R - like ripening and aroma Part 2

In which phases does the barrel maturation of whisky take place?

The maturation of whisky in an oak cask can be divided into three stages, namely additive, subtractive and interactive maturation. These three stages of maturation do not follow a strict sequence, but overlap and sometimes merge into one another.

What is meant by subtractive maturation?

This is best illustrated using a charred oak barrel. Oak behaves like a sponge and soaks up liquid. An oak barrel made from new, unused staves ("virgin oak") in particular soaks up spirit eagerly as soon as the barrel is filled with it. The spirit also penetrates more or less deeply into the staves during barrel maturation, depending on the season. The spirit passes through the charring layer formed on the inside of the staves by the charring of the barrels. This charring layer acts like a filter and has the ability to remove the unpleasant and odorous sulphur compounds through adsorption. The cask therefore subtracts substances from the maturing distillate that are considered undesirable in the later whisky.

Does the black charcoal layer also contribute to the color of the whisky?

No. The activated carbon layer contributes little to nothing to the color or flavor of the maturing spirit. Its role is to remove unwanted flavors in the new make through a combination of adsorption and oxidation to ultimately improve the quality of the maturing whisky. In addition, charring breaks up the structure of the oak more, allowing the spirit to penetrate even more easily and deeply into the staves.

What does additive maturation mean?

As the maturing distillate penetrates the barrel staves, the aroma-rich wood decomposition products, which are formed in the wood during the toasting or charring process, are released over time. However, the natural ingredients of the wood, which make up around 10 to 12 percent of the dry matter in addition to cellulose, hemicellulose and lignin, are also released from the staves by the distillate. These include tannins, oils, fats, resins, acids, ring-shaped esters (lactones), vanillin and many other aromatic compounds, such as eugenol, which smells of cloves. All of these substances are released from the wood into the later whisky, i.e. they are added to the spirit.

Are the wood constituents continuously released into the distillate?

No. The release of the substances naturally occurring in the oak wood and formed by heat ("toasting, charring") to the maturing distillate during barrel maturation is not continuous. The release is therefore not linear, but follows a logarithmic curve. It has been scientifically proven that in an observation period of three years, half of all substances are released from the wood to the maturing spirit after the first 30 to 50 days of barrel maturation and that this release rate decreases significantly thereafter.

What happens during interactive maturation?

Oxygen comes into play during the interactive maturation of a New Make into whisky. The air contains around 21 percent oxygen. Due to the porosity of the oak wood, air passes through the microscopically small channels of the wooden staves from the outside into the inside of a cask. The oxygen in this air can then enter into chemical reactions, known as oxidations, with certain substances in the distillate. For example, oxygen oxidizes the alcohol ethanol to acetaldehyde. This is a compound that has a pungent aroma of unripe, green fruit. This aldehyde can be further oxidized with oxygen to acetic acid. This acid can in turn undergo an esterification reaction with ethanol and react to form acetic acid ethyl ester. A compound that smells of green apples in small quantities, but has an unpleasant adhesive odor in higher concentrations.

Is only ethanol oxidized by atmospheric oxygen?

No. Of course, other alcohols, such as fusel oils from fermentation, can also react with oxygen via the aldehyde stage to form the corresponding acids. These in turn can form new, mostly fruity esters with various alcohols. These then appear in the matured whisky with notes of apples, pears, apricots, peaches, cherries, tropical fruits and many more.

Do substances other than alcohols also react with atmospheric oxygen?

But yes. For example, the tannins also react with oxygen. This reaction is catalyzed, i.e. supported, by copper ions that originate from the still and enter the distillate in traces during distillation. During the reaction of the tannins with oxygen, some strongly coloring substances, so-called quinones, are formed, which are released into the maturing distillate. This is how part of the color of the later whisky is created.

How quickly do oxidation reactions take place in the barrel?

Although the process of oxidation begins as soon as a barrel is filled, it takes at least five to seven years before the effects of oxidation are clearly recognizable both olfactorily and gustatorily.