Words by Simon Difford
The maturation of wine, spirits and even Tabasco sauce in wooden casks greatly improves their flavour. This has been understood for 100s if not 1000s of years and was probably discovered by accident. Goods were stored and transported in casks and it was fairly obvious that this improved the flavour of the contents. However, the science behind cask maturation is now well understood.
Firstly it's important to appreciate the difference between age and maturity. Two teenagers may be exactly the same age but one far more mature than the other. The same applies to cask-aged spirits - two casks of single malt whisky may have been filled on the same day from the same distillation but numerous variables can result in the contents of one cask being far more mature than the other. A greater ageing period does not always equal greater maturity. Beware also over-aged spirits. Too long in a cask can be far more detrimental to a spirit than too little time in cask.
Before spirits are filled into a cask to age they are usually diluted to below 65% alc./vol. (63.5% is standard in the Scotch whisky industry) and the lower maturation strength so the better the extraction of flavours from the cask. However, this results in a greater volume of spirit to mature, so requires more casks, more manpower to move those casks and more warehouse space in which to store them.
The warehouses and cellars used to store maturing casks vary the world over, from caves to underground bunkers and large buildings built of masonry, wood or tin sheet-clad timber. They can be anything from a single storey with casks stacked three high on top of each other to palatalised buildings as high as the reach of forklift trucks and modern robotic systems allow. Some of these warehouses, particularly in the bourbon industry have floors and temperatures between the top and the bottom floors can differ by as much as 35°C (95°F) and on what level a cask is stored greatly affects the flavour of the finished bourbon. The top floors of these buildings are dry and hot and here casks evaporate water quickly, so gain a couple of per cent in alcohol strength. Conversely, the bottom floors are cool and moist so here casks evaporate more alcohol than water so lose strength.
Temperature plays a key role
Heat expands the wood of the cask and the wine or spirit stored inside causing it to seep into the wood's pores; conversely when the temperature drops and the wood contracts the liquid is forced out. This action is referred to as a cycle. Nature provides around four cycles per year due to seasonal changes. Some distilleries artificially heat their warehouses to increase the number of cycles, so giving the spirit more flavour and colour.
These cycles cause interaction with the wood allowing a spirit stored within to extract flavoursome chemicals from the cask known as wood-derived congeners. Different timbers contribute different flavours and over some 2,000 years, white oak has become the timber of choice in which to mature wine, spirits and even beer.
White oak contributes luscious flavours yet is relatively neutral compared to pine and other woods. When quarter sawn, white oak becomes water liquid-tight - unlike red oak and many other woods. The cell structure of white oak is also ideally suited to being worked to make a cask.
Brad Boswell is a fourth-generation American cooper who when asked, "Why oak", replied: "I've tried making barrels with just about every type of wood you can name. And it turns out cherry wood doesn't taste like cherries and apple trees don't taste like apples."
Different types of oak
There are some 600 species of oak but four different types of white oak are most prevalent for maturation of wine and spirits: American white oak (Quercus alba), European oak (Quercus robur), French oak (Quercus sessiliflora) and Japanese oak (Quercus mongolica), each with different compositions of lignin, cellulose, tannins and hemicellulose (see below) so will influence a wine or spirit very differently.
American oak casks are usually seasoned with bourbon while European oak casks tend to be seasoned with sherry and sometimes port or even table wines. The tradition of heavily charring the inside of American oak bourbon casks but only lightly toasting European oak casks also influence differences in maturation.
American white oak (Quercus alba)
Native to eastern North America the Quercus alba white oak is the continent's pre-eminent hardwood (along with sub-species and hybrids including Quercus bicolor, Quercus lyrata and Quercus macrocarpa). Trees grown in forests in colder regions such as northern Minnesota are preferred for cask making as climatic conditions cause the oaks to grow more slowly so consequently have a tighter grain.
Since the end of the Second World War casks (usually Standard Barrels) made from American white oak have dominated other timbers used for cask maturation of spirits. American oak imparts a lighter colour to spirits than European oak.
American white oak has high levels of vanillins and when used for the maturation of spirits imparts distinctive vanilla and coconut flavours. Secondary flavours include tropical fruit, spices (mostly ginger), caramel, fudge, honey, almonds, hazelnuts, butterscotch. Scotch whisky distillers say bourbon seasoned casks also help remove soapy green flavours from the spirit.
European oak or Spanish oak/English oak/Russian oak (Quercus robur)
European oak, most commonly from Spain and northern Portugal is mostly made into casks for the wine industry and when used by whisky or other distillers, European oak casks are mostly used for a final secondary period of ageing known as finishing. European oak imparts rich dried fruit cake, currant, date and clove flavours to spirits.
Spanish oak casks which have been seasoned with sherry in Jerez are usually shipped 'wet', meaning that although the sherry has been emptied the bung will have been replaced so keeping the inside of the cask moist. Contrary to belief the flavours derived from ageing whisky in ex-sherry casks come from the European oak, not the sherry. Remember it is the oak which originally flavoured and coloured the sherry.
French oak or Sessile oak (Quercus sessiliflora)
Like Quercus robur, this is mainly used by the wine industry, particularly for the maturation of cognac. French oak generally comes from the forests of Tronçais and Limousin. The Tronçais forest, in Allier, provides darker, softer, finer-grained wood, which is particularly porous to alcohol while the Limousin forest produces medium grained wood, harder and even more porous. Tronçais oak has fewer tannins and more lignin so is noted for producing softer cognacs, whereas Limousin oak is sought for its robust flavours as eau-de-vie matured in Limousin oak casks tends to extract more tannins.
Japanese oak (Quercus mongolica)
Also known as Mizunara oak, Japanese oak has extremely high levels of vanillins but is a soft and very porous wood so casks made from Japanese oak are liable to leakage. It imparts flavours of vanilla, honey, fruit (particularly pears and apples), and spice (particularly nutmeg and clove). It unusually also contributes floral blossom notes.
Composition of oak
Oak is made up of four basic flavour compounds; tannins, cellulose, hemicellulose and lignin.
Think of tannins as giving a similar flavour to chewing on a softball glove; it's bitter and astringent. As unpleasant as that sounds, bitterness in small doses is incredibly important for both flavour and for protecting a tree from bugs and insects, attracted by sweetness. American oak is made up of only 1% tannin whereas French oak can be as much as 8 to 10% tannins. Tannins extracted from the wood contribute to the mouthfeel of matured spirits.
Made from a long chain of sugar molecules, cellulose is often described as the skeleton of a tree. It is the main component of plant cell walls and gives oak its remarkable strength. A barrel weighing 100 pounds will comprise of 50 pounds of cellulose. Cellulose fibres in oak are bound in lignin, a complex polymer. Toasting and charring the cask breaks down the Lignin, releasing the cellulose, vanillins and other compounds. In turn, heat also breaks down the cellulose to release sugars.
Out of that same 100-pound cask, 25 pounds will be hemicellulose and that's where the sugars are. Human beings love sugar so this is crucial to the flavours casks impart. Caramel, basically burnt sugar, is even more appealing to the human palate and when a cask is charred or toasted this produces the desirable caramel flavour. Hemicellulose also adds weight. People tasting aged spirits often describe the liquid as smooth or silky, saying a spirit or wine has weight and viscosity. Hemicellulose adds a touch of sweetness which in turn also makes the liquid feel good on your palate.
Lignin or Lignen (named after the Latin word for wood, lignum) forms part of the tree's cell walls (xylem cells) and comprises a quarter to a third of the dry mass of wood. It is one of the most abundant biopolymers on Earth. Lignin accounts for the final 25 pounds of that cask's weight and contributes a similar proportion of the flavour the oak imparts. Most vanilla used in cooking doesn't come from vanilla beans; rather it comes from lignin which is industrially processed to make vanillin. To create this fake vanilla, lignin is separated from cellulose and cooked. So similarly to hemicellulose in casks, when lignin is charred it produces vanillin.
Added to a savoury dish, vanilla will add complexity without impacting on the flavour but if too liberally added then it will stifle other flavours. Dr James Swan, a much-resopected expert in whiskey maturation, once compared vanilla to salt on French fries. "If you douse your fries with way too much salt they will taste unpleasant. A light dusting is perfect. That is comparable to vanilla in drinks, and casks subtly and slowly impart their vanilla flavours."
Another vital chemical found in American white oak is lactone, in particular, coconut lactone. This is added to sun cream to give it that tropical smell. Simply put, American oak is known for having a sweeter toasted character than European oak and this comes from lactone. European oak does not have lactone.
White oak, unlike other woods, adds the organoleptic properties of vanilla, caramel, spice, smoke, coconut, coffee and mocha, all depending on the exact makeup of hemicellulose, tannin and lignin. Other substances in the wood also contribute, such as eugenol which contributes spicy aromas.
Cask shape and size
The shape and size of a cask will greatly influence how its contents age and develop. Smaller casks have a greater surface area to liquid ratio than larger casks so have an accelerated ageing effect.
Many people refer to wooden casks as being barrels but a barrel is actually a particular size (180 to 200 litres) and shape of cask used by the American whisky industry. Properly termed an American Standard Barrel (ASB) bourbon distillers believe this offers the perfect wood to liquid ratio. However, some Scotch whisky producers, Diageo included, believe larger hogshead (225 to 250 litres) are better, to the extent that they buy ASBs, break them down and rebuild them into larger hogshead casks.
For detailed information about casks and their various shapes and sizes please see our encyclopedia page on casks
Oak is also particularly suited to cask making because the oak's natural porous structure also allows the cask to breathe. Oxygen can come in and out of a cask through the oak while the slightly larger liquid molecules are held back. Over time, this oxygenation will mellow a wine or spirit, rounding any harsh edges.
The layer of charcoal inside a cask from toasting or burning mellows spirits such as whiskey by absorbing sulphur and other undesirable substances. The degree of char will affect both the flavour and colour of a spirit. The degree of cracking caused by charring also allows a spirit to more easily reach sugars in the subsurface below the toasted layer. More on cask toasting and charring.
Cask activity & rejuvenation
Whisk(e)y and rum distillers the world over use casks which have previously been filled with wine or bourbon. When these second casks are first filled with whisky or rum they are termed first fill casks and in the case of Scotch whisky, such casks are usually filled with grain whisky or light malts, unless the casks are ex-sherry casks in which case they will immediately be filled with malt.
After they have completed this first ageing period the cask will be emptied and refilled, so becoming what is termed a . The influence (activity) of the oak on the spirit it holds reduces with each refill as the outer layer of charred/toasted oak starts to become saturated with the compounds it has extracted from the whisky. A good cask will be capable of another two fillings and periods of ageing before it stops contributing and is deemed exhausted.
Due to the high cost of oak casks, some distillers rejuvenate exhausted casks by flaying, scrapping or routing back the interior surface to new wood then re-toasting/charring. Such rejuvenated casks are capable of sustaining a further three to four periods of spirits maturation. These rejuvenated casks tend to produce a spicier whisky.
Many distillers measure the colour (tint) casks contribute between fills, more active casks will impart more colour to while exhausted casks give little colour. A good oak cask will last for 80 to 100 years, around the same time it takes for an oak tree to reach maturity.
In the whisky industry, first fill casks tend to be filled with grain whisky or lighter new make malt spirit and left for a relatively short period (around 5 years). Then when these casks are refilled they will be used to mature malts for an average of 8 to 12 years per fill.
Cognac and Armagnac producers use new virgin oak casks but also have to be aware of how active a cask is. Spirit may be placed in new casks for as short a period as two weeks and then moved to less active casks to continue ageing.
Understanding of how active casks are, when they are exhausted and careful management of what they are filled with and for how long is crucial to the production of good quality spirits (and indeed wine).