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Making Single Malt Scotch Whisky is part science and part art. As a scientist, I rejoice in the artisan aspect of whisky-making and sincerely hope that the human senses and the human mind will forever be the final arbiter of sensory maturity and sensory excellence rather than some mechanical device. Nonetheless, over the past 50 years especially, science has revealed some of the mysteries of whisky making and exploded some of the unhelpful myths without, I hope, ruining the mystique and the majesty of Scotland’s Single Malts.
This section of Annandale’s website comprises a series of short Technical Notes on various aspects of Single Malt Scotch Whisky production, from barley through to the mature product. The topics are presented alphabetically, and the series will be expanded over the coming months.
I’m deeply grateful to the late Dr Jim Swan and to Dr Gordon Steele for educating me in the science of whisky making and to Mark Trainor (Head of Production at Annandale Distillery) for schooling me in the practical subtleties of making the very finest Single Malt Scotch Whisky.
Professor David Thomson
Co-Founder – Annandale Distillery
We’re often asked why we don’t mature our whisky in quarter casks (125 litres), octaves (50 litres), and firkins or blood tubs (~30-40 litres). (200 litre ex-bourbon barrels are the smallest casks we fill at Annandale Distillery.)
To address this question, we need to consider the role of oak in whisky maturation:
Oak adds to maturing whisky as well as taking something from it. If the cask has previously held bourbon, Scotch whisky, sherry, wine, port, rum, cognac, etc., some of this maturing liquid will have become impregnated in the staves (staves are narrow lengths of wood with chamfered edges that form the body of a wooden cask). This liquid remains, impregnated in the stave after the cask is emptied. When new-make spirit is filled into the emptied cask, some of the previous occupant will be extracted from the staves, adding flavour and colour to the maturing liquid, often quite quickly.
The maturing spirit also extracts various naturally compounds from the oak, including vanillin (vanilla/sweet/confectionery flavour), tannin (astringency and golden brown colour – as in black tea) and various sugars (principally xylose but also arabinose, galactose, ribose rhamnose and glucose).
Whisky casks are usually charred inside. Charring has a three-fold effect:
1.) It produces a layer of carbon on the inner surface of the stave. Carbon is very porous, which allows it to adsorb (as opposed to absorb) various compounds from the maturing spirit. Some of these adsorbed substances may be undesirable flavour and odour compounds.
2.) It increases the porosity of the surface of the stave which encourages deeper penetration of the spirit into the oak.
3.) It induces chemical reactions in the oak that are implicated in the conversion of hemicellulose (one of the principal structural components of oak) into the aforementioned sugars (although glucose arises from elsewhere).
The sum total of all of these effects is known as ‘cask effect’ (or sometimes ‘oak effect’).
Probably the most important consideration when choosing casks for whisky maturation, is the extent to which the distiller wishes the flavour of the final whisky to be influenced by cask effect. The nature and extent of the cask effect depends on three factors:
The ‘freshness’ of the oak (i.e. how often the cask has been filled previously and for how long). The fresher the oak, the greater its potency in terms of delivering vanillin, tannin, xylose and other oak components.
The nature of the previous occupant(s). Sherry is likely to have a fairly potent effect. However, if the previous occupant had been Scotch whisky, the impact would typically be much less.
The ratio of the inner surface area of the cask to the volume of liquid held within the cask. The smaller the cask, the greater the ratio of the inner surface area of oak to volume of liquid inside, as shown in the bottom row of the table below. The smaller this ratio, the bigger the cask effect.
Firkin - Volume/litre 40 | Surface area/square metre 0.82 | Volume/square metre (litre) 48.8
Barrel - Volume/litre 200 | Surface area/square metre 2.23 | Volume/square metre (litre) 89.7
Hogshead - Volume/litre 250 | Surface area/square metre 2.59 | Volume/square metre (litre) 96.5
Sherry Butt - Volume/litre 500 | Surface area/square metre 4.03 | Volume/square metre (litre) 124.1
By way of example, a 40 litre firkin has the equivalent of 48.8 litres of spirit per square metre of inner cask surface area (i.e. a ratio of ~50:1) whereas, with a 200 litre barrel, it’s 89.7 litres/square metre (~90:1). In other words, there’s much more oak contact in a firkin than in a barrel (x 1.84) and therefore the ‘cask effect’ is likely to be bigger. The difference between a barrel and a sherry butt is also considerable (89.7 versus 124.1 litres/square metre, respectively). There’s even a noteworthy difference between a barrel and a hogshead.
Make no mistake, cask effect is often used quite deliberately, as means of expeditiously bringing Scotch whisky to an acceptable level of sensory maturity, at an early age. There’s nothing wrong with this as such (in my opinion) provided the resulting whisky remains within the boundaries of what is an acceptable/credible Single Malt Scotch Whisky and provided the defining character of the distillery’s spirit is still evident. (These are the criteria applied at Annandale Distillery). However, it’s totally unacceptable if some sort of cask effect (e.g. ‘finished in Madeira wood’) is presented as an intriguing adjunct, when the real reason is to mask bad, imbalanced, rough or otherwise immature spirt. Sadly, this happens. I’m always very suspicious when multiple cask effects are used to finish a whisky, especially a young whisky.
Amongst purists, there’s even a point of view that once-used ex-bourbon barrels (i.e. barrels that have only been used once to mature bourbon) exert too much cask effect on the flavour of the whisky. Whilst this point of view may be a little extreme, it’s certainly possible to ‘over bourbon’ Scotch whisky (again, in my opinion).
To summarise, at one end of the spectrum, the most benign maturation vessel for Scotch whisky (i.e. with minimum cask effect) would be a several-times-used ex-bourbon cask (ideally of known provenance). The whisky may take longer to mature but the intrinsic character of the distillery’s spirit will surely shine through (warts and all). During the first three years of production, Annandale Distillery laid down ~50% of its stock in several-times-used ex-bourbon casks with a view to longer-term maturation. The other end of the cask effect spectrum would be represented by a never-used-before, 40 – 50 litre cask made from virgin Spanish oak and conditioned for two years with either oloroso of Pedro Ximinez sherry. In this case, it’s entirely possible that the maturing spirit will be ‘over-oaked’ (i.e. excessive cask effect) before reaching the legal 3 year minimum, and any evidence of distillery character will have been overwhelmed.
At Annandale Distillery, we’re entirely confident in the quality of our new-make spirit*, so we don’t want to (and we don’t need to) obliterate its intrinsic character using excessive cask effect. That’s why we don’t mature our whisky in small casks.
(*We’re so confident about the quality of our new make spirit that we bottle it at 63.5% ABV and sell it as Rascally Liquor®. Why not give it a try?)
Co-Founder – Annandale Distillery
The Scotch Whisky Regulations (2009) specify that Scotch Whisky can be matured in a variety of different casks provided that the cask is made of oak and provided that it is no more than 700 litres in volume. The reason why oak is specified in law is because it has a particular effect on the maturing spirit that gives Scotch Whisky its characteristic flavour (sensory characteristics). In essence, oakwood maturation is fundamental to the flavour of Scotch Whisky.
Most of the maturation casks used in Single Malt Scotch Whisky have previously matured either bourbon, Tennessee sour mash (e.g. Jack Daniels), cognac, brandy, rum, tequila, and a multitude of wines including various red wines, white wines, sherry, Port, Madeira, etc.
Oak has three very specific effects on the maturing spirit:
1.) Extraction – the spirit extracts various flavour components from the oak, predominantly, vanillin (vanilla, sweet), various lactones (coconut, cocoa, toasted, nutty, creamy, woody, maple) and tannin (astringency and colour).
2.) Extraction – the spirit extracts residues of the prior occupant – the barrel staves will have become impregnated with vestiges of the previous occupant.
3.)Absorption – the oak absorbs various undesirable flavour compounds from the maturing spirit (especially sulphur compounds produced during fermentation).
In other words, the cask both ‘gives’ and ‘takes’ from the maturing spirit. The balance of these three effects is critical to the flavour of Scotch Whisky.
Virgin oak casks (i.e. casks that haven’t been used previously to mature other spirits or wines) are rarely used in Scotch Whisky maturation, simply because of the potency of the virgin oak extracts. In contrast, the regulations governing bourbon production in the USA (Federal Standards of Identity for Distilled Spirits – 27 CFR 5) stipulate that charred virgin oak casks must be used for maturation. It’s the extracts from virgin oak that give bourbon much of its character. (If Scotch malt spirit was matured in virgin oak, it would eventually become very bourbon-like.) The US bourbon regulations create a mountain of once-used ex-bourbon casks, the vast majority of which are bought by the Scotch Whisky industry. Consequently, much of the Single Malt Scotch Whisky produced in Scotland has a ‘hint of bourbon’ due to further extraction of flavour components from the oak, as well as the extraction of residual bourbon from the barrel staves.
In more recent times, Scotch whisky distillers have become increasingly adventurous and innovative in their choice of oak casks (see above) to either mature or finish their whisky. These different cask types produce Single Malts with flavour profiles that are distinctly different from those matured in ex-bourbon barrels.
The STR is one such innovation! STRs are ex-Burgundy (red) wine casks that have been shaved (S), toasted (T) and re-charred (R). After a cask has held Burgundy for several years (possibly as much as 7 years), the oak barrel staves will have become heavily impregnated with the wine. If such a wine cask was subsequently filled with malt spirit, it’s possible that the resulting Single Malt would be too ‘winey’ in character, too red in colour and, because the inner surfaces of the cask may have been stripped of all of its flavour compounds, there could be too little oak character.
It was the late Dr Jim Swan, a great friend and guiding light to Annandale Distillery, in collaboration with Bodegas José Y Miguel Martín (Spain), who developed the idea of carefully shaving the inside surface of Burgundy casks (to a depth that’s a closely guarded secret) to remove some of the wine-impregnated oak, thereby bringing some of the active oak, held deeper in the stave, closer to the surface. The inner surface is then toasted to break down certain structural elements in oak (particularly lignin and hemicellulose) into sugars which are subsequently caramelised by the heat (via Maillard reactions). Toasting also releases a variety of potent flavour compounds including vanillin. In the Swan/Martin STR process, toasting involves the burning of oak chips milled from redundant wine barrel staves. The temperature and duration of toasting, and the precise source and mix oak chips, is another closely guarded secret. Finally, the inner surface of the barrel is charred on a gas burner to create a charcoal (crocodile skin) surface. This is essentially activated charcoal, which has a huge surface area (due to the charring), and phenomenal adsorptive capacity for soaking-up undesirable flavour compounds. (You’ve guessed it – the extent of charring is another commercial secret!)
Whilst cask rejuvenation via shaving and re-charring isn’t new, the ‘clever’ part of the Swan/Martin STR process is leaving enough residual ‘wine effect’ whilst reactivating the oak by toasting and re-charring. This is a very difficult balancing act. It’s also worth noting that thermal reactivation, whether it’s by toasting and/or re-charring, does not recreate exactly the flavour potential of virgin oak. In particular, lactones (coconut, cocoa, toasted, nutty, creamy, woody, maple), tannins (astringency and colour) and eugenol (cloves) are not recreated. Consequently, the balance of wood extractives in regenerated casks, such as STRs is rather different from those of a new cask, which makes them interesting and different.
The Swan/Martin STR process undoubtedly brings another flavour dimension to whisky maturation…but does it create great-tasting Single Malts? Jim Swan was heavily involved in the development of Kavalan Distillery in Taiwan. In 2015, Kavalan Solist (Vinho Cask) won the accolade of the ‘Best Single Malt Whisky in the World’ (World Whisky Awards). Working with the team at Kavalan, Jim matured Solist in Swan/Martin STRs! Solist is variously described as having extra-fruity notes (melon, mango, citrus, vanilla, oak spice and dates). Not bad considering just how young this whisky was at the time of the award!
In late January 2017, Jim Swan met with David Thomson and Teresa Church, Co-Founders of Annandale Distillery. We talked about wood policy and nosed a lot of Annandale’s peated and unpeated young, maturing spirit. Jim was convinced that our spirit would mature very well in his STRs. His parting words, as he left the distillery for yet another overseas consultancy trip, were…… “Don’t forget to order a consignment of STRs from Miguel Martin!” We invariably took Jim’s advice, and so 90, 230 litre STR hogsheads were ordered. Just over two weeks later, Jim passed away. We were broken-hearted.
Fast-forward three years and Annandale’s spirit, filled into Swan/Martin STRs, has metamorphosised into peated and unpeated Single Malt Scotch Whisky. Jim was right! The union between Annandale’s malt spirit and Jim’s STRs is a marriage made in heaven.
In mid-July 2020, Annandale Distillery will launch Swan/Martin STR-matured, peated (Man O ‘Sword) and unpeated (Man O’ Words) Single Cask, Single Malts under its Founders’ Selection label. It’s our tribute to the great man.
Founders’ Selection Man O’ Sword (Peated) - Cask 2017/355:
Wonderfully rich golden amber hue
Peat is subtle at first but very reminiscent of tobacco smoke and smoky bacon. This is followed by a hint of coconut and sweet almond cherry Bakewell.
A huge peat explosion, bringing in earthy notes with smouldering green foliage, creamy vanilla, fresh peach and a slight tickle of black pepper on the back of the throat.
Founders’ Selection Man O’ Words (Unpeated) - Cask 2017/321:
Rich, golden amber
Spicy hints of cinnamon and nutmeg precede creamy vanilla, butterscotch and sweet caramelised demerara sugar
Hot melted butter initially which coats the palate, followed by hints of fresh conference pear, summer strawberries and fresh cream, Braeburn apple, blackberry, prunes and cool fresh honeydew melon
Professor David Thomson
Annandale DistilleryBuy an STR Cask