most distillers like to make a big splash (yuk-yuk) of the “unique” source of water that supplies the distillery. This is usually a stream that runs near the distillery, with its purportedly distinctive set of minerals and characteristics. Many people take this quite seriously. Again, I’ve seen charts and plots of water mineral content across the Scottish Highlands, for example. This does get pretty laughable though when two or more nearby distilleries – using the same water source – produce dramatically different tasting whisky styles, and yet all point to the unique water properties!

distillery3Again, it is worth keeping in mind that whisky is distilled, which doesn’t seem to leave a lot of room for huge flavour effects from the water. There is obviously some water still left in the newmake distillate (or added back in before casking), and this could have an effect. But it is debatable how much this relatively low water level really influences the general taste of the final whisky, relative to all the other aspects that went into (and came out of) the still.

As an aside, some people like to “rescue” a greater role for water by pointing out that the relatively high proof whisky in the vatted casks eventually gets diluted down to a lower ABV before bottling. But with the exception of a tiny handful of distilleries in Scotland, these casks get sent off to commercial bottlers elsewhere who dilute it with filtered and deionized water. So much for water contribution to terroir at this step!

The Truth Shall Set You Free

It is worth examining for a moment why this largely unhelpful local environment-based terroir idea continues to hold such sway in the industry. No one expects honesty from the marketing and PR arms of major distillers (although some degree of shame would be nice). But why don’t all those whisky enthusiasts/reviewers – who typically know better – set the record straight?

Some do, although it is usually buried in the footnotes of their sites. Some instead go to great lengths to torture the English language so as to make it seem like they are supporting this largely bankrupt idea (when they know better). It is painful to watch the efforts some go to in an effort to slip out from under a direct falsehood (while none the less imparting the desired wrong impression). I can only assume they worry that the truth would somehow diminish the cachet of the product that they have their livelihoods invested in. They shouldn’t be – the fascinating world of whiskies can only be enhanced by a better understanding of what contributes to the unique flavours of this human-engineered product.

What’s wrong with admitting that it is the historical traditions of fermenting, distilling, barreling, and blending at the distillery in question that determines the main characteristics of whisky flavour – not what piece of rock is located within view of their front door?

If I seem to be getting a bit worked up here, let me turn to an appropriate agriculture metaphor from Thoreau:

If I boast more than is becoming, my excuse is that I brag for humanity rather than for myself; and my shortcomings and inconsistencies do not affect the truth of my statement. Notwithstanding much cant and hypocrisy – chaff which I find it difficult to separate from my wheat, but for which I am as sorry as any man – I will breathe freely and stretch myself in this respect … I will endeavour to speak a good word for the truth.

Why Barrels Matter

So, let’s get down to the essential question – why does wood barrel aging impart so many of the major flavour characteristics of whisky? Recall that by law, most jurisdictions require a significant period of barrel aging before the product is called whisky (otherwise, it is simply moonshine). And despite the relative resurgence recently of this so-called “white whisky”, it is the wood aging that really matters the most.

As a point of clarification, whiskies tend to be aged in various sized wood barrels (usually made of oak), often having contained other spirits previously (e.g. various types of wine, sherry, other whiskies, etc.). The barrels can also be called casks or butts, depending on their size and previous use. I will use many of these terms interchangeably below, but see this blog post at the Whiskey Reviewer for a good explanation of all the different barrel types and their origins.

barrels4The point is that wood is a complex natural product. Its tough structure keeps the whisky protected from the outside environment – but only to a degree, since it does “breathe” (i.e., allows gaseous exchange with the environment). It imparts new chemical compounds into the whisky – and helps filter out other less desirable ones, as described here. And the extended time in the barrel allows for interactions to occur within the whisky, forming new compounds and breaking down others.

Most people tend to focus on the extraction of compounds from the wood, but this is only one aspect to consider.  Note as well that the inside surface of the barrels are frequently charred or “toasted” – which basically refers to how much the luminal surface is burnt. The main role of this charring is to filter out unpleasant compounds in the whisky (especially sulphur). But it also provides a barrier from the wood that alters the rate of exchange both ways, favouring long-term storage for gradual aging (which experts seem to believe imparts the best flavour profile). Finally, the charring will also help introduce new flavour compounds into the whisky (e.g. various lignins).

So, to put it simply, wood helps remove some of the chemical “impurities” left behind in the rough distilled product, and imparts new ones. These compounds (called congeners) are left behind for a reason – to flavour the whisky. Otherwise, we’d all be enjoying vodka right now. The exchange process in wood is designed to take considerable time, and is dependent on the size of the barrel and the environmental conditions present during storage.

This does rescue a small role for local environment during barrel aging (i.e., gas exchange works both ways). But again, you have to apply some critical reasoning as to the likely magnitude of this effect. Contrast the relative influence of “sea air” near the distillery, for example against the actual chemical components left behind by the specific distillation methods, and the effect of physical contact for years with used wood containing previous spirits. The main point is simply access of air to the whisky, with the ensuing oxidation, evaporation and concentration that entails (i.e., the result of the Angel’s share, as they call it).

Size Matters (for Barrels Anyway)

It is worth noting that the size of the barrels/casks is similarly mandated in law – usually not to exceed 700 cubic liters. Most barrels are in fact 500L or 200-225L, although smaller ones are possible (e.g., so-called “quarter casks” – see this post for more info). The point here is one of surface area-to-volume: the smaller the cask, the faster the whisky will exchange with the wood – and vice-versa.

But it gets even more interesting than that, which brings us to our next point – diffusion. Typically, Scotch whiskies are matured in used barrels. This is not about simply encouraging recycling. Whisky producers long ago realized that the previous contents of a barrel alter the flavour profile of the new spirit. The reason for this is simple – there is still a lot of the old product left behind in the wood staves of the barrel! One estimate that I’ve seen is that up ~7L is left behind in the wood of a standard 500L barrel. Over time, that ~1% volume will mix and exchange with the new whisky, further altering its flavour. This is known as diffusion, is another critical feature of wood barrel storing (although honestly, this one happens much faster than the others described above).

This helps explain why ex-sherry butts (especially first-fill) impart such distinctive flavours (and colour) to the new whiskies they hold. Try adding 1% sherry (by volume) to your favourite single malt and watch/taste what happens! Now, most Scottish whiskies are aged in ex-bourbon barrels from the US (which, coincidentally, are mandated by law not to be re-used for bourbon production). This creates a very symbiotic industry, where the practices in one jurisdiction complement another. Sherry butts are often re-used many times, and are sometimes only used for final “finishing” of a whisky that has spent most of its time in ex-bourbon barrels.

Of course, there is a lot more than just simple extraction and diffusion going on in barrels. Barrel aging is complex, with a lot of variables at play in determining the final flavour of the whisky.  The experience of tropical environments on whisky making is particularly interesting, where the higher temperatures accelerate all the chemical reactions that take place during maturation (though not consistently).

I hope the above begins to give you a general feel for why whisky has the characteristics it has. To be clear, I am not discounting all effects of local terroir for the grain or water used in whisky production. Instead, I am trying to point out that these are relatively minor players in setting the major flavours in whisky – which mainly come down to distillation method, choice of barrels for maturation, and blending.

The next stage in understanding how a consistent whisky product finds its way to your local store shelves is explained on my Single Malts vs Blends page.

Further Reading

For a further discussion of the role of wood in whisky aging, you can check out this page of the Beginner’s Guide on the Malt Madness website. The Science Whisky blog has good pages on the underlying source of flavours after distilling and through barrel aging. And of course, I will be providing an explanation of how to categorize whisky flavours on my flavour classification page.


  • Hi selfbuilt.
    I have read this post, and it is very interesting and timely. However I do find some statements not to be correct, or at least inaccurate, and oversimplified.
    You state:
    “With the exception of American straight bourbon, which places a limit by law of distilling to no more than 160 proof (i.e., 80% alcohol), whisky distillers in other jurisdictions don’t typically have limits. They could distill the grain up to near-vodka levels of purity if they wanted (though they usually don’t).”
    The Scotch Whisky Regulations 2009 ( clearly state an upper limit to the distillate:
    (b)that has been distilled at an alcoholic strength by volume of less than 94.8 per cent so that the distillate has an aroma and taste derived from the raw materials used in, and the method of, its production;
    For single malt Scotch Ben Nevis distillery has the newmake with the highest alcohol content in Scotland at 80% ABV. The rest are below this.
    “Again, it is worth keeping in mind that whisky is distilled, which doesn’t leave a lot of room for huge flavour effects of the water.”
    “But with the exception of a tiny handful of distilleries in Scotland, these casks get sent off to commercial bottlers elsewhere who dilute it with filtered and deionized water. So much for water contribution to terroir!”
    It is worth to mention that newmake even at 80% ABV contain 20% water. Obviously the reduction of the newmake to casking strength ie 63,5% ABV, needs water, and most, I think, (not a handful) distilleries cask their newmake for their single malts at the production site.
    So local water may indeed play a role in flavour.
    “The heavy emphasis on peat-smoke flavours is also a relatively new preference among whisky drinkers – it wasn’t always so highly prized and emphasized historically.”
    Historically the malt drying prosess used peat as their main source of energy.
    “Under Scottish law, all whisky must be produced in used barrels.” Check The Scotch Whisky Regulations 2009 again.

  • Thanks for your comments. I appreciate you taking the time, and am glad to clarify.

    On the issue of the upper limit of the distillate, the “94.8%” under Scotch Whisky Regulations is not actually a choice to limit, but a recognition of physical chemistry principles (i.e., you can’t distill ethanol beyond ~95-96% purity). “Aroma” and “taste” are subjective, and not defined. As such, the regulation does indeed allow distillation up to what I called “near-vodka levels” of purity (i.e., they would only have to claim some minuscule amount of “flavour” below 94.8%). I hope none do this, but the regulations would allow them to get away it – which is the point I was making about the lack of strict regulations (i.e., the quoted statement above remains accurate). I do appreciate you including the link, it is good idea to include here.

    On the issue of water, it is true that local water could play a role is whatever portion goes into the cask with the newmake. I agree that most distilleries likely cask their newmake on site. Of course, it is unclear if they filter that water first. It’s also unclear at what strength they all cask (since an upper limit is not specified in the regulations, unfortunately). But fair enough that I will put a point to that effect in the post above. My main point is that local water has no effect at the time of dilution to bottle strength, since that is done remotely. And more generally, I am trying to get across that water is only one aspect of what goes into (and comes out of) the still, and its debatable how much it particularly influences final flavour.

    On the issue of peat, I agree that it was indeed the main source of heat energy for many distilleries, and thus long present in many Scotch whiskies. But my point in that sentence was that the modern emphasis on heavy peat-smoke flavours in the final whisky is relatively new. Based on historical descriptions, it appears that peated whiskies were milder in smoke flavour previously, and this is something makers go out of their way to emphasize now.

    On the issue of used barrels, that is a good point – I was going by secondary sources for that statement. I don’t see it in the regulations, so I will remove it. As an aside, that is not surprising to me – again, the regulations are not overly strict in setting standards (which is an underlying theme here). But in trying to get down to what are the main flavour elements of whisky, barrel maturation is key.

    All of this gets back to my main thesis here – it is the people who make whisky that matter, using their historical processes and materials. In particular, it is the distillation method, barrel selection, and blending processes that are the main contributors to whisky flavour, and not the precise geographical location of the distillery. This point should be obvious, yet is frequently obfuscated by those who like to claim special status for minor contributing elements.

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