|Demonstration casks at Springbank distillery|
The first, and most obvious, is that compounds are extracted from the wood by the alcohol. This is influenced by a number of factors:
•The range of compounds that can be extracted from the wood - which is influenced by the type of wood, where it was grown, how long it was seasoned (left out to dry before being shaped into a cask), how the cask was toasted or charred (more heat breaks bigger compounds into smaller ones, producing new compounds), and how many times and for how long the cask has held other liquids before (hence the new wood/first fill/refill/etc terminology one finds in whisky info).
•The ABV, as some compounds will be more soluble in ethanol while others will be more soluble in water, so the strength will shift the sets of compounds that are extracted, everything else being equal.
•The size of the barrel, as the interaction between spirit and cask is limited by the surface area. So generally a smaller barrel will increase the rate of extraction as there is a higher surface area:volume ratio (hence why some distillers use smaller casks to 'speed up' maturation), while the opposite is true for bigger casks.
•Temperature fluctuations will cause the spirit to expand and contract, pushing and pulling it out of the wood. So a climate with broad temperature extremes will increase the rate of extraction, while a climate with narrow temperature extremes will slow down the rate of extraction. This is why some rum distillers in the Caribbean will actually heat their warehouses, to prevent the barrels from cooling down at night and increasing the rate of extraction. On the flip side, this is one reason why cool, maritime Scotland tends to have lower rates of wood extraction.
This can generally be thought of as 'additive' aging, whereby new compounds are added to the array present in the liquid when it is first placed in the cask.
Second, compounds are extracted out of the spirit by the wood, more so in casks that are heavily charred, by the layer of charcoal on the inside of the barrel. Some compounds will be absorbed into that layer of charcoal in the same fashion as household water purifiers. This is one form of 'subtractive' aging, whereby compounds that are present in the liquid when it is added to the cask are removed.
Third, compounds evaporate from the cask as it interacts with air. This is one reason why Diageo's notorious 'cling-film' experiment never went very far - there needs to be a certain amount of interaction with air to allow high boiling compounds that made it past the foreshots cut to evaporate. Otherwise the whisky would be left with more 'immature' and off-putting odors and flavors. Additionally, water and alcohol also evaporate, depending on environmental conditions, changing the volume and ABV of the liquid inside, which will influence its extractive potential as noted in point 1.
Fourth, compounds react, both with the wood, with other compounds within the liquid, and with the oxygen in the air.
•One of the main ways in which compounds within the liquid react with each other is via the formation of esters. Put simply, an ester is a combination of an acid and an alcohol that gives off water in the condensation process. Some esters are created during fermentation or the distillation process, but they can also be created from free volatile acids and alcohols that react as the spirit matures. Additionally, recombination will happen, especially as ethanol displaces other alcohols in esters via mass action. Additionally, as ethanol is oxidized to acetic acid, acetate esters will also become more common. Putting the two together, ethyl acetate tends to be the dominant ester in all spirits. This will be influenced both by the concentration of alcohol (primarily ethanol), temperature, and the rate of oxidation.
•Oxidation will transform molecules within the spirit. Alcohols will become aldehydes and ketones, then aldehydes will become acids. Unsaturated compounds will be cleaved into aldehydes and ketones. This is influenced by the oxygen tension in the cask, the rate of gas exchange, and the ambient temperature (a general rule of thumb for chemical reactions is that they will go 2x faster for every 10º C that the temperature is raised). But as these reactions are generally uncatalyzed and molecular oxygen is not a particularly effective oxidizer of organic molecules on its own, they will be rather slow.
•Alcohol will help to break down the macromolecules that make up the wood, increasing the range of substances that can be extracted into the spirit. This is influenced largely by the concentration of alcohol in the spirit.
|Sherry butts at Bruichladdich|
Sherry begins as a white wine, produced largely from palomino grapes, which is then fermented to dryness at ~15% ABV. Most sherries are then fortified to between 15.5% and 20% ABV with neutral grape spirit.
Aging sherry focuses primarily on the recombination of compounds already within the liquid and, for some varieties, on oxidation. This means that the casks are basically inactive, acting as contains rather than as direct participants in the process. If you've tried sherry before, you will have noticed that it doesn't have the tannic notes of, say, a California cabernet. This is because the casks used by sherry bodegas are first seasoned with lower quality wines that are later used for making sherry vinegar and the like, to extract the bitter tannins before they are used for higher quality sherry. That is not to say that the casks play no role - storing sherry in truly inactive containers of glass or stainless steel would not produce the same product.
The casks are host to microbial flora that interact with the wine and are critical in the formation of flor - a waxy layer of yeast that forms on top of sherry when the concentration of alcohol is around 15-16% ABV. It acts to exclude gas exchange, protecting the sherry from oxidation. This is critical for fino and manzanilla sherry to retain their freshness, even after prolonged time in the cask. On the flip side, amontillado and oloroso sherries rely on the gas exchange afforded by casks, aging oxidatively. These sherries are fortified to 17-20% ABV, which kills the flor, allowing oxidation to occur. This develops color and new flavors in the sherry that are not found in fino and manzanilla sherries.
In addition to primary cask aging, sherries are blended in a process called a solera. This is formed from layers of casks - wine for bottling is withdrawn from the bottom level, which contains the oldest wine. The casks on the bottom are then refilled from the level above, continuing upwards until the top layer is filled with new wine. This is a process of fractional blending, where some of the old wine always remains in the solera, adding complexity to the finished product. Solera casks also tend to be extremely inactive - if they contained significant amounts of extractable compounds, the final product would become intolerably bitter as more tannins were leached into the wine.
|Duty paid sample sherry casks at Lagavulin|
In fact, as noted by Whisky Advocate in their article about sherry, there was a period in the 1980s and 1990s when distillers were buying solera casks from the bodegas. The wood was, as noted above, rather inactive, so these casks would have added a layer of sherry flavor on top of the whisky, but would not have contained the other extractable compounds that distillers seek. In addition, they likely would have been leaky, necessitating extra work by the coopers.
So what constitutes a good sherry cask for a distiller? For much of the 19th and 20th centuries, these were casks left over from transporting sherry from Spain to England, where it was bottled by British firms for British consumption. These could range from local grocers buying a cask or two, to large firms like Harveys, who would bottle cases upon cases. After the sherry was dumped, there were a lot of casks left over. It didn't take long for Scottish distillers to realize that not only were these cheap containers for storing their spirit, but they also made it tastes a whole lot better. The critical element for distillers, beyond price and availability, was that the transport casks often would have been new wood, rather than the inactive casks preferred by the bodegas. So the fresh wood would be impregnated with sherry for a relatively short amount of time before being turned over to the distillers. Transport casks eventually ceased to be an option, due to sherry producers beginning to bottle their own products in the late 19th and early 20th century and the eventual ban of transporting bulk wine over 15.5% ABV within Europe in 1981 (I've seen it stated as 1986 elsewhere).
In the early 20th century, DCL figured out that they could 'improve' the process by adding a thick, syrupy form of sherry called paxarete to a cask, then subjecting it to high pressures and temperatures, to artificially 'inject' sherry into the wood. Especially after Prohibition was lifted in America and ex-bourbon barrels became extremely cheap due to regulatory requirements, this became a way to create a new 'sherry cask'. This was helpful, both because it was even cheaper than transport casks (by that time it would have been clear that they still had value) and consistency, both in terms of the output and in ensuring a steady supply, due to the decreasing availability of transport casks. The practice of using pax was fairly common from roughly the 1920s until the 1980s, when the Scotch Whisky Association banned it. Again, this would often be carried out on relatively new wood, either a freshly made cask or an ex-bourbon barrel that still contained a lot of extractable compounds.
|Used sherry casks at Springbank|
One of the most important aspects of making good whisky is achieving the proper balance of extractable compounds in the wood. New wood (sometimes known as virgin oak) is, with rare exceptions, considered to be too active for scotch, being used only as a finish for whisky that spent most of its maturation in casks that had previously held some other liquid. Distillers usually seek a balance of extractable compounds - enough to impart flavors of vanilla and coconut (these tend to be the dominant elements of American oak) or spices (these tend to be dominant in European oak) to the spirit without completely overwhelming it. The sweet spot is an ex-bourbon or ex-wine cask that is being filled with whisky for the first time (a slightly misnomered 'first-fill' cask) or second time (also slightly confusing 'refill' or 'second-fill' cask). First-fill casks are perfect for whiskies that will be aged a relatively short time, say 8-15 years. The more active first-fill casks will impart their flavors more quickly, adding a significant amount of richness to the spirit, but there is also the risk of going too far and making the spirit overly oaky. Refill or second-fill casks are more suited for longer periods of time, where the wood will impart flavors, but then hit a point where the wood has given up all it can, allowing the other axes of maturation to proceed without overwhelming the spirit with extracted flavors. There are always exceptions to these rules of thumb - first-fill casks will not always stamp a heavy mark on the spirit or may be well-suited to a particular distillery with intrinscially weighty or flavorful spirit (Mortlach or Ledaig, for instance), while refill casks will on occasion provide more richness than a first-fill cask. Additionally, distillers will often continue to use a cask for 3-5 fills, especially for lower quality or grain whiskies that are destined for less refined blends, though these also sometimes end up in the warehouses of independent bottlers. But increasing attention is being paid to getting the right amount of extraction out of wood, providing the right amount of flavor from the wood to balance the character of their spirit. For instance, Laphroaig will discard their casks after a single fill because the heavy character of their spirit requires active casks to balance it out (though there are also cases where they may have overshot the mark).
The take-away from all of this is that aging alcohol in oak casks requires an understanding of what the wood will or will not impart to the liquid, given its state and the amount of time it will be spending in the cask. Aging is a complex process and focusing on one element or another will require different sorts of wood. A good distiller or venenciar will know how to use the casks to achieve the final product they desire.