Why Bourbon Gets Stronger and Scotch Gets Weaker with Aging
Four Roses Distillery bourbon barrels
In the bottle at home, neither Scotch nor bourbon will reduce in volume over time, unless the cork comes loose or you drink it, as we tend to do. In the barrel before it gets there, on the other hand, the amount of whiskey that goes in at the beginning can be significantly different to the amount that comes out after aging.
The “evaporative loss” as some of the barrel’s contents disappear into the atmosphere is more romantically known as the “angel’s share,” and there are really two things going on. The total volume of liquid decreases over time, and the ratio of alcohol to water changes.
While aging, both bourbon and Scotch lose liquid in the barrel due to evaporation, bourbon more than Scotch. But after a few years aging in Kentucky, bourbon comes out the barrel at a higher strength than it went in. After years aging in Scotland, Scotch whisky comes out of the barrel at a lower proof than it went in. Let’s talk about what’s going on.
Hot or Not?
Stacked Dickel barrels in rickhouse
Many factors influence how much whiskey or other spirit evaporates out of a barrel, including how tightly packed together barrels are in the warehouse, whether air circulates between the barrels, the type of building material of the warehouse, the number of windows, the warehouse height, the type of wood used in the barrels, the entry ABV, and more. But temperature and humidity have the most influence on evaporation.
Ambient (environmental) temperature will significantly impact the total volume of liquid evaporating. In Kentucky, it usually gets very hot in the summers, and a larger volume of liquid is lost compared with continually cool Scotland. (I should note that bourbon can be aged anywhere in America, from Alaska to Florida, but for this article assume that we’re talking about Kentucky.) Figures for the angel’s share vary, but many producers cite a 2% annual evaporative loss in Scotland and closer to 4% in Kentucky. According to distillers, the loss is also not linear – more whiskey evaporates per year in the early years of aging.
Diffusion Confusion
Cold and wet Glenmorangie barrels
It’s easy to understand that hotter temperatures result in more evaporation. After all, the clothes drier works faster when set on ‘hot.’ Now let’s talk about humidity and its impact.
You may or may not recall from high school science class the concept of diffusion, which is movement of anything from a region of higher concentration to a region of lower concentration. If you add salt to water, the salt molecules will spread out evenly into the water so that it is evenly salty. Similarly, if you have a porous membrane separating water with lots of salt and water with less salt, the salt will cross the membrane until it reaches equilibrium (equal saltiness) on both sides. In whiskey, the wood barrel is the membrane.
Alcohol vapor is crossing the membrane into the surrounding air, where there isn’t alcohol naturally. So too is water – but the outside air already has some water in it. When the air outside the barrel is very dry (low humidity), lots of water evaporates until in theory the amount of water vapor inside the barrel and outside of it would equalize. When it is very wet outside the barrel (high humidity), then less water evaporates comparatively.
Summing Up
Lagavulin in Islay
So, in Kentucky when it’s hot and sometimes dry, lots of alcohol and water evaporate, but relatively more water into the surrounding air. The ABV of the spirit in the barrel rises.
In Scotland where it’s quite cool and humid year-round (especially in Islay warehouses close to the ocean), evaporation of both water and alcohol occurs at a slow rate, but less water escapes the barrel compared with alcohol. The ABV decreases.
A frequently cited figure is .5% ABV loss per year aging for Scotch whisky. For very long aged Scotch, producers need to ensure that it doesn’t reduce in alcohol strength so much that it drops below 40% ABV. You could no longer sell it as whisky!
So remember: where it’s humid more water stays, and where it’s dry it goes away.