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Cask Types and Whisky Maturation

Cask Types and Whisky Maturation


Whisky would not be what it is were it not for oak. While the malting and distillation processes that result in new spirit form a baseline character, much of what defines a whisky comes down to what happens as it ages in casks.

As whisky matures in oak, series of complex chemical reactions occur. Over time, wanted flavour compounds are introduced and unwanted flavours are eliminated. The precise details of this process have not yet been fully understood, but what is certain is that during that time in oak, a delicious transformation occurs: a stunning development in colour, flavours, and aromas.

A great deal of skill goes into selecting casks appropriate for the spirit, and just what flavours develop to form the fully mature whisky depends significantly on the kinds of casks used.

The most popular type of cask used for Scotch whisky maturation is ex-bourbon barrels. They are readily available because only virgin-oak casks can be used in bourbon production. They lend an appealing flavour profile of creamy vanilla and sweet tropical fruits, and an enticing golden colour - our Treasurer's Selection Auchroisk 17 year old is an outstanding example of this type of casking.

The other common type of cask is ex-sherry. They are generally associated with sweetness and rich dark fruit flavours, but different types of sherry do give a varied influence. The two most common are Oloroso, which lends dark, nutty, dried fruit notes, and Pedro Ximenez, which gives a syrupy, raisin sweetness – what might be described as a Christmas cake in a bottle. If that’s the kind of thing that whets your appetite, Glenfarclas 10 year old and our Single Cask Speyside 21 year old are both must-haves. Sherry casks generally produce a gorgeous rich amber colour.

Different types of cask maturation can be combined within a single whisky, with stellar results. For example, the Balvenie Doublewood is a brilliant balance of sherry and bourbon influences. A whisky might be aged entirely in one type of cask, or, for example, it might be first matured in bourbon casks and then transferred to sherry for a period of time ranging from months to several years.

This option of allowing the spirit time in a second type of cask opens an exhilarating range of options. Cask finishing, also known as second maturation, is a relatively recent innovation, begun only in the 1980s, and it has made possible a wide scope for experimentation. Various types of cask finishes have come out in recent years, showcasing unique influences, from dessert wine to rye casks. There are some rules limiting the playing field – for a Scotch whisky to be allowed that designation, it can only be aged in casks deemed to be traditional. That means no cider or tabasco barrel ageing, though, apparently, beer barrels are fine (see the Glenfiddich IPA, for example - fresh, sweet, and unbelievably smooth).

Port-finished whisky is a classic. While tawny port barriques lend a spicy, dry richness, ruby port finishes give the vibrant, puckery sweetness of red fruit, such as strawberry and raspberry. The Dalmore Portwood Reserve is an exceptional recent release, its burst of juicy sweetness making it a supremely likeable dram.

A variety of other types of finishes crop up. Red wine cask finishes are often from specific regions, such as Bordeaux. The tannin dryness paired with notes of red fruit and berries can result in a mouthwatering depth and vivacity, as in the case of the Deanston 9 year old Bordeaux cask finish. White wine casks also offer intriguing possibilities. A Chardonnay finish lends a silky, buttery mouth-feel, Glen Moray 10 year old being an excellent example. Sauternes casks bear the high sugar content of the wine, and they lend honeyed fruit and acidic notes, with abundant apricot and peach. Rum finishes give plenty of delectable tropical fruit and brown sugar sweetness - the Glenfiddich 21 year old is a noteworthy example.

While Scotch whisky traditionally has been aged in casks that previously held another spirit, another trend that’s begun to take off is the use of virgin oak casks. The powerfully spicy oak character fresh wood can provide in a short period of time is a way to give depth and maturity to younger spirits. Virgin oak also provides loads of vanilla and sweetness. With an increase in no-age-statement whiskies to meet market demand, there is an impetus to try to achieve maturity quickly. But because the flavours virgin oak lends are so powerful, this type of cask typically is used only sparingly.

Appropriate use of the many types of casks available for use in whisky production is a highly complex task that requires the skill of a master distiller. But when the right spirit spends the right amount of time in well-chosen casks, the result is the unparalleled depth and nuance enjoyed by whisky lovers around the world.

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