What are the phases of whisky maturation in a cask?
The maturation of whisky in an oak cask can be divided into three sections: additive, subtractive, and interactive maturation. These three phases of maturation do not follow a strict sequential scheme but rather overlap and sometimes transition into one another.
What is subtractive maturation?
This is best illustrated with a charred oak cask. Oak behaves like a sponge and soaks up liquid. An oak cask made from new, unused staves ("Virgin Oak"), in particular, eagerly absorbs spirit as soon as the cask is filled with it. Even during cask maturation, the spirit penetrates more or less deeply into the staves, depending on the season. In doing so, the spirit passes through the charcoal layer formed by charring the staves on the inside of the cask. This charring layer acts like a filter and has the ability to remove unpleasant and strong-smelling sulfur compounds through adsorption. The cask thus 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 charcoal layer contributes little to no color or flavor to the maturing spirit. Its role is to eliminate unwanted aromas in the new make through a combination of adsorption and oxidation, ultimately improving the quality of the maturing whisky. In addition, charring breaks down the structure of the oak wood more strongly, allowing the spirit to penetrate even more easily and deeply into the staves.
What does additive maturation mean?
As the maturing distillate penetrates the cask staves, the aroma-rich wood degradation products, which were formed in the wood during the toasting or charring process, are gradually extracted. But also the natural components of the wood, which, in addition to cellulose, hemicellulose, and lignin, make up about 10 to 12 percent of the dry mass, are extracted from the staves by the distillate. These include tannins, oils, fats, resins, acids, cyclic esters (lactones), vanillin, and many other aromatic compounds, such as eugenol, which smells like cloves. All these substances are released from the wood into the later whisky, thus being added to the spirit.
Are the wood components continuously released into the distillate?
No. The release of substances naturally occurring in oak wood and formed by heat ("Toasting, Charring") into the maturing distillate during cask maturation is not continuous. The extraction therefore does not occur linearly but follows a logarithmic curve. It has been scientifically proven that within a three-year observation period, half of all substances are released from the wood into the maturing spirit within the first 30 to 50 days of cask maturation, and this rate of release then significantly decreases.
What happens during interactive maturation?
During the interactive maturation of a new make into whisky, oxygen comes into play. Oxygen makes up about 21 percent of the air. Due to the porosity of the oak wood, air enters the inside of a cask through the microscopic channels of the wooden staves from the outside. The oxygen in this air can then undergo 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 fruits. This aldehyde can be further oxidized with oxygen to acetic acid. This acid, in turn, can undergo an esterification reaction with ethanol to react to ethyl acetate. A compound that smells like green apples in small quantities but has an unpleasant glue smell in higher concentrations.
Is only ethanol oxidized by atmospheric oxygen?
No. Of course, other alcohols, such as the fusel oils from fermentation, can also react with oxygen, via the stage of the respective aldehydes, to form the corresponding acids. These, in turn, can form new, usually fruity esters with various alcohols. These then manifest in the matured whisky with notes of apples, pears, apricots, peaches, cherries, tropical fruits, and many more.
Do other substances besides alcohols react with atmospheric oxygen?
Yes, absolutely. For example, 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 trace amounts during distillation. The reaction of tannins with oxygen forms some highly coloring substances, called quinones, which are released into the maturing distillate. This is how part of the color of the later whisky comes about.
How quickly do oxidation reactions occur in the cask?
Although the oxidation process begins as soon as a maturation cask is filled, it takes at least five to seven years for the effects of oxidation to be clearly perceptible both olfactorily and gustatorily.
Leave a comment
This site is protected by hCaptcha and the hCaptcha Privacy Policy and Terms of Service apply.