Wood Is Where the Magic Happens (Or Is It?)

It seems like all of the best things in life are aged in wood: Cognac; Armagnac; Tequila; Wine; Rum; etc. What is it about wood (oak in particular) that makes it so useful for aging alcohol? Well, oak is dense, yet porous. Oak is also just flexible enough to support seasonal expansion and contraction. What the oak provides is a way to allow alcohol to escape — very gradually — while imparting wonderful flavors to the liquid inside. In the case of Scotch, the imparted flavors strongly depend on what was in the container before.

We have to take a detour to America for a while. It turns out that making bourbon and rye whiskey has a lot in common with making Scotch whisky. Let’s be generous: There is a grain that provides carbohydrates to a solution that is then fermented with yeast before distillation, and the resulting spirit is then aged in oak barrels. And, importantly, the barrels used in bourbon production can only be used once. The ex-bourbon barrels are happily bought by Scotch manufacturers who use them to age their product. A Scotch that has been aged exclusively in ex-bourbon barrels will be lighter in color than one that has been aged in, for instance, a Sherry butt. (No matter how many times I see those two words together, I can’t help but remember a girl I knew in high school. Enough said.)

Sherry is a fortified wine that is produced in Spain and it imparts much darker colors and completely different flavors to the Scotch. As with any oak barrel, the oak’s role is to provide the flexible membrane through which the alcohol can slowly escape. The thing about these containers is that they are quite large compared to bourbon barrels. They hold about 3x the volume of liquid, and due to the geometry of the container, don’t have 3x the surface area exposed to the whisky. So the effect is that the whisky takes a longer time to acquire the effects of this wood when it’s in these containers.

What does Sherry have to do with Scotch? Well, it used to be common to ship alcohol in wood barrels. And the residents of the UK used to drink a lot of Sherry. So there were a lot of empty Sherry butts…perfectly good, but unused. It’s not too surprising that an opportunistic distiller took advantage of this situation.

I’m not sure when the bourbon law took effect that mandated the one-use system, but once that happened, the US had lots of excess bourbon barrels. They weren’t as convenient as the Sherry butts, but they are ideal for aging Scotch, so eventually a brisk export business was set up to provide empty bourbon barrels to Scotland. There is basically an an unending supply: As I wrote in an earlier posting, the US domestic whiskey market is also hot, in fact it is so hot that the coopers here are running out of mature oak trees from which to fashion bourbon barrels. We are almost literally drinking our oak forests! With all the bourbon being produced, as long as we can figure out how to grow more oak trees, there will always be enough barrels for Scotch producers.

The folks that actually work in the distillery would take issue with my title. It’s said that they make the spirit, and the oak makes the whisky. Well, yes, but most people don’t find new make spirit palatable. And on the other end of the scale, it’s certainly possible to have too much oak influence, which overwhelms the spirit. The master distiller and blender will test regularly to see when each cask is done aging. I almost can’t imagine a better job.

Now it’s late and I am tired, but I wanted to touch on sulphur. Or sulfur (both are correct, depending on where you live). Sherry butts don’t necessarily transport themselves to Scotland anymore. Most Sherry is transported in bottles these days. So the butts are in Spain, and since Spain is hot, there is a chance of spoilage. Since each of these butts is worth probably $1000, there is a desire to make sure they don’t spoil. One way to make sure they don’t spoil would be to get them to Scotland as quickly as possible. However, some combination of human laziness and inventiveness gave someone the bright idea to burn sulfur candles in containers to act as a preservative. If only sulfuric odors weren’t reminscent of rotten eggs, this might be ok, but those odors are NOT what you want in the flavor profile of a fine Scotch! I haven’t heard of this issue with the bourbon as much, perhaps because Sherry is much sweeter and therefore leaves a surface that is almost literally a perfect breeding ground for microorganisms. What I do know is that I don’t want sulfur in my Scotch. Some distilleries take a very proactive stance on this, and manage their barrel supply as if it really matters to them (it does!).

As I said earlier, the oak is flexible and porous. This is ideal in Scotland’s gently evolving seasons. As the days get longer, the warmer periods cause the barrels to expand, then contract at night. The very slow expansion and contraction oscillation of the barrels is a kind of subtle encouragement to the osmosis process that squeezes out the alcohol and/or water. In a humid environment like Scotland, the net result is that more alcohol than water escapes, diluting the strength of the whisky. In a drier place like Kentucky or Bangalore, the net result is that more water escapes, which strengthens the contents. And the latter places have more significant temperature swings so there is a more rapid aging process. What takes 30 years in Scotland might only take 5-7 years in Kentucky (or 4 years in Bangalore!).

Bruichladdich uses a variety of woods in a variety of ways. If I were more awake I could write about them, but I’ll sign off for now and add to this posting tomorrow.


The usage of the wood is in some sense related to the type of Scotch being produced. The master distiller has in mind a certain flavor profile for the finished product and knows that various casks each contribute a specific flavor profile (bourbon casks certainly yield lightly colored spirits with floral notes as well as notes of vanilla, caramel, and other baking references; sherry butts will contribute dark fruit or dried fruit like raisins, plums, figs, etc.; port casks that held port will contribute even stronger flavors and smells, like fruit cake or Christmas pudding; Scotch producers have been known to use rum casks and pretty much anything else you can imagine, with varying success).

The wood notes combine with the spirit notes and you can get other notes from the aging process, including iodine, seaweed, etc. After all, the warehouses at Bruichladdich are very close to Loch Indail. In fact, you might not know that all Bruichladdich spirit is produced, aged, and bottled right on Islay. No other Islay Scotch whisky distiller does this. They take Islay seriously: They even try to buy all their barley locally. Other notes like leather, tobacco, and other things definitely impact on my nose, but I don’t know whether they come from the wood, the char, the former contents, or the spirit, or some of the above.

Back to the wood: The size of the vessel matters, too — a smaller cask has proportionally more surface area exposed to the enclosed volume, so it makes its flavor and aroma contributions more quickly. Wood contributes so much to the whisky, but this is the sum of the source of the casks (i.e., type of oak, char level, etc.) and what they previously contained. The total package is what’s important. I mentioned char a couple times without defining it. Bourbon barrels are rather severely charred before use, creating a nice charcoal layer inside that acts as a filter. If you have a pool or a fish tank, you know that charcoal is used for filters and the char layer in the bourbon barrel provides the same role here.

Note that historically some casks were used more than one time by Scotch producers, and you can expect that the flavor contribution decreases with each use. That’s the other factor that’s important: Whether or not the cask is first-fill, fourth-fill, whatever.

One other final note about the “Angel’s Share” (the alcohol that escapes through the wood). Osmotic membranes allow a flow from one side to the other to balance the concentration. Since Scotland is humid, and since the inside of the barrel is humid, not much water is lost to the surrounding air. But since there is little alcohol in the surrounding air (well, normal air is not very alcoholic — but air in a whisky or wine warehouse is noticeably elevated in alcohol!), the net effect is that alcohol is expelled from the casks at a slow rate.

The initial fill of new make spirit is at about 70% ABV. Fortunately for us consumers, the kind folks at Bruichladdich bottle the product when it’s ready, before the angels take all of the good stuff, and still has a bit of “kick.” We consumers appreciate that. 🙂

Next time I’ll start to peel apart the wide variety of expressions made by Bruichladdich. It would be too difficult to take a bottom-up approach: There are too many variables that are being used to make the components of the final product — a previous posting had the quote from Mark Reynier where he said that they are running four stills with something like 8 different barleys at 4 different peating levels and all the resulting spirit is going into specific casks for prescribed amounts of time. Oh, and I forgot that they do double, triple, and quadruple distillation, too! At least a top-down approach has a shot at making sense of things. What I hope to achieve is a matrix or chart that shows which of the expressions are similar to each other. I have only my own interest at heart: I need to know how to branch out from what I know I like into stuff I might like as well.

Somewhere along the way I am also going to write about the actual equipment used in a distillery.



  bourbonfan wrote @

I believe the bourbon law around using casks only once took effect on May 4, 1964. Great blog so far – looking forward to continued reading !


  tmaufer wrote @

It might be bad form to comment on your own blog, but I’ll risk it. Thanks for the info on bourbon casks, and if you like this blog, please tell your friends! I’m having a lot of fun writing it.

[…] at 01:37 · Filed under Aging, BruichladdichBlog I’ve written about wood before. The previous article talked in detail about casks because they are important to so many different […]

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