Inside of one of Springbank's aging warehouses. Notice the variety of cask sizes.
The Importance of the Cask
By Brett Mullins
As whiskey enthusiasts, we not only enjoy the flavor of whiskey, but spend many hours pondering what makes each whiskey different. What are governing factors that create the diversity and individual complexities of each whiskey? We want to understand these things so that we can use that information to choose whiskey that is similar to those we already like. Comprehending these facets of whiskey making can be like trying to figure out how a magician does his trick. There is much mystery about the whiskey making process, and specifically the aging process. Here, we will attempt to demystify the effect of the cask on your whiskey.
Let us first look at the wood itself. The vast majority of whiskey casks are made of oak. There are a few specialty whiskies that are aged wholly or partially in casks made of other wood such as maple, hickory, cherry, or apple. But these are used only in tiny numbers, often experimentally or to impart a specific flavor. In the whisky industry as a whole, the number of whiskies aged in these alternate wood types is statistically insignificant, so we won’t cover them in this article. We will instead focus on oak, whose varieties are used in millions of casks per year. Oak is perfect for whiskey casks because it does not contain resins, like pine wood does for instance, which could taint the whiskey. There are three species of oak used for whiskey casks. Let’s examine the qualities of each.
Quercus Alba - This is American White Oak, which is by far the most used wood for whiskey casks. White Oak is a fast growth oak that produces flavor compounds when toasted that are reminiscent of vanilla, oak, and coconut. This is the wood which, by law, must be used to age bourbon. Additionally, bourbon must be aged in a new barrel, so the barrel can only be used once to produce bourbon. These used bourbon barrels are then sold and reused worldwide to age other types of whiskey. Some might ask, “Why isn’t American Red Oak used for casks?” Red Oak casks would leak due to the wood grain’s cell structure. Red Oak, because of its permeable nature, is used for packaging crates for dry goods instead.
Quercus Petraea – This is a European oak, referred to as Sessile Oak. While found across Europe, the trees found in France are used heavily to make casks for wine. Sessile Oak is a slow growth oak and offers fine tannins and vanillins as flavor compounds.
Quercus Robur – Another European Oak commonly called Pedunculate Oak. In the whiskey industry, this Oak is referred to as Spanish Oak, although it is found across Europe. This species is especially harvested from the Limousin Forest in France, where it is the most numerous species. It is a fast growth Oak and imparts raisin-like flavor compounds, and has the most tannins of the three species. It is used primarily for sherry and cognac casks.
Casks of each of these wood types are used, sometimes in combination, to age whiskies that do not require a new cask to be exclusively used. In general, American Oak is considered to impart more sweetness to the spirit, while European Oak imparts more spice. The wood options chosen by a whiskey producer are the first factor in shaping the character of a particular whiskey.
The next important factor for the cask is how the wood is dried. Wood has a moisture content of about 50% when it is cut. This makes the wood too unstable to make casks from, so it must be dried. It can be air dried, in which it is stacked to let air circulate around it over months or years, or kiln dried, in which the drying process is accelerated using heat, dehydration, or a vacuum apparatus. Wine makers usually use air dried wood. However, bourbon casks are often made with either kiln dried wood or a combination of air and kiln dried wood. Air drying typically makes for a less astringent whiskey, so it is often the more desirable method. However, this astringency effect lessens after the first fill of the cask.
After drying, the wood is cut and formed into the cask. Cask construction is standardized and does not vary in a way that has significant impact on the aging spirit. However, the treatment of the cask prior to filling with spirit has a significant effect. The cask will undergo one or two heat processes in preparation for filling: toasting and/or charring. There is a huge difference between the two.
Toasting – Not all whiskey casks are toasted, although most are. Some bourbon producers, for example, skip the toasting step, while others do not. Toasting is a process in which heat is applied for a long period of time to the inside of the cask. The exact specifications for toasting are a carefully guarded secret amongst individual whiskey making companies and their coopers. No matter the exact specifications, the purpose of all toasting methods is to liquefy the sugars contained in the wood and force them to the surface where they will contact the spirit during aging. The heat turns the sugars into a caramel-like compound which not only naturally flavors the whiskey, but also imparts color to it. The specific variables of toasting, such as temperature and time, are likely instrumental in which casks are most desirable for refills. Obviously, the greatest impact of toasting goes to the first spirit filled into the cask, which is one reason why bourbon generally matures more quickly than Scotch, which is aged almost exclusively in used casks. But, a high quality toasting should, theoretically, be more prized for 2nd and 3rd fill. It is possible to re-toast a cask, extending its usability for further fills, but the benefit is greatly lessened from the first toasting.
Charring – The other heat related process is called charring. This is a quicker and hotter process in which an actual fire burns inside the cask, literally burning the inside surface of the cask. So, what does charring the inside of the cask do? The charring leaves behind a layer of wood that is now the combustion reaction’s carbon byproduct. There can be more or less carbon byproduct depending on the level of charring being performed. Carbon is a natural filter and neutralizer, and removes some of the unwanted flavors in the unaged whiskey such as Sulphur. In addition, depending on the level of char, the characteristics of vanilla, spice, and tannins imparted to the spirit will be altered. The length of charring varies from only seconds to about a full minute, and is measured on a scale of 1 to 4, with 4 being the deepest char. Not all casks are charred. It is a legal requirement for bourbon and Tennessee whiskey to use charred casks, but not for other kinds of whiskey. As with toasting, a cask may be trimmed on the inside and re-charred for additional fills. Wine casks are not charred at all, so whiskeys such as scotch, which may be matured completely in an ex-wine cask, may never be exposed to charring at all.
To summarize, a whiskey cask may be toasted, charred, or both. It depends on the intent of the distiller and the type of whiskey being aged. There are also numerous undisclosed processes for toasting, and four different possible levels of charring. These variables all play a role in the huge range of taste profiles in whiskey. It also explains why distilleries who utilize used bourbon or wine casks consistently purchase casks from the same source year after year. A change in the source of casks could greatly alter the taste of their whiskey.
The final and most difficult factor to address is cask size. Why difficult? Because there is no universal standard for cask sizes. For example, the term “butt” could mean either 500 liters or 477 liters, depending on the context. The size of a cask has significant impact on the whiskey because larger casks contact less of the whiskey at any given moment, while small casks contact the whiskey more. To understand this concept, let’s examine the chart below:
Name Liter US. Gallon
American Standard Barrel/Bourbon Barrel 200 53
Butt 500 132
Quarter Cask 125 33
Standard Hogshead 238 63
Puncheon 320 85
Madeira Drum 650 172
Port Pipe (tall) 500 132
Sherry Hogshead 245 65
Cognac type 300 79
Bordeaux type 225 59
Barrique cask 225 59
Bloodtub 50 13
Looking at the size of the Port Pipe and Madeira Drum, it is easy to see that whiskey could be aged longer in these without as much worry of “over-oaking” than in the Bourbon Barrel, which is a fraction of the size. By contrast, the Quarter Cask at only 125 Liters would make contact with a larger percentage of the whiskey and would therefore make its impact at an accelerated rate.
This means that comparing two whiskies of the same age would be largely irrelevant unless you knew the comparative size of the casks each was aged in. This is even more complicated since many whiskies are aged in more than one cask or have the batch split into different types of casks that are married together in large vats at the conclusion of aging. Knowing this, simple age statements on bottles, while still valuable, can have diminished usefulness when comparing whiskies that were matured wholly or partially in disparate casks.
In conclusion, it is hard to overstate the impact of the cask on the whiskey’s final result. While the cask is only one component in the whiskey making process, it is obviously an important one. Many whiskey makers will rightly assert that the wood is the greatest factor in determining the quality of their whiskey. Hopefully, if you made it this far, you know more about casks than before and were not bored by the long explanations. While knowledge is power, it is humorous to realize that this information could easily make it even harder to make our whiskey choices, as we now have another handful of things to consider when comparing whiskeys on the shelf!