MATURATION OF NEW MAKE WHISKY IN CASKS

The Influence of Wood as Casks on Whisky

Maturation in Casks

Oak barrels have been used by humans for thousands of years to store and transport material. Early settlers of the United States in Kentucky began charring the interior of new white oak barrels prior to ageing distillate to create the distinctively flavoured spirit known as bourbon whiskey. Their life in the US over, these barrels were bound for Scotland to mature Scotch whisky. Despite the unique flavour and cultural significance of Scottish Spirit and America’s Spirit, little is known about the wood-distillate interaction that shapes Scotch whisky and bourbon whiskey, or other whiskies for that matter.

Whisky is matured for several years in casks of different origins. What influences the maturation process? Why do whiskies from one distillery sometimes taste so differently? Numerous experts have tried to identify different flavours with physico-chemical methods. Even the smallest amount can account for big differences in taste. That's how sensitive our senses are. Researchers measure the various substances like esters, tannins, lactones, vanillins, etc., in ppm (parts per million) and ppb (parts per billion). These small quantities are hard to grasp in a nano-dimensional world.

Glenfiddich predominantly matures their whiskies casks made from American white oak that have previously been used to mature Bourbon whiskey. In contrast, distilleries like Macallan or Bowmore use mainly casks that held Spanish sherry before. One malt whisky from Springbank became a legend: it was matured in ex-rum casks from the Caribbean and took on a green colour.

The smallest changes in the maturation process also have an enormous influence on the taste of the whisky; this implies that the wooden cask is the most important component of the maturation process.

The Wood

A wooden cask is a masterpiece of craftsmanship. Although more and more machines are used by coopers today, the actual manufacturing is still done by hand. The planks for the staves mustn't simply be cut at right angles from a log like construction timber. The grain direction of the wood must be taken into account so none of the radial vessels of the wood penetrates the side of the cask. Otherwise too much alcohol evaporates or the cask may even start to leak. Only oak wood is suitable for cask production. Softwood contains resin, which prevents the cask from breathing. Other types of wood emit unpleasant flavours that make the whisky awkward or even unpalatable. Oakwood from trunks with an age of 70 to 200 years is ideal.

There are two fundamentally different species of oak: American white oak (Quercus alba) and the various European oak species (Quercus in general). American white oak grows faster and has a mellower, finer and more contained aroma, while European oak provides full, intense aromas and more tannins. An American oak can be cut down after 70 years, while the slower-growing European oak must at least grow for 100 additional years. It is far more expensive than American oak, up to 10 times.

Wood doesn't only contain annual rings but also vessels that lead from the core to the bark radially. The tree transports water and nutrients through these vessels. For whisky, however, these vessels are inconvenient since they make the cask staves leaky. Therefore the wood must be cut according to special patterns (star cut, mirror cut or rift cut) so the annual rings stand vertically. With this method far less usable wood can be cut from one log, so a cask stave is much more expensive than a normal plank.

Timber cut vs Star cut

These planks are then made into staves with trapezoidal wanes (according to the roundness of the planned cask). The newly made staves must then be dried until they reach a level of less than 10% residual moisture. Whether this is left to nature and solar heat or done quickly in modern drying chambers doesn’t affect the quality of the cask.

If the casks were made from this wood, you would get a tight container, but the whisky couldn't mature. From a maturation standpoint, the wood is still dead. Only the following thermal treatment breathes life into the wood. This is a combined process. Only with heat can the wood can be bent into the typical cask shape. During 'toasting', the wood is heated up to 200°C in a big oven for approximately 30 minutes, and the firm wood structure is broken up, cellulose is split into wood sugar and caramelises, and the Lignin is partially converted into Vanillin. The cask begins to live in terms of maturation. After the cask has been bent into shape, the inside of the cask is burned (charred) for 3 to 5 minutes and extinguished with water.

The composition of a bourbon whiskey barrel.

Diagrams showing a stylised bourbon whiskey barrel (scale bar: 15 cm), a cross section of the charred barrel interior and a stave. The sampling scheme highlights the charred surface of the barrel stave (C); the inner portion of wood, which undergoes thermal degradation and distillate penetration (P); the red line (R), which indicates the depth to which the distillate penetrated the stave; and the outer portion of the stave (O). Note that the thickness of the red line and its distance from the outside of the stave vary both within the stave and among staves.

Researchers employed an inverse method to measure the loss of specific wood polysaccharides in the oak cask during ageing for up to ten years. They found that the structural cell wall wood biopolymer, cellulose, was partially decrystallised by the charring process. This pyrolytic fracturing and subsequent exposure to the distillate was accompanied by a steady loss of sugars from the cellulose and hemicellulose fractions of the oak cask. Distinct layers of structural degradation and product release from within the barrel stave are formed over time as the distillate expands into and contracts from the barrel staves. This complex wood-sugar release process is likely associated with the time-dependent generation of the unique palate of both Scotch and Bourbon.

The practice of ageing spirits and wine in charred oak barrels dates back to Roman times and while today’s food and beverage industry has adopted modern technology to increase productivity and improve consistency, the traditional oak barrel is still held in high regard. First selected for their unique physical and chemical properties, barrels constructed from the heartwood of 80- to 120-year-old oak trees (Quercus) reliably hold a variety of liquids of variable viscosity. In addition, they contribute to the unique flavour that has become characteristic of the wine and spirits stored within, resulting in their continued use.

The contribution made by the oak barrel to the flavour of the bourbon whiskey is affected by the cultural practices associated with its production. First of all, the staves of an oak barrel are seasoned in the open air. During this time, hydrolysable tannins are lost and macromolecules, such as lignin, are degraded leading to an increase in aromatic compounds in the wood. The staves are then bent into shape assisted by steam. After the barrel is constructed, its interior is charred. Charring, a form of fast pyrolysis, is the process during which the interior of the barrel is exposed to a natural gas flame that reaches 1950°C, leading to the modification of the biopolymers, cellulose, hemicellulose and lignin, which are the major constituents of oak heartwood.

Each biopolymer can withstand a different intensity of heat, with hemicellulose (30% of the cell wall and dominated by xylan and glucomannan) breaking down first at around 225–325°C, followed by the Β-1,4 linked glucan structure of paracrystalline cellulose at 315–440°C and finally by lignin, which requires temperatures exceeding 400 °C to induce degradation. The barrel is now ready to hold the distillate, which will become whisky only after it has been aged for a sufficient amount of time – a minimum of 24 months for Kentucky straight bourbon whiskey and 3 years for Scotch. The ageing of whisky takes place in a large multi-tiered warehouse, traditionally made of limestone but presently constructed from modern building materials. Though these structures shield the barrels from the weather, they are relatively uninsulated and allow for seasonal temperature shifts.

The effects that these changes in temperature have on whisky has been documented with studies demonstrating variation in the amount of sugars, the phenolic content and the accumulation of various other volatile and nonvolatile compounds. These changes are thought to be due to the expansion and contraction of the distillate as it heats and cools. When warm, the distillate expands and penetrates the staves of the barrel and when it subsequently cools, components of the wood are drawn from the staves, giving rise to the flavour profile associated with the whisky. It is clear that both charring and ageing affect the interaction between the oak barrel staves and the distillate. However, the majority of studies to date have focussed on the changing flavour profile of the whisky.

In simple terms, it is necessary to investigate the oak cask, focussing on whether recalcitrant cell wall polysaccharides such as cellulose and hemicellulose are extracted during barrel ageing. Such extraction would provide an unusual source of chemical building blocks.

Usually, the whisky is filled into the cask with an alcohol content of 63.5%. Over the years some of the cask content evaporates through the cask walls. Alcohol is more volatile than water so it evaporates more quickly. The alcohol content of the whisky decreases by 0.2% to 0.6% annually. This evaporated alcohol is called 'The Angel’s Share'. The fluid level decreases by 2% each year. It is measured with a square wooden ruler that has four scales on each of its four sides corresponding to the various cask sizes. The scales indicate the target level for each year. With this method, even the smallest leaks can be detected. Experienced controllers tap on the cask ends with a long-handled wooden hammer and deduce the fluid level by the resulting sound.

Due to the evaporation and the absorption of flavours from the cask wood, the whisky becomes mellower with each year. Samples are taken regularly from each cask to find out when the whisky has reached its prime. The size of the cask is important, too. Larger casks have a smaller surface in proportion to the content, and fewer flavours can be extracted from the wood. Therefore whisky in large casks must be stored longer in order to reach the same level of maturation! 

Small oak barrels can age spirits like whisky quickly. Because they expose a greater surface area per volume of liquid to the wood, they can more quickly diffuse the woody flavor of oak – a product of lignin, vanillin, and small traces of tannins that are extracted from the cell walls of the wood – throughout the drink inside, resulting in a finely aged spirit, wine, or even beer in a fraction of the time that would be spent in an industry-standard barrel. Mathematically speaking, the volume of small oak barrels increases by a power of three relative to dimension, while surface area increases by a power of two.

How do Sherry Casks Flavour Whisky?

Like Bourbon casks, Sherry casks have been used to age Scotch whisky for well over 200 years. But what are distillers looking for when choosing Sherry casks, and how are they able to produce specific flavours?

In the 18th century, Sherry was a fashionable drink in Scotland. Sherry was shipped to Glasgow and Leith in casks, coinciding with a dramatic expansion of the whisky industry. As there were no domestic forests in Scotland to use for casks, distillers turned to those being landed on the docks. These included rum and wine, as well as Sherry, but it was the latter that became the preferred option then, whereas all types of casks are in use today.

The casks used for transporting Sherry were, however, different from those used to mature it. They would tend to be made from fresh wood (although fino would be transported in old casks so there was no wood influence) and would only be used for a few trips between Jerez and Scotland before being turned over to distillers. The result was that the Sherry casks used for whisky had residual power and flavour.

While Sherry comes in a variety of styles, including dry fino and sharp, salty manzanilla, these are only occasionally applied to whisky. The darker-hued and aromatic oloroso is the more traditional choice, with sweet, rich Pedro Ximenez a tantalizing option. The influence of Oloroso Sherry casks is identifiable as red fruit, figs, dates, and raisins, with Macallan and Glenfarclas as perhaps the most famous examples.

In the mid- to late 19th century, as whisky makers needed more casks, there was a decline in the volume of Sherry imports into Scotland. Blenders imported new oak from America, coopered the casks in Glasgow and then treated them with Sherry before passing them on to distilleries.

In 1986, all Sherry had to be bottled in the Jerez region by law, meaning that shipping casks ceased to exist. Distillers then began working with cooperages in Jerez to make new casks to their specifications (air-dried European oak, seasoned with oloroso Sherry). Again, these are imitation shipping casks.

What influence does the Sherry have on whisky? All of this might seem as if it is the oak which is the main driver in terms of flavour in Sherried whisky. That’s not quite true. What happens during the seasoning process is that the Sherry modifies the flavour compounds in the oak. Up to 20-30 litres of Sherry soak into the wood of a Sherry butt. Not only does the ‘Sherried’ character come down to the Sherry itself, but also oxidation and the way in which the Sherry has interacted with and changed compounds in the oak. All of these then interact over time with the maturing spirit.

In very simple terms, there is more of an impact from the Sherry (or rather Sherry changed by oak and air) in the whisky’s youth, while in older examples you notice more of the oak (which is itself changed by the Sherry). This explains why a Sherry-finished whisky has more of a wine-driven character when compared to a whisky matured exclusively in ex-Sherry casks.

Sherry casks that are filled with whisky for the first time can inflict a powerful surge of chestnut and burnt red colours, and dark fruit flavours, making them a popular choice for finishing whisky—a brief stay in a secondary cask. If reused as their second fill, Sherry casks are more subdued and suitable for long-term maturation.

A first-fill bespoke European oak ex-Sherry cask will have the maximum impact of clove, resin, dried fruit and tannin. The same cask filled for a second time will have less of those compounds available for the whisky. You can find ex-Sherry casks which have been refilled so many times that the ‘Sherry’ element is virtually invisible.

The Sherry Spectrum:

Sherry-applied Scotch whiskies, tasted from lightest to boldest help single out sherry notes:

Touch of Sherry—Glenmorangie Lasanta: sultana, citrus, ginger

Some Sherry—Bushmills Black Bush: sweet malt, black fruit, cocoa

Very Sherried—Macallan Rare Cask: vanilla, raisin, chocolate

Sherry Bomb—Aberlour A’bunadh: dried fruit, candied orange, spice

A list of various distillers and distilleries that talk about their barrels:

https://www.youtube.com/watch?v=lTCV-5RVsIc
https://www.youtube.com/watch?v=0OLwKUmBCq4
https://www.youtube.com/watch?v=8wDAUIty17M

https://www.youtube.com/watch?v=7uc1GG3pTPk deals mainly with the Edradour distillery. Ralfy explains in more detail at https://www.youtube.com/watch?v=T7tuZOf2DrA. 

With thanks to Gollihue, Richmond, Wheatley, Victoria Pook, Meera Nair, Isabelle Kagan & Seth DeBolt.