Water molecules are small
Alcohol molecules are much larger
Since a water molecule is very small, it is easier for it to escape through the barrel than the much larger alcohol molecule. The atmosphere at the top of a barrel storage area is very hot and dry, allowing the water molecules to escape much faster than the alcohol molecules, thus driving the ABV up in those barrels and concentrating the flavours inside the barrel.
Barrels at the top of a Barrel Storage Area therefore ages at an accelerated pace due to Water Molecules in those barrels escaping easier – the ABV Raises to around 70 to 75%
The atmosphere at the bottom of the Barrel Storage area is moist and cool. Since a water molecule is very small, water can penetrate the barrel from the surrounding moisture and that drives the ABV down in those barrels.
Barrels at the bottom of a Barrel Storage Area therefore ages at a slower pace due to Water Molecules absorption – the ABV drops to around 55%
Barrels at the middle of the Barrel Storage Area, what the late Booker Noe of Jim Beam called the “Center Cut”, stay more constant, varying just a little, as you will not experience extreme fluctuations.
1 High, 2 High, 3 High
In a typical American Barrel Storage Area (called a Rackhouse) there are 9 floors, with three barrels on each floor – we call those the one high, two high, and three high. Between the 9 floors in the Rackhouse, that makes it 27 barrels tall in total. ABV Percentages vary from floor to floor due to temperature and humidity. Barrels on the inside of the Rackhouse also ages differently than those on the outside as well, since the barrels on the outside get more heat closer to the walls. Where you put the barrels therefore affects those barrels in different ways.
Single Barrel vs. Small Batch
Some distilleries use the location of the bourbon in the rackhouse as their small batch. Many barrels are mingled together for a small batch so that those have a consistent taste from bottle to bottle and batch to batch over the years.
A single barrel is (supposed to be) bottled from only one barrel at a time. So, if we bottle whisky from a barrel two high on the 3rd floor – and then bottle from a barrel one high on the 6th floor – the two sets will taste very differently.
At Buffalo Trace Distillery, they have wood and tin, brick, and concrete block Rackhouses (3 different types).
Whisky ages differently in each type of house, so they put certain bourbons in certain types of houses, and that’s what creates the differences in their small batch and single barrels.