A Bourbon Backbone

A Bourbon Backbone

Originally published in Unfiltered #74

Words: Lee ‘Connas’ Connor


Before we started hearing about ‘wood policies’ and ‘cask management’, whisky has always needed to be aged. For that, the most popular vessel of choice has, is, and for the foreseeable future always will be the humble refill bourbon hogshead. SMWS ambassador Lee ‘Connas’ Connor continues his maturation journey with a deep dive into the workhorse of the Scotch whisky industry


While fully accepting the important role that European oak (Quercus Robur) plays in the maturation of Scotch, we must acknowledge that without its American cousin (Quercus Alba) we simply would not have the vibrant, forward-thinking, international industry that we enjoy today.

According to the Scotch Whisky Association there are “some 22 million casks lying maturing in warehouses in Scotland waiting to be discovered — that is around 12 billion 70cl (700ml) bottles”. And for many reasons, not least that by law bourbon producers are only permitted to use the cask once, it’s fair to assume that up to 90 per cent of those casks originally came from the US. Put simply — in terms of maturation — the bourbon cask is the vessel on which the Scotch producer sails. That vessel also provides low levels of tannins and astringency, and high levels of cis-oak lactones, resulting in the coconut and gorse flavours intrinsically linked to the experience of imbibing Scotch all over the world.
When analysing the singular role that Quercus Alba plays in maturation, the first thing to note is that it is naturally disposed to coopering. This variety of oak tree grows relatively straight and contains few knots, making for a liquid-tight container less likely to form leaks. When it comes to implications for creation of colour and flavour, how effectively the cask is created in the cooperage plays a key role.

Manufacturing flavour

Once the staves are sawn to length and assembled at one end, the inside of the cask is slowly toasted over a period of 40 to 60 minutes to make it softer and easier to form into a finished cask. Although this process was originally for the purposes of physically forming a cask, we now know that specific control of toasting needs to be in place to ensure that the maximum amount of desirable flavour and colour are delivered.

Specifically, heat breaks down the naturally occurring lignin compound (which has a complex makeup, to say the least!) into smaller compounds such as guaiacol, vanillin and eugenol. Qualities such as raisin, fudge, vanilla, clove and wood spice are all contained therein. If toasting is completed efficiently, there is a better chance that these characteristics will contribute to the maturation.

A willing exorcist

Once the staves are assembled, bourbon casks are fired using a gas burner. An aggressive heat, applied very quickly, physically sets the staves alight. Once cooled, this leaves a crust of char above the layer of toasting previously created. This char cover is neutral in terms of active flavour compounds. However, the carbon skeleton formed within it acts to filter out some of the sulphury, sour, cereal and oily notes contained in the spirit — while happily leaving the more desirable fruity compounds intact.

Kentucky christening

As the name suggests, the first time the cask is used it is to mature bourbon. The alcohol content in the spirit goes to work breaking down the abundant flavour compounds in the new wood and seeps through the char and toast coverings back into the cask. The result is that some of the original more aggressive notes are tempered, and some of the potential bourbon lies in the pores of the oak before it is transported (usually intact) to Scotland.

As we know, here in the Scotch industry, it is permitted to fill and refill the cask as many times as we wish. But with each fill, the vessel will lose a certain amount of the potential influence it has over the spirit inside it. And although more recently various methods have been developed to rejuvenate and somewhat refresh used oak, there comes a point where it simply has no more to give. It is in the skill of the maker that we find the ability to find the right cask, for the right spirit, at the right time.

A culmination of craft

So, if you’re lucky enough to delve into Cask No. 88.24 When fruit & cream collide from this month’s Outturn, you may pick out notes of peaches, apricots, oranges or subtle hints of charred oak. That’s in no small part down to the skill of our Spirits Team in matching the spirit with the right bourbon cask — a refill hogshead, in this case — to both enhance the desirable characteristics and strip out the less enticing aspects from the original liquid at exactly the right time to deliver the optimum whisky. It’s not just luck!


Unfiltered Magazine is The Scotch Malt Whisky Society’s premium whisky knowledge magazine delivering quality content exclusively to members in an immersive multimedia format monthly. To view Unfiltered #74 in its entirety (as well as all back issues), log in and access the members’ portal or join The SMWS today — the world’s most colourful whisky club.


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