How does cask filling strength affect whisky maturation?

How does cask filling strength affect whisky maturation?

The alcoholic strength at which a spirit enters a cask, and its effect on the final product, is a subject of some debate for distillers and whisky connoisseurs

Production | 13 Oct 2023 | By Matt Strickland

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One often-discussed topic in circles of whisky erudition is that of cask fill strength and its effects on the final liquid. However, to say the topic is complicated is putting it mildly.


Over time, various whisky traditions have evolved a cultural list of best practices and that often includes the alcohol level that a cask is filled with. For example, most malt whisky distillers in Scotland place their new-make spirit in the cask at around 63.5% ABV. In the US, for bourbon and rye whiskeys, the legal maximum fill strength is 125 proof (62.5% ABV).


There are possible economic benefits to a higher filling strength. Proportionally more spirit can be fit into a cask if it is filled at a higher alcohol level, thus lowering the number of casks needed and saving the distillery money in cooperage. Fewer casks mean less warehouse space is needed and subsequent labour costs go down. But there are chemical and quality issues that are affected by fill strength, too.


In the simplest chemical terms, whisky is a binary combination of ethanol and water. Of course, there are dozens of flavour and aroma compounds mixed in to give whisky its unique character, but those only amount to about 1 per cent of the total mixture. Water and ethanol serve as the lattice work upon which all these other compounds are structured.


Water and ethanol are both solvents. However, some things are more naturally soluble in water than ethanol and vice versa. In whisky making, this phenomenon is perhaps most pronounced inside a cask during maturation. Wooden casks (generally but not exclusively oak) contribute a number of important extractive elements that help to make up the final character of a whisky. Chemical groups such as colour compounds, tannins, and wood sugars tend to be highly soluble in water. So-called whisky or oak lactones are more soluble in ethanol.


What this means for distillers is that they have some degree of additional control over how their whisky matures inside the warehouse. For example, there has been a slow but steady move among American craft distillers towards barrelling at lower proofs. Peerless in Louisville, Kentucky barrels most of its spirit at 107 proof (53.5% ABV). Leopold Bros in Colorado fills a lot its casks even lower, at around 100 proof (50% ABV). While the technique may seem novel, this is actually the way a lot of grain spirit in the US and elsewhere was barrelled a century or more ago. In the late 1800s, prior to mass manufacturing of glass bottles, whiskey was usually sold by or directly from the cask straight to the consumer. Drinkers back then may not have had easy access to ice, or even potable water, to dilute their whisky to a drinkable strength, so the lower fill strength helped ensure a more palatable tipple right from the cask.


Distillers such as Peerless and Leopold Bros are taking advantage of some basic chemical principles. A lower alcohol percentage means there is more water inside the barrel. The higher water content means that water-soluble compounds such as wood sugars, colour compounds, and tannins are extracted more quickly, effectively giving the whiskey a greater amount of cask character in less time.


It should be noted that oak extraction is not the same as maturity and any illusions a distiller might have about fast tracking Mother Nature’s long-term effects on whisky to get a bottle out sooner should be abandoned. There are numerous other reactions that require more time to take place, and should the distiller opt for a lower fill strength, they should aim to produce a distillate that holistically marries with their maturation programme, alcohol percentage included.


While some distillers are moving towards lower fill strengths, there are still solid arguments for reaching higher – supported by the science of flavour as well as economics. As an example, let's look at single malt Scotch whisky , which as previously mentioned has an average fill strength of about 63.5% ABV. The vast majority of single malt is matured in ex-bourbon casks. That bourbon character, which contributes many of the nuances of a fine malt, has been shown to be best extracted with higher fill strengths. Another benefit is that some compounds such as oak lactones (responsible for some of whisky’s impactful coconut and fruity aromas) are more alcohol soluble and thus extract better at these higher fill strengths. Finally, in Scotland’s cool and humid maturation climate, a higher fill strength helps to hedge against too much lowering of a cask’s alcohol volume as more ethanol than water is lost to the angels’ share in these damp conditions. Conversely, in Kentucky’s hot and dry warehouses, more water is lost, and so lower fill strengths can help to stop the ABV of spirit in a cask creeping too high during the maturation process.


It's easy to see that cask fill strength is not a one-size-fits-all philosophy for distillers. The best whisky makers in the world understand that it is just one factor out of dozens that affect the character of their wares. They must take into account the character of the new-make spirit, cask, maturation climate, and more in order to make the best educated decision on what strength to fill the cask with.

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