Aging of spirits

Aging is the 3rd (and optional) step in the production of alcohol. Before you can mature your spirit, it must first be fermented and distilled.

Aging is when a spirit is stored in wooden casks for a longer period, which is especially known from whiskey and dark rum. Aging spirits, wine, and even Tabasco sauce in barrels improves flavor. It is said that for whisky, 60-80% of the final flavor comes from the casks. This has been known for centuries (if not millennia) and was probably discovered by mistake. Although the method has been known for a long time, it is only now that the science behind it is understood.

The age of the barrel, the wood it is made of, the size, as well as the level of charring of the inside are some of the factors that determine the speed and overall effect of the aging.


Oak casks on storage Where the casks are stored varies greatly. It can be anything from caves and underground bunkers to warehouses made of stone, wood, or e.g. tin. Each type has its advantages and disadvantages, as temperature and humidity have an enormous say on the final product. A dry heat evaporates the liquid and the spirit's alcohol percentage increases. A damp cold, on the other hand, evaporates the alcohol so that the alcohol percentage falls. Heat with high humidity can keep the evaporation of liquid and alcohol roughly equal (here Diplomatico Reserva Exclusiva is a good example). The inevitable evaporation is also called the “Angel's share” as it disappears into the air. In Scotland, producers of whiskey must write off 2% of their production volume annually on the basis of “angel's share”.

Heat expands the casks and the wax it contains, which allows the liquid to penetrate the wood's pores. Conversely, cold will cause the wood to contract and force the liquid out. From nature's side, due to the seasons, there are a few changes in temperature during a year. Some distilleries choose to heat their warehouses to create these cycles, as it adds more flavor and color to the spirit.

Making wooden barrels

Only oak is suitable for wooden barrels. Unlike red oak, and many other types of wood, white oak becomes waterproof when it is quarter sawn. The cellular structure of white oak is perfect for this, as it is very compact. Oak trees are "ring porous", which means that the vessels that carry water up the tree are found in the outer growth ring. As the tree matures, the older vessels become clogged with crystalline structures called tyloses, and as a result the center of the tree (heartwood) does not conduct water at all. In addition, oak often grows more vertically than other trees. Softwood is not suitable as it contains resin which prevents the barrel from breathing. Other types of wood give off unpleasant flavors to the spirit. Brad Boswell, a fourth generation cooper, was asked “why oak?” to which he replied, “I've tried making barrels out of every type of wood you can think of. It turns out that cherry trees don't taste like cherries, and apple trees don't taste like apples.

There are two different types of oak that are mainly used: American white oak (quercus alba) and various European species (generally quercus). The American white oak grows faster and has a milder, finer and more contained aroma. The aroma of vanilla and coconut is also more dominant in the American oak. The European oak gives a fuller and more intense aroma and contains more tannins. An American oak can be cut down after 70 years, whereas the European one needs about 100 years more.

The cutout on the right ensures a watertight barrel, whereas the one on the left does not take into account the grain of the wood and will make the barrel leaky. The wooden barrels are in themselves a masterpiece. The direction of the grain must be correct so that the barrel stays tight and not too much alcohol evaporates. Furthermore, wood has veins from the core of the tree to the bark, which can make the barrels leaky. This means that the wood must be carved in a special way, such as a star cut seen here on the right. The wood is cut into trapezoidal sticks, which are subsequently dried until they have about 10% of their moisture left.

After this, a process begins where you "roast" the wood. The wood is heated to 200 degrees Celsius in a large oven for about 30 minutes. During this process, the structure of the wood is broken up and sugar grains in the hemicellulose (caramelized carbohydrates in the cell walls) are caramelized. This forms volatile aroma compounds such as furfural (almonds), maltol (toast), cyclotene (caramel). The same happens with lignin, which breaks down into aldehydes and phenylketones (vanilla), guaiacol (smoke), eugenol (cloves) and other aromatic compounds. In addition, there is also a list of wood extracts, such as lactones (own coconut), polyphenols (eg tannins), terpenes, lipids, acids, etc. After heating the wood, it is bent into the final shape of the barrel and it is charred inside for 3 to 5 minutes.

The charring makes the spirit softer in taste, as it absorbs sulfur and other unwanted substances. The charred layer acts in a way like a carbon filter. The cracks in the wood caused by the charring also make it easier for the liquid to reach the sugar hidden in the wood.

Wooden barrels can be reused 3 to 4 times before the taste disappears from the wood. The first filling of the barrel is therefore most interesting, as it extracts the strongest flavor from the wood. The charred layer is simply filled with the unwanted substances from the spirits.

By scraping the charred layer from the inside of the cask, it can be renewed. Behind the charred layer hides wood that has not been in contact with spirits. After the charred layer has been scraped away, the "new" wood can be charred, and in this way you have a renewed barrel. This can be repeated as long as the barrel allows. Exactly how old a barrel can become is not known, as the process has only been practiced for the past 10 to 20 years. Even though the barrel is being renewed, and it is actually "new" wood that affects the spirit, the cask still gives a spicier taste than before.

The size of the wooden cask is also important. A small barrel gives the liquid more surface to work with, which means that aging takes place faster. For economic reasons, the use of small casks is not widespread.


After aging in oak casks, the spirit is bottled. Some manufacturers choose to filter the liquid through e.g. carbon filter, to get the last unwanted substances out of the spirit. However, it is very individual what the different manufacturers choose to do. You can read more specific details about individual products on our review page, where we describe the journey of the various spirits from crop to final product.

The Solera method

Solera is a method for maturing spirits, typically wine, beer and brandy, where you transfer part of the liquid from one cask to the next over several rounds. In this way, you end up with a product that is a mixture of different liquids that have matured over different periods of time. Solera, which in Spanish means "on the ground", refers to the lowest level of barrels. The method consists in the fact that the liquid from the top barrels after some time is moved to the next layer and so on until the liquid reaches the barrels on the "floor". Although this suggests a stacked structure, the barrels are not necessarily stored that way today, instead the barrels are extensively marked.

None of the barrels are ever completely emptied and therefore it can be said that the 1st distillate that was poured into the barrels is still there. Any differences that could be between the productions are therefore also equalized and the Solera method produces a uniform product.