Why is one whisky at 40% and another at 60%?

Forty percent is a legal minimum: rule 3 of the Scotch Whisky Regulations 2009 requires bottling at a minimum of 40 percent alcohol.

Regulation (EU) 2019/787 does the same in Annex I, category 2, for all whisky in the European Union: a minimum of 40 percent alcohol, at least three years of maturation in wooden casks of at most 700 litres, no addition of alcohol and no sweetening or flavouring.

In the United States, 27 CFR 5,143 likewise prescribes a minimum of 40 percent, designated there as 80 proof. For Scotch it also applies that it may be distilled to at most 94.8 percent.

Cask strength means the whisky is bottled at the strength at which it comes out of the cask, without any water added.

American regulations lay this down strictly. Under 27 CFR 5.87, the terms barrel proof and cask strength may only be used if the bottling strength is no more than two proof degrees, i.e. one percent alcohol, lower than the strength at the moment the cask was emptied.

The same rule also covers original proof, original barrel proof, original cask strength and entry proof. Those may only be used if the strength at filling the cask and at bottling is the same. In the European Union the whisky regulation names only the minimum of 40 percent, and forbids bringing it up to strength with added alcohol.

Humid Scottish air slows the evaporation of water, so proportionally more alcohol disappears from the cask and the strength drops by about 0.5 percent per year.

Whisky goes into the cask at around 63.5 percent, with water making up about a third of the volume. Within 48 hours of filling, the staves already absorb about 2 percent of the volume, and after that about 2 percent of the ethanol evaporates each year.

At 0.5 percent per year, a fill of 63.5 percent ends up at around 48 percent after thirty years. The type of warehouse plays a part: a dunnage warehouse with an earth floor, thick stone walls and a slate roof holds the temperature better than a racked warehouse with a concrete floor, thinner walls and a metal roof.

Dry air mainly draws water out of the cask, so the share of alcohol rises instead of falling.

The American rules take that into account. Bourbon and rye may be distilled up to 160 proof (80 percent) and go into the new, charred oak cask at no more than 125 proof (62.5 percent). For whisky that has the same strength at filling and at bottling, 27 CFR 5.87 uses the separate term entry proof.

Heat speeds everything up. In Goa, 8 to 10 percent loss per year is reckoned and bottling already takes place after three years, whereas in Scotland maturation almost comes to a standstill at temperatures below 2 degrees.

The label states the actual alcohol content by volume to at most one decimal, followed by % vol, determined at 20 degrees Celsius.

Annex XII of Regulation (EU) 1169/2011 allows a tolerance of 0.3 percent by volume for drinks with more than 1.2 percent alcohol by volume, other than beer, wine, cider and fruit wine. A bottle stating 46.0 percent may therefore be between 45.7 and 46.3 percent. Whisky may not be bottled below 40 percent.

For tasting, the strength makes a difference. Research in Scientific Reports from 2017 shows that the smoke compound guaiacol stays at the interface of liquid and air up to about 45 percent by volume, and above 59 percent by volume actually moves into the liquid.