Shaved, Toasted & Re-Charred Red Wine Casks (STRs)

The Scotch Whisky Regulations (2009) specify that Scotch Whisky must be matured in Scotland in oak casks of no more than 700 litres in volume. Oak is specified by law because it has a particular effect on the maturing spirit that’s fundamental to the development of characteristic Scotch Whisky flavour.

Oak has three effects on the maturing spirit:

  • 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 also colour).
  • Extraction – the spirit extracts residues of the previous occupant, impregnated into the cask staves.
  • Adsorption – the charred oak surface inside the cask adsorbs various undesirable flavour compounds from the maturing spirit (especially sulphur compounds produced during fermentation).

In other words, the cask both ‘gives to’ and ‘takes from’ the maturing spirit. This is sometimes called ‘oak effect’. The balance of these three effects is critical to the flavour of Scotch Whisky. A maximum volume of 700 litres is specified because beyond this, the ratio of the inner surface area of the cask versus the volume of maturing spirit would be too small to achieve the desired oak effect. (See Technical Note on cask size for further information on surface area versus volume.)

Most of the casks used to mature Single Malt Scotch Whisky will have previously held bourbon. However a variety of other oak casks are also used including those that previously held Tennessee sour mash (principally Jack Daniels), cognac, brandy, rum, tequila, and a multitude of wines including various red wines, white wines, sherry, Port, Madeira, etc.

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 oak effect from virgin oak. 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. Indeed, 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.) Inevitably, the US bourbon regulations create mountains (literally) of once used ex-bourbon casks. A huge proportion of these are bought by Scotch Whisky distillers and, as a consequence, much of the Single Malt Scotch Whisky produced in Scotland has a hint of ‘bourbon character’ 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). When a cask has held Burgundy for a period of time (possibly as long as 7 years), the oak barrel staves become heavily impregnated with wine. If such a cask was subsequently filled with malt spirit, it’s possible that the resulting Single Malt may be too ‘winey’ in character, too red in colour and, because the inner surface of the cask may have been stripped of most of its flavour compounds, its capacity to impart oak effect may have diminished greatly.

It was the late Dr Jim Swan, a great friend and guiding light to us at 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 exposing active oak that’s held more deeply within the stave. Thereafter, the inner surface is toasted to break down structural elements in the oak (particularly lignin and hemicellulose) into sugars. These sugars are subsequently caramelised by the heat (via Maillard reactions). Toasting also releases a variety of potent flavour compounds, including vanillin, from the newly exposed oak. In the Swan/Martin STR process, toasting involves the burning of oak chips, some of which are milled from the staves of scrapped barrels. The temperature and duration of toasting, and the precise source and mix of the 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. The charred inner surface 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 yet another commercial secret!)

Whilst cask rejuvenation via shaving and re-charring isn’t new, the ‘clever’ part of the Swan/Martin STR process is in leaving enough residual ‘wine effect’ whilst reactivating the oak by toasting and re-charring. This is a very difficult balance to find, hence the secrecy! 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, as is the effect on whisky flavour.

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 advocated that Solist should be matured in Swan/Martin STRs! Solist is variously described as having extra-fruity notes (melon, mango, citrus, vanilla, oak spice and dates). Not bad considering how young this whisky was at the time of winning the award!

In late January 2017, Jim Swan met with David Thomson and Teresa Church, founders of Annandale Distillery, during one of his frequent visits. We talked about wood policy and nosed a lot of Annandale’s peated and unpeated maturing spirit. Jim was convinced that our spirit would mature very well in Swan/Martin STRs. His parting words to us, 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 x 230 litre STR hogsheads were ordered. 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 superb 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 the Founders’ Selection label. It’s our tribute to the great man.