In this Unfiltered feature from 2010, we examined the ins and outs of cask strength whisky with the help of Brian Kinsman, master blender at William Grant & Sons

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As members of the Society, we are spoilt for choice with regular access to some of the finest single cask, single malt whisky in the world. But this hasn’t always been the case.

Wind the clock back more than 30 years or so (now almost 35!), before the Society existed, and cask strength whisky was a rare beast.

The irony is that if you were to wind the clock back even further, by another 100 years or so,

whisky drinkers would have been able to take their bottles straight to their local distillery and have them filled directly from the source. But the onset of regulations on alcohol strength, mass production techniques and retail requirements saw that direct link between the drinker and the distillery cut off.

Today, that link is alive and well – but what exactly is cask-strength whisky? Here, we take a look at how it is made and its key features.

The mainstream

While the Society bottles at cask strength, the majority of whiskies are reduced in strength through dilution with water once the maturation process is completed – mainly to reduce the alcohol content to make it more accessible to a wider spectrum of drinkers.

Most whiskies are reduced to 40% abv – the legal minimum. There are also financial reasons behind this – whisky companies pay higher duty according to the higher percentage of alcohol.

From water to waterfall

The first major step on the journey to producing cask strength whisky comes when the new-make spirit is collected – or cut – from the still before it is put into casks for maturation.

At this stage, the strength, or ABV (alcohol by volume), of the spirit can typically range from 60% to 75%.

“Each distillery will cut at a different point depending on how they run their still or the design of the still,” says Brian. “At Grants, our spirit is generally about 70% abv.”

Balancing act

Some distillers dilute their spirit before it is filled into casks, believing that water can ‘open up’ the whisky and help the maturation process. The standard filling strength is 63.5% abv, though this can vary from company to company.

“A lot of academic work has been done over the years on the optimum filling strength and it is generally accepted that whisky natures better in the 60-65% region,” says Brian.

“Some of the flavour compounds in the oak that you want to get out of the cask and into whisky are not overly soluble in the higher strength alcohol. By adding water at this stage, you are trying to get that balance of alcohol and water to help the maturation process.”

The tick-tock of time

“The strength of the whisky taken out of the cask after maturation will obviously depend on the length of time it has spent in the wood and the conditions is has been kept in,” says Brian. “Typically, the evaporation of water and alcohol during maturation will take the strength down to about 50-60%. If you take 12 years as an example, you probably lose about 6% by ABV in terms of strength.”

Back-to-basics bottling

At this stage, the whisky is at cask strength and the ABV can vary considerably. The majority of casks will be between 50% and 60%, but some can be much lower – but not below the minimum legal requirement of 40% for Scotch whisky.

Two Society bottlings highlight this variation. Cask No. 27.79: Maraschino, mon cheri, a 19-year-old from a refill ex-bourbon barrel, was just 40.6% abv. At the other end of the scale is a young cask from an Islay distillery at 68.2% abv – Cask No. 23.6: Rustic, roasted and smoky is a 9-year-old from a refill hogshead (ex-bourbon).

The extremes in strength are likely to have been caused by the fact that the cask at 40.6% was probably leaky, meaning that the spirit evaporated faster. Water may also have got in to further reduce the strength.

The others

Some whiskies are reduced to 43% abv – these tend to be for specific markets such as South America or South Africa, where 43% is set as the legal minimum. Others are reduced to 46%.

The conclusion

Cask-strength whisky would have been considered the norm 100 years ago, but there is no doubt that it is now considered a prestigious product for connoisseurs. Brian believes the fact that it is not diluted gives it a unique cachet.

“It gives people a little window and a real insight into the industry so they can experience what whisky is like at the point it comes out of a cask.”