The interaction between wood and whisky is one of the most interesting, if not completely understood components of the whisky production process.
The following article is based primarily on a presentation entitled “A Wood Primer” that John Glaser of Compass Box Whisky created and was kind enough to share with me – much of the content of that presentation was based on the work of Dr. Jim Swan who is well known for his work as a consultant in the whisky industry and who is currently working with the Welsh Whisky Company in the production of Penderyn. Additional information came from a recent article by Ian Wisniewski entitled America – the Stave in issue 52 of Whisky Magazine; the work of Dr. Masaharu Minabe of Suntory; Graeme Richardson of William Grant & Sons; and Dave "Robbo" Robertson of the JMR Easy Drinking Whisky Company. Enjoy!
One of the most frequently asked questions is “Why do whisky makers use Oak?”
The reason that Oak is utilized is its unique physical and chemical nature. Oak has strength – physically, its wide radial rays give strength when shaped for a cask; Oak is also a "pure wood" as opposed to pine or rubber trees which contain resin canals that can pass strong flavors to maturing whisky.
But it’s not just the Oak itself, it’s the transformation that happens to the Oak as a result of the seasoning and heating treatments during the coopering process – these result in the production of pleasant-tasting Oak lactones.
Oak has three broad effects on the spirit:
- Additive – Adds organoleptically (a $0.50 word meaning the taste and aroma properties of a food or chemical) desirable elements from the cask. For example: vanillin, Oak lactone (coconut, bourbon character), toastiness, wood sugars and color.
- Subtractive – Removes undesirable elements from new make spirit. For example: sulphur compounds and immaturity.
- Interactive – Adds extractive wood elements from the cask and converts them to organoleptically desirable elements. For example: change tannins to acetals; change acetic acid to fruity esthers.
Dr. Swan defines 5 specific constituents of Oak and identifies how they influence maturing spirit:
- Cellulose – Which has virtually no effect other than to hold the wood together.
- Hemicellulose – Which consists of simple sugars that break down when heated and provide:
- Body: through the addition of wood sugars
- "Toasty & carmelised aromas & flavors"
- Color (unaged or "new make" whisky is a clear liquid)
- Lignin – The binding agent that hold the cellulose in wood together which, when heated yield:
- Sweet, smoky and spice aromas
- Oak Tannins* – Which play an essential role in maturation by enabling oxidation and the creation of delicate fragrance in spirits. Tannins combine with oxygen and other compounds in the spirit to form acetals over time. According to Dr. Swan, acetals:
Have a strongly ethereal influence on the product giving it delicacy and top-note…without it, spirits are dull and flat.
*Naturally occurring preservative compounds with a slightly puckery, astringent taste in the mouth, similar to the effect of strong black tea or fresh walnuts.
- Oak Lactones – Resulting from lipids in the Oak, they increase dramatically during toasting and charring and can pass on a strong woody and perhaps coconut character; lactones give bourbon its distinctive character; and occur in higher concentrations in American Oak than in European varieties.
Will any Oak do?
So any Oak tree can be used when making a whisky barrel? No. Of the hundreds of Oak species, just three species are used for wine and whisky cooperage:
Quercus Alba, “White Oak” (America)
- Commonly referred to as “American Oak”
- The most commonly used variety in whisky cooperage
- More vanillin than European varieties
- Fast growth
- High in lactones, which when toasted, provide woody, vanilla, and coconut flavors
Quercus Petraea, “Sessile Oak” (Europe)
- Found across Europe, notably in France
- Most commonly used for wine cooperage
- Slow growth, fine tannins and more vanilla (compared to Pedunculate)
- Most common species in Tronçais forest
Quercus Robur, “Pedunculate Oak” (Europe)
- Found across Europe
- Spanish Oak generates more raisin, prune-like flavors
- Most commonly used for cognac and sherry cooperage
- Fast growth, more tannins, thus more oxidative characteristics in the matured products (compared to Sessile)
- Most common species in Limousin forest
Now that that’s out of the way, there are a number of other factors in how wood affects whisky. Chief among them are:
- Growth rate of the "donor trees";
- Method and length of time to dry the wood;
- Toasting and charring during cooperage.
Impact of Oak Growth Rate: Slower is Better
Winemakers are convinced of the relationship between Oak growth rates and the flavor and quality of their wines; while in whisky, this factor is not widely considered. It is known that slow growth Oak has more of the “good stuff” – especially vanillins and Oak lactones. White Oak is "fast-growth."
Tis’ the season
Once the wood is cut, the method used to season (dry) the wood has a huge impact. The wood MUST be dried before being used to make barrels – the drying process converts chemical compounds in the wood to more desirable types. How the wood is dried and for how long has a direct impact on the quality of the spirit.
It’s accepted that air seasoning is better than kiln drying (it reduces tannic astringency as well as releases more vanillin), yet, while the barrels used to age wine may be made of staves which have been air dried for as much as 24 months – most bourbon barrels are made from wood which has been kiln dried in a matter of weeks.
Why? Some distillers think that the method for drying the wood is only important for the first-fill of a spirit aged in a new cask, (e.g., wine or bourbon) and has little or no impact when maturing spirits in previously used casks – and of course, Scotch is aged in previously used casks.
The Heat is On
The application of heat is integral to the process of making the barrel – wood fibers behave much like plastic polymers – they want to be straight. In order to bend the staves, they need to be heated. The straight staves are arranged inside a metal hoop and heated. I have heard that either an open flame or steam may be used. As they are heated they become more pliable and are shaped – hoops of various diameters are added to each end – six in total – which are hammered down, towards the middle. Each hoop is held in place by the pressure exerted by the staves as they try to straighten themselves. The casks are then toasted which caramelizes the wood sugars.
This is where the construction of bourbon casks and sherry casks diverge.
Bourbon Vs. Sherry
The barrels, once formed, are charred – the inside of the cask is set on fire for a short period of time, which creates a black charred layer. There are various levels of charring which will have different affects on the spectrum of compounds and
flavors the Oak will impart to the maturing spirit: more vanillins,
lactones, "toastiness," spice characters, and tannins.
Charring casks causes further transformation. Char (carbon) removes sulphur compounds and immaturity from new spirit. Bourbon casks are typically charred for 40 seconds to 1 minute, but some distilleries have experimented with charring times of up to 3-4 minutes. The result of charring is dramatic changes on the surface – for example, wood sugars are caramelized, which will leech into the maturing spirit.
Sherry casks are only toasted and not charred. The casks used to mature Oloroso are the most popular with the Scotch industry. Sherry casks can be made of American Oak, but this is usually for Fino Sherries and are generally not used by the Scotch industry. It’s accepted that European Oak adds more flavor than American Oak – sherry cask matured whiskies tend to be more full-bodied than bourbon cask matured ones, and this is likely the result of the type of wood, just as much as the type previous liquid occupant.
The wide-spread use of bourbon barrels is a fairly recent occurrence – a result of the difficulty in sourcing sherry casks during the Spanish civil war in the late 1930’s. Currently any where from 300,000 – 400,000 bourbon casks are acquired for use in the maturation of Scotch whisky – in contrast to only about 18,000 sherry casks.
Contrary to popular belief, very few whiskies are aged exclusively in bourbon barrels – most ex-bourbon aged malts are vatted with a (varying) percentage of whisky which was aged in ex-sherry barrels. Laphroaig, Glemorangie 10, Ardbeg 10, Glenlivet 12, are among those few "pure" ex-bourbon matured whiskies.
It’s not the size of the cask (or is it?)
There are three cask commonly used by the Scotch whisky industry:
- Barrels – 190 liters/50 gallons
- Hogsheads – 250 liters/66 gallons
- Butts – 500 liters/132 gallons
Butts come from the sherry industry while the majority of barrels and hogsheads originate in the bourbon industry. All things being equal, the larger the cask the slower the maturation. Conversely, a smaller cask means that the maturing whisky is exposed to more wood and maturation is quicker – the Laphroaig quarter cask is an example of this.
One last thing
Once a bourbon cask has completed its "first life" that is, it has been used to age bourbon, it is ready for its second life as a whisky aging vessel. It is broken back down into separate staves and shipped to Scotland. In Scotland, coopers reassemble the staves into casks which will be used to age the whisky that you will enjoy in a few years. Some bourbon casks and all sherry casks are generally shipped whole – not broken down into separate staves.
It’s not common, but some companies re-char ex-bourbon casks before use.
Casks may be used for as many as four fills, i.e., filled with four separate batches of new make spirit. Generally, though, casks are retired after their second, or third re-fills. Sometimes when a cask has reached the end of it’s their useful life – after it has been filled and re-filled so many times that the spirit has taken out all the "good stuff" from the wood, some distillers will shave down the the inside of the cask to reach fresh wood and then the cask will be re-charred.
Below are pictures of this being done at a Diageo Cooperage in Carsebridge. The first picture shows the machine that shaves down the inside of the cask. The second picture shows the recharring – which lasts 30-45 seconds at this cooperage.
How ex-sherry casks are treated, once whisky distillers get their hands on them, differs by distiller. Most will empty the cask of any residual sherry, nose the cask (to ensure the casks smells fresh, and then fill with new spirit. Dave Robertson doesn’t believe any one would char fresh sherry casks unless the sherry cask does not smell "right", in which case they might char, or may simply reject the cask.
I’m sure that in the future I’ll delve more into the nuances of maturation, bottling, etc. But for now, I suggest that you pick up issue 52 of Whisky Magazine. In his article, Ian discusses the various charring techniques – focusing on "how the bourbon barrel influences the taste of whisky." A thoroughly interesting article.
As a matter of fact, if you live in the US and you don’t currently subscribe to Whisky Magazine, you may want to consider a subscription; especially since Paragraph publishing offers readers of The Scotch Blog a 22% discount off a new subscription. Simply go to Whisky Magazine and use the code BLOG1205 when placing your order.