The normal filling strength in the Scotch whisky industry is 63.5% ABV (alcohol by volume), a standard which enables companies to easily trade casks of the same age and volume ‘like for like’ as, when dealing in new spirit, alcoholic concentration is intrinsically linked to value. The capacity to ‘swap’ with other distilleries has historically been a necessity for whisky makers, as all need to have access to a wide range of flavours and spirit styles for blending purposes – though this requirement as it concerns the major blends has been somewhat alleviated through the creation of large conglomerates such as Diageo and Pernod Ricard, which own large numbers of distilleries producing a varied style of makes. Nevertheless, trading still goes on and the industry standard has stuck.
As new make settles in the spirit receiver at what’s called ‘receiver strength’, typically around 70% ABV, water is added to reach this lower filling strength. However, it should be noted that this is hardly a universal approach and at Glenfiddich, Balvenie and Kininvie, for example, new-make spirit at 71% ABV is reduced to two different filling strengths of 63.5% ABV and 68.5% ABV. “These are our traditional filling strengths for as far back as we can see in our records and, once it’s a tradition, it’s a very bold move to change it. Every cask at either filling strength could be used in any expression. Glenfiddich 12 Years Old, for example, comprises casks filled at both strengths,” says Brian Kinsman, William Grant & Sons master blender.
Meanwhile, according to master distiller Billy Walker, the team at The GlenAllachie fill at four different strengths: 63.5%, 65%, 67% and 69.3% ABV, depending on the cask type being filled and the intended maturation period. Another option is filling at receiver strength and this practice is common at a number of grain and malt whisky distilleries; Bruichladdich, for example, has filled at receiver strength of 69% ABV since re-opening in 2001.
There are a few factors influencing a distiller’s decision to fill at a particular strength. Firstly, there are the practicalities. The higher the filling strength is, the lower the final volume of liquid, which requires fewer casks and therefore less warehousing space – both of which result in lower overall cost to the distiller. Correspondingly, lower filling strengths create greater volumes of liquid, meaning more casks and warehousing space, increasing costs. The fact that lower filling strengths mean adding greater amounts of water has brought about a flippant industry saying: “Why age water?”
However, this ‘common-sense’ view neglects the scientific realities of maturation. It has been proven that a combination of alcohol and water is more effective at extracting desirable lavours than alcohol alone and there is a direct relationship between entry proof and the ‘speed’ of maturation in the first 10 years, at least when measured by tracking certain key maturation indicators. Though, as is often the case in whisky, there’s still some room for debate.
Flavour compounds must solubilise (dissolve) in order to be extracted from the cask. Some of these are more soluble in alcohol, others in water, though every compound has a degree of solubility in both. A test case is vanilla, one of the most universal flavours in whisky. This derives from vanillin, which is more alcohol soluble.
“Different filling strengths result in a different flavour profile, as the interaction of spirit and wood differs. At a higher strength you draw a broader range of flavours from the wood, and you have the potential to age for longer before the strength would drop below 40% ABV,” says Adam Hannett, head distiller at Bruichladdich. “Octomore’s high peating level and high filling strength work well together; filling at 63.5% ABV wouldn’t give the same intensity in the resulting malt. The greater level of vanillin extracted at a higher filling strength also complements the phenolics very well.”
However, this doesn’t necessarily translate into perceivable differences in the whisky when it’s nosed and tasted, as Brian Kinsman explains: “The level of vanillin can be greater in casks filled at higher strength, but once diluted for bottling there’s no difference in the level of vanilla notes on a sensory evaluation, and the level measured in PPM (parts per million) is the same.”
Meanwhile, Kinsman points out, evaporation from the cask includes water and alcohol, which means the alcoholic strength is continually changing: “Evaporation is two per cent per annum, which includes an annual loss of 0.5% ABV in both our filling strengths. This two per cent is indicative up to 12 years, then slows down to one per cent for the next 10-15 years. After that it’s very cask specific as it continues to flatten out, but there is always some evaporation.”
It’s often said that higher filling strengths require longer to mature, while lower filling strengths mature more rapidly. One explanation for this is that the rate of reactions between the spirit and the cask slows down at higher strengths, and accelerates at lower strengths. However, only some reactions seem to be impacted by this (for instance, research suggests that ester formation remains constant regardless of fill strength) and the issue of exactly how much filling strength needs to vary in order to alter the rate of development changes depending on who is asked. Some say as little as 2% ABV is enough to impact maturation character, while others claim that a change of 5% ABV would only make a negligible change to the resulting whisky. So far, research seems to suggest that both can be right, depending on the exact maturation characteristics that the distiller wishes to emphasise.
Whether the filling strength has a particular influence on specific styles of malt whisky is another consideration. Peated malts are an interesting test case, being at one ‘extreme’ end of the flavour spectrum: “We sometimes fill Ardbeg above 63.5% ABV, which promotes a smokier whisky; filling below 63.5% ABV makes it fruitier,” says Brendan McCarron, Glenmorangie’s head of maturing whisky stocks. “If you change anything, the result will be different. The question is how different.”
Conducting experiments to reach a definitive verdict on filling strengths is challenging. Even casks of the same type don’t have an identical influence, which means differences in the resulting malt whisky can be down to cask variability rather than filling strength. Similarly, even two warehouses next to each other can have microclimates that modify the influence of the cask and, of course, the overall maturation environment (meaning seasonal weather and local climate conditions) plays a significant role too.
Nevertheless, research is ongoing and more may be revealed in the coming years. Perhaps Torabhaig, operational since 2017, will provide answers. They have been filling new-make spirit at various strengths, including 58%, 61.5% and 63.4% ABV, and according to Neil Mathieson, whisky maker at Torabhaig, each cask is labelled and set up in such a way that the team can directly compare casks where the only variable is the filling strength: “There are distinct patterns in the flavour profiles gained at each strength... But we can only muse on the results until we have actual empirical data.”
As is so often the case in the world of whisky, it looks like we’ll just have to wait and see.