Whiskey Science: Looking at the Angels’ Share

Editor’s Note: The Whiskey Wash kicks off a few new whiskey writing focuses this month, the first of which is the science behind whiskey. To help us with this, we welcome Paul Hughes, an assistant professor of food science and technology at Oregon State University, and the head of OSU’s new distilled spirits academic program. He’ll be offering occasional insight into this interesting topic. 

The combination of celestial beings and whiskies isn’t usually to be recommended, but in the case of the angels’ share, we should make an exception. The term refers to the loss of liquid from within a cask of maturing spirit, and these losses are significant.

In Scotland, these losses occur at a rate of about 1 – 2% of volume per year, so around two to four liters each year from an American Standard Barrel (191 liters). In hotter climates, the losses can be more substantial. In India, for instance, evaporation approaches a percentage in the double digits.

Angels' Share

The angel considers her share (image via Bruichaddich)

However that’s not the full story. The alcohol and water are lost at different rates from the cask. Sometimes the overall concentration of alcohol in the cask increases; other times, it decreases. Why is that?

The first thing to realize is that the concentration of alcohol in the cask in molecular terms is always lower than water. This might not make much sense – after all, the typical alcohol concentration for Scotch whisky maturation is 63.5% alcohol by volume, and so, of course, the alcohol concentration is higher than water in volumetric terms.

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However, this alcohol percentage is equivalent to around 11 moles per liter (a mole is a convenient way of talking about numbers of molecules: one mole is 6.022 x 1023 molecules, to be exact). The concentration of water at 63.5% ABV is about 20 moles per liter. These differences are due to the different sizes (strictly speaking, masses) of the alcohol and water molecules. Alcohol molecules are bigger than water molecules, and so it takes fewer of them to occupy the same volume – they simply take up more space.

So, in the example of a 63.5% maturing Scotch whisky, water is close to twice the concentration of alcohol. But alcohol is more volatile, so it evaporates more easily from the cask. In fact, the relative differences in the evaporation rate from the cask are dictated by the environmental conditions of the maturation warehouse.

If humidity is high in the warehouse, then the losses of water are slowed and alcohol is lost preferentially. This is the typical situation in Scotland, where relative humidity is often 80 – 90%. In drier climates, though, the losses of water can be faster than alcohol, resulting in an increase in alcohol concentration over time in the spirit maturing in the cask. So, humidity is the main driver of differences in the relative rates of alcohol and water losses from casks

The overall losses due to the angels’ share are affected by three main factors:

  1. The tightness of the cooperage
  2. The amount of airflow within the warehouse
  3. The ambient temperature profile.
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These factors are, to some extent, self-explanatory. If the joins of the cask are not sufficiently tight, or indeed if there are any leaks, then, of course, volume losses will be greater.

If there is greater airflow within the warehouse, then the atmospheric concentrations of alcohol and water will be lower, which has the effect of speeding up evaporation from casks. (This was noted to spectacular effect in a warehouse in Scotland where a heavy snowfall collapsed a warehouse roof and, because of safety issues, the warehouse was left exposed to the environment for weeks. The impact of the evaporative losses was painful…)

But, the higher the ambient temperatures, the faster water and alcohol will evaporate. Think of drying laundry on a clothes-line. Breezy, warm days will dry the laundry more quickly than cooler, dank days.

Finally, it’s worth bearing in mind that there is more to maturing whiskey than alcohol and water. In terms of the angels’ share, it was once thought that dimethyl sulfide (smells of cooked corn or tinned tomatoes), which boils at around 68°F, was lost by evaporation. It turns out that this loss during maturation is actually due to oxidation – but that’s another story.

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Paul Hughes

Paul Hughes is an assistant professor of food science and technology at Oregon State University, and the head of OSU's new distilled spirits academic program. He'll be offering occasional insight into some of the science behind our favorite drink.