E - for earth barrel maturing

What is meant by barrel maturing?

With earth barrel ageing, the barrels are completely surrounded by earth and stored in the ground for a certain period of time - in our case, between two and three years.

At what depth are the barrels buried in the ground?

There are no specifications. In our experiments, the barrels were buried one to two meters deep in the ground.

What are these barrels filled with?

The casks that are stored in the ground can either be filled with new make or with pre-matured spirit. Of course, a second maturation of whisky in buried barrels is also conceivable.

How many earth barrel maturations have there been at St. Kilian so far?

So far, we have carried out three experiments with underground barrel ageing. In 2017, two 50-liter barrels were filled with mild St. Kilian New Make and, after three years, dug out from under the ground and bottled. A year later, we filled two 30-liter barrels with smoky St. Kilian New Make. These were also buried and bottled after three years. The third experiment started in 2021. 29 small, 30-liter barrels were filled with two-year-old, smoky St. Kilian spirit that had already been pre-matured in fresh Palatinate oak barrels and buried in the ground of our Bunker City in the Odenwald for another two years.

When was the whisky from the last earth cask experiment bottled?

The 29 small barrels in total, 15 of which were stored in three original wooden coffins and 14 of which were in direct contact with the ground, were excavated in October 2023. All the casks were then emptied, with some of them being used for our Grave Digger - Exhumation earth cask maturation. The remaining whisky is stored in interim containers, waiting for the right time for a fourth St. Kilian earth cask maturation.

What is so special about barrel ageing?

Buried in the ground, the exchange between the barrels and the air from the surrounding area takes place only with difficulty. This is because, depending on the storage location and depth in the ground, there is little to no oxygen. This means that it cannot reach the inside of the barrel and cause any oxidation reactions with the maturing distillate. If oxidation is omitted as part of the maturation process, the maturation process of the whisky underground is limited to the interaction of the spirit with the cask. This interaction is intensified by the use of small casks, because the smaller the cask, the greater the ratio of wood surface area to spirit volume. A smaller cask therefore results in more intensive contact between the maturing distillate and the wood. For this reason, we have so far only buried 30 liter and 50 liter barrels.

Does water penetrate the buried barrels from the outside?

This cannot be ruled out. It is true that barrel wood, especially oak, behaves like a sponge and soaks up liquid - both from the inside and the outside. However, this liquid (alcohol and water inside the barrel, only water on the outside) only penetrates a few millimetres to centimetres into the wooden staves and therefore does not pass from the outside to the inside and vice versa. Otherwise a barrel would not be tight and liquid would leak out. However, the bunghole is a critical point in the drum. Although it is sealed with a food-safe silicone stopper, water from the ground can still get into the inside of the cask via microscopic openings around the bunghole and "dilute" the maturing whisky, so to speak. You have little influence on this, as the cask is exposed to the prevailing elements in an uncontrolled manner underground.

Why does an earth barrel lose volume and alcohol content?

During barrel ageing, liquid is lost from the inside of a barrel through evaporation (keyword: Angels' Share). This liquid escapes as gas through the fine pores of the oak staves, through the microscopically small gaps between the staves and through the bunghole, which is sealed with a stopper. In the case of earth barrel maturation, there is often a lot of moisture in the immediate vicinity of the barrel, especially after a rainy period. According to the laws of physics, this means that relatively more alcohol than water reaches the outside through evaporation from the inside via the pores of the oak staves. As a result, the alcohol content of the whisky often decreases more significantly during earth maturation than during normal maturation in a warehouse.

What is the alcohol content of an earth cask matured whisky?

This is difficult to predict, but the alcohol content will drop significantly in some cases during the maturation process. In our first experiment, the cask-matured whisky only had an alcohol content of 40.6 % vol. after three years underground, down from an initial 63.5 % vol. and therefore slightly above the legally prescribed minimum alcohol limit of 40 % vol. The second smoky single malt whisky, bottled in 2020, reached 45.0% vol. after three years of maturing in earth casks, while the Grave Digger - Exhumation single malt was bottled at a hefty 49.4% vol.

What is the cost of an earth cask matured whisky?

From our own experience, we can say that this effort is enormous. Earth has to be dug out for the barrels, which - depending on the number of barrels - also requires heavy equipment in the form of an excavator. In the last experiment in particular, original wooden coffins were also buried, in which some of the barrels were placed. We also shot a promotional film for the Grave Digger - Exhumation Single Malt during the excavation, which was also challenging from an economic point of view. We dressed up as gravediggers and dug the liquid treasures out of the ground in the deepest Odenwald.

Why does St. Kilian go to such great lengths?

Because it's incredible fun. Because we are extremely keen to experiment in Rüdenau. And because we are incredibly excited to see what the end result will be. Just imagine the following: After two or three years in the ground, you dig the barrel out again full of excitement and expectation and then you're really happy when something great comes out of it. This is something very special for us and, of course, for our St. Kilian fan community. And that is exactly what drives us, that is our motivation for further experiments of this kind.

What does an earth cask matured whisky taste like?

Very tasty! For our latest experiment, Grave Digger - Exhumation, our master blender Mario Rudolf artfully blended the contents of Oloroso sherry, Amarone red wine, Ruby port and American white oak barrels. The result is a mystical essence of dark smoke, incomparable depth and a complex, subtly earthy and spicy taste.