E – as in earthenware cask maturation

What does "Earth Cask Maturation" mean?

With earth cask maturation, the casks are stored completely surrounded by earth for a certain period – in our case, so far between two and three years – in the ground.

https://youtu.be/bdOo8FR5rR0

At what depth are the casks buried in the ground?

There are no specific guidelines. In our experiments, the casks were buried one to two meters deep in the earth.

What are these casks filled with?

The casks stored in the ground can be filled with either New Make or with already pre-matured spirit. Of course, a second maturation of whisky in buried casks is also conceivable.

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

We have conducted three experiments with earth cask maturations to date. In 2017, two 50-liter casks were filled with mild St. Kilian New Make and, after three years underground, were dug up and bottled. A year later, we filled two 30-liter casks with smoky St. Kilian New Make. These were also buried and bottled after three years. The third experiment started in 2021. A total of 29 small, 30-liter casks were filled with already two-year-old, smoky St. Kilian Spirit, pre-matured in fresh Palatinate oak casks, and buried in the ground of our Bunker City in Odenwald for another two years.

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

The total of 29 small casks, 15 of which were stored in three original wooden coffins and 14 casks next to them in direct contact with the soil, were exhumed in October 2023. All casks were then emptied, with a portion of them being used for our Earth Cask Maturation Grave Digger – Exhumation. The remaining whisky is stored in interim containers and awaits the right time for a fourth St. Kilian Earth Cask Maturation.

What is special about earth cask maturation?

Buried in the earth, the exchange between the casks and the surrounding air takes place with difficulty. Depending on the storage location and depth in the ground, there is little to no oxygen. This means it cannot enter the inside of the cask and cause oxidation reactions with the maturing distillate. If oxidation, as part of the maturation, is absent, the maturation process of the whisky underground is limited to the interaction of the spirit with the cask. This interaction is enhanced by the use of small casks, because the smaller the cask, the larger the ratio of wood surface to spirit volume. With a smaller cask, there is more intensive contact of the maturing distillate with the wood. For this reason, we have so far only buried 30-liter and 50-liter casks.

Does water from outside penetrate the buried casks?

That cannot be ruled out. Although the cask wood, especially oak, behaves like a sponge and soaks up liquid – from the inside as well as from the outside – this liquid (alcohol and water inside the cask, only water outside) only penetrates a few millimeters to centimeters into the staves and therefore does not pass from the outside to the inside and vice versa. Otherwise, a cask would not be sealed and liquid would leak out. However, the bung hole represents a critical point in the cask. Although it is sealed with a food-grade silicone stopper, water from the ground can indeed enter the inside of the cask through microscopic openings around the bung hole and, so to speak, "dilute" the maturing whisky. There is little control over this, as underground the cask is uncontrollably exposed to the prevailing elements.

Why does an earth cask lose volume and alcohol content?

During cask maturation, liquid is lost from inside a cask through evaporation (keyword: Angel's Share). This escapes as gas through the fine pores of the oak staves, through the microscopic gaps between the staves, and through the bung hole sealed with a stopper. In the case of earth cask maturation, there is often a lot of moisture in the immediate vicinity of the cask, especially after a rainy period. According to the laws of physics, relatively more alcohol than water escapes by evaporation from the inside through the pores of the oak staves. As a result, the alcohol content of the whisky during earth maturation often decreases more significantly than during normal maturation in the warehouse.

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

This is difficult to predict, but the alcohol content will sometimes decrease significantly during maturation. In our first experiment, after three years underground, the cask-matured whisky had an alcohol content of only 40.6% vol, down from an initial 63.5% vol, and was thus minimally above the legally required minimum alcohol limit of 40% vol. The second smoky single malt whisky, bottled in 2020, reached 45.0% vol after three years of earth cask maturation, while the Grave Digger – Exhumation Single Malt was bottled at a robust 49.4% vol.

How much effort is involved in an earth-cask matured whisky?

From our own experience, we can say that this effort is enormous. Earth has to be excavated for the casks, which – depending on the number of casks – also requires heavy equipment in the form of an excavator. Especially in the last experiment, original wooden coffins were also buried, in which some of the casks were placed. In addition, we shot a promotional film for the Grave Digger – Exhumation Single Malt during the excavation, which was also economically challenging. We dressed up as gravediggers and extracted the liquid treasures from the soil in the deepest Odenwald.

Why does St. Kilian put in all this effort?

Because it's incredibly fun. Because we are extremely experimental in Rüdenau. And because we are incredibly curious about what the result will be in the end. You just have to imagine this: after two or three years underground, you dig up the cask again with excitement and anticipation, and then you are overjoyed if something great has come out of it. That is something very special for us and, of course, for our St. Kilian fan base. And that is exactly what drives us, that is our motivation for further experiments of this kind.

How does an earth-cask matured whisky taste?

Very delicious! For our latest experiment, Grave Digger – Exhumation, our Master Blender Mario Rudolf skillfully married the contents of Oloroso Sherry, Amarone Red Wine, Ruby Port Wine, and American White Oak casks. The result is a mystical essence of dark smoke, incomparable depth, and complex, subtly earthy-spicy taste.