The Science of Whisky Maturation

There are over 100 whisky distilleries in Scotland and yet the New Make spirit is unique and its chemical composition differs from distillery to distillery. This colorless distillate already contains some of the compounds contained in the final aroma of the matured whisky, such as long-chain alcohols, phenols, esters, lactones, aldehydes, fusel oils, compounds containing sulfur and nitrogen. But the really interesting things happen in the wooden barrel, in which the whisky gets its unique and round taste. During barrel maturation one tries to find the balance between the spirit character and the maturation character.

Quite a few distillers believe that up to 60-80% of the flavor components in the finished Scotch Malt Whisky come from barrel maturation and especially from the oak wood used. This is not at all surprising, since the New Spirit remains in the barrel for an average of 10 years and more before bottling. During this long phase, the maturing whisky has a lot of time to interact intensely with the wood. If you have ever tasted the fresh distillate from the Spirit Safe and compared to the corresponding 10-year-old single malt, then there are enormous differences in taste. The matured whisky has become much softer, more pleasant and rounder and the unpleasant smells and flavors in the New Spirit have disappeared or at least are less noticeable. During the ripening process, various aromas are released from the oak wood by the alcohol, in return, undesirable compounds in the New Spirit are released to the wood. With the oxygen from the surrounding air in the warehouse, chemical reactions create a multitude of new compounds that lead to a complex aroma spectrum in the whisky. It is therefore all too understandable that the highest quality is important when choosing the wood and that wood management in the respective distillery is of enormous importance. With the oxygen from the surrounding air in the warehouse, chemical reactions create a multitude of new compounds that lead to a complex aroma spectrum in the whisky. It is therefore all too understandable that the highest quality is important when choosing the wood and that wood management in the respective distillery is of enormous importance. With the oxygen from the surrounding air in the warehouse, chemical reactions create a multitude of new compounds that lead to a complex aroma spectrum in the whisky. It is therefore all too understandable that the highest quality is important when choosing the wood and that wood management in the respective distillery is of enormous importance.

Why oak wood?

Oak is considered pure wood, which contains no resin channels. The lack of these resin channels is important in two ways. On the one hand, no unwanted aromas are released from the wood to the fresh distillate and, on the other hand, it gives the wood porosity and thus enables the barrel to “breathe”, which is essential for the maturation of the whisky it contains. In addition, oak is flexible and extremely durable, and it can be used for many decades. Wood from oak trunks aged 80 to 200 years is ideal as a raw material for a whisky barrel. The freshly felled oak wood has to be stored in the air for 18 months before it is processed further. The moisture of the rain, the warmth of the sun and the drying by the wind gently mature the wood, whereby unwanted, hard tannins are washed out of the cells. Enzymes and microorganisms cause a chemical change in the wood, which has a decisive influence on the later whisky aroma.

In the 1990s, it was shown that Scottish malt whisky matured very slowly in bourbon barrels that were made from staves that had been industrially accelerated compared to barrels that were made from naturally dried oak. This led to the so-called “oak wood specification” for the Scottish whisky industry, in which a proportion of at least 75% staves from naturally dried oak wood in the whisky barrel is required.

There are over 400 different oak species of the Quercus genus, of which only a dozen are suitable for storing alcoholic beverages. With a few exceptions, only two types of oak are used in Scotland for the storage of fresh distillate, which will later become whisky: the American white oak ( Quercus alba ), as well as the European style ( Quercus robur ) and the European sessile oak ( Quercus petraea ). Japanese oak ( Quercus mongolica ) is one of the somewhat exotic, but still important, oak species, which is becoming increasingly important, especially with Japanese distillers.

Quercus alba

The American white oak has a decisive advantage over the European oak. It is more productive which is to say that it grows much faster and the wood is denser. Due to the lower porosity, they can be sawed lengthways. This means less waste when cutting out the staves. American production of bourbon is a great advantage and a constant source of good barrels for Scotland. Due to legal regulations, these barrels may only be filled once by the bourbon industry. A real bourbon whisky always comes from a new oak barrel, which was also charred before filling. The lower tannin content (see below) in the American white oak and the typical carbonization of the bourbon barrels are responsible for the formation of the coveted vanilla and coconut aromas and a sweeter texture when these barrels are used for the maturation of Scottish whisky. American white oak also leaves only a light straw color. The of circa ~18 million whiskies currently maturing in Scottland today it’s estimated thatnearly 95% lies in American white oak!

Quercus robur and Quercus petraea

European oaks, which are mainly sourced from France and Spain, on the other hand, grow more slowly (approx. half as fast) and therefore have a lower bulk density compared to the American white oak. European oak should not be sawn to obtain the staves because the barrel made of it would leak due to its more porous structure. Therefore, they have to be split in the fiber direction in a much more complex manner. Slowly growing wood essentially has a slightly different interior structure, which favors a larger, more complex range of aromas. European oaks also contain more tannins than American white oaks and have a much stronger and more robust aroma. They also give the maturing whisky a much darker color that resembles sherry. The tannins give the whisky a characteristic rough note of dryness, which causes the tissue and mucous membranes to contract - the so-called astringency. Chemically speaking, tannins consist of different phenols, which are linked to form a real network via different sugar molecules. These tannins are released from the wood to the whisky and are often perceived as interesting leather or walnut flavors.

Quercus Mongolica

The Japanese oak, also known as Mizunara oak, occurs in the East Asian region, but is rather rare in Japan. It is not very popular among carpenters due to the nature of the soft wood and the tendency to leak. Due to the lack of European sherry barrels in Japan after the Second World War, the domestic raw material had to be used more and more and corresponding barrels filled with New Spirit. Decades later, however, it was discovered that the maturation in Japanese oak barrels gave the whisky a unique spectrum of flavors that was previously unknown in whisky, ranging from aromatic wood tones such as sandalwood, cedar and agarwood to notes of camphor, mint and Coconut stretched out. For this reason, today, to a certain extent,

What influence do the different oak woods have on the aroma and taste in Scottish whisky?

Primary aromas in oak

The reason for the differences in aroma and taste in the whisky is, among other things, the chemical structure of the wood used and the interaction of wood, distillate and ambient air. The main components of the wood are cellulose, hemicellulose and lignin, with cellulose having the largest share with 40-55%. The percentages of hemicellulose and lignin are approximately equally distributed. Added to this are tannins and the substances contained in the wood from previous fillings, such as sherry, port, wine, etc.

Cellulose, hemicellulose and lignin

Cellulose is a long-chain macromolecule made up of a large number of sugar molecules. The individual sugar strands are linked together to form large (macro) molecules via electrostatic bonds.

In contrast, hemicellulose is not a long-chain macromolecule, but is characterized by short chains and branches of sugar molecules. Hemicellulose serves as a scaffold for the cell walls in plant cells.

Lignin consists of a group of phenolic macromolecules that are composed of various aromatic building blocks to form a three-dimensional, amorphous network. All of these three substances per se are odorless and tasteless.


Quercetin belongs to the polyphenols - its name refers to a relationship to the oak (Latin Quercus) - and is found in higher concentrations in onions, apples or broccoli. The plant dye is said to have strong antioxidant and anti-inflammatory properties.


The heartwood of European oaks is rich in low-molecular organic ingredients, with the hydrolyzable polyphenols, in particular the tannins (gallotannins, ellagannins), being the most important extract fraction in terms of quantity. Tannins give the oak a bitter, woody and astringent taste. They react with other vegetable dyes, which are also found in oak wood, the so-called anthocyanins in the presence of atmospheric oxygen, which enters the barrel from outside through the pores of the wood during storage, to form complex polymeric color pigments, which are partly responsible for the corresponding color of the whisky are.

Organic acids

Tart, acidic and bitter-bitter aromas are caused by hydroxycinnamic acids (such as caffeic acid). Other primary components in wood are organic acids, which are very common in the plant world, such as acetic acid, benzoic acid and cinnamic acid. They give the wood a pleasantly acidic, balsamic and cinnamon taste.

Whisky lactone

An essential and extremely important aroma in oak is whisky lactone (chemical: cis - and trans - β- methyl- γ- octalactone), which is also called quercus or oak lactone due to its origin. It is a ring-shaped ester and is the leading substance for the oak taste. Whiskylacton has a pleasant smell and taste that is reminiscent of coconut, whereby American white oak contains about 20 times more whisky lactone than the European oaks. Interestingly, only two of the four possible stereoisomeric whiskylactones are found in oak.


The ionones belonging to the terpene ketone class occur in numerous isomeric forms (α-, β- and γ-ionones) in certain fruits and plants as important aroma components. Ionones come from the degradation of the carotenoids also found in oak wood - yellow to red, fat-soluble pigment pigments of vegetable origin. Some Ionone smell priority violet-like of cedar and low dilution, while others the wonderful fragrance of f exude driven picked raspberries. Ionones are also responsible for the smell of fresh hay.

Secondary aromas in oak

Toasting and charring the barrels The charring out of new barrels has a major influence on the later whisky aroma, which, in addition to the physical change in the wood (flexibility), results in important chemical reactions of the wood components. But before we take a closer look at this process, I would like to go into another important advantage of carbonizing American oak barrels.

Sulfur flavors

The treatment with large burners, which takes only a few minutes, and is then extinguished with water The charcoal layer formed on the inside of the barrel, which is only 2-4 millimeters thick, later acts as an activated carbon filter and removes the unwanted aromas and flavors, such as the volatile organic sulfur compounds, from the fresh distillate. Because these molecules are noticeably noticeable in the slightest concentration due to their characteristic smell of cooked vegetables, cabbage, asparagus, lit matches and even rotten eggs. They originate from virtually every phase of whisky production. The primary source of sulfur is the natural amino acids cysteine ​​and methionine, which are contained in the barley proteins.

Furan derivatives

As already mentioned, the charring of new barrels leads to important chemical reactions of the wood components. The large, cross-linked sugar molecules of hemicellulose are broken down and caramelized when the oak barrels are roasted or toasted. Hemicellulose thus ensures the sweetness and color of the whisky. These formed, smaller sugar molecules can in turn be further broken down to furan derivatives, such as furfural and 5-hydroxymethylfurfural, which are responsible for fine almond, caramel and aromas of toasted black bread.


From the other wood component lignin, the heat treatment of the wood creates a wealth of different aromatic compounds, such as vanillin and guaiacol, which contribute the characteristic and valued vanilla and spice aromas. The organic compound vanillin belongs to the orchid family, which is obtained from their fermented capsules. Vanillin is one of the characteristic elements in whisky. It is already noticeable in extremely low concentrations, and the smell does not increase noticeably, regardless of the increasing concentration of vanillin.


For many, the smoky and peaty aromas that are particularly pronounced in the island malts are the easiest to identify. The ‘smokiness’ of a whisky is determined by the phenols and cresols that result from the burning of peat. The more the malt was peated during kilning, the clearer the aroma in the finished malt. However, peat is not the only source of these flavor components. Degradation processes in barrel wood can also contribute to the formation of phenols and thus a smoky, peaty aroma. Through the thermal treatment of the wood, phenol derivatives such as 4-ethyl- and 4-vinylphenol are formed from lignin. These are descendants of the para-Cumaric acid and smell woody, smoky and give the whisky spicy aromas. 4-vinylphenol occurs naturally in the thorn apple. In the course of the burning out of oak wood, the eugenol also arises, which occurs naturally in clove oil and is said to have antiseptic, antibacterial and analgesic properties . Chemically speaking, all phenols are based on the benzene molecule. This hydrocarbon molecule consists of a ring of 6 carbon atoms, each of which carries a hydrogen atom. Phenols are extremely stable - they cling to the malted barley when kilned, dissolve in hot water during mashing, survive the fermentation and distillation process, and then, after 10 or 20 years in the barrel, they are still present. However, the intensity of the phenol taste does not remain constant right from the start. The ppm share (parts per million) of phenol decreases continuously in the course of the manufacturing and ripening process.

Guaiacol derivatives

Another thermal breakdown of the lignin takes place via ferulic acid, an organic compound that is also found in many plants such as dill, rice and grasses. This organic phenolic acid produces aromatic aldehydes (oxidized alcohols) such as 4-vinyl-, 4-methyl- and 4-ethylguajacol, which draw attention to themselves with a smell of chimney air as well as with sooty woody and bacon notes. Far more pleasant aromas are formed via the chemical compound syringaldehyde. This substance also belongs to the class of aromatic aldehydes, but has a pleasant scent and taste reminiscent of wild berries.

The influence of oxygen

Another important, not to be underestimated factor for the formation and design of aromas is oxidation, ie the chemical reaction with atmospheric oxygen. The oak, which is more porous than other types of wood, facilitates the supply of atmospheric oxygen from the external environment of the barrel. The oxidation promotes the complexity and intensity of pleasant aromas in the whisky, in particular the delicate, fruity, spicy and minty notes. These arise from a complex sequence of chemical processes. Tiny traces of copper left during the distillation get from the still into the fresh distillate, serve as a catalyst. The copper atoms convert the atmospheric oxygen into hydrogen peroxide, a strong oxidizing agent, which is also known as a bleaching agent for the hair.

Aldehydes and esters

When alcohol is exposed to the air, atmospheric oxygen removes two hydrogen atoms (hydrogen) from the alcohol. This oxidation process is also referred to as dehydration and this class of oxidized alcohols is referred to as aldehydes. The compound that is formed by dehydrating ethanol - the drinking alcohol commonly known as alcohol - is called acetaldehyde. This occurs naturally in many fruits (berries) and vegetables, as well as in wine, where it is formed by the influence of yeast and oxygen. Acetaldehyde can be further oxidized to acetic acid. This also forms when beer or wine become acidic. In addition, the acid is the main component in the vinegar. Chemical reactions of alcohols with acids lead to a new class of compounds, the so-called esters. Esters mainly form during fermentation, but also during barrel maturation. Esters are the stars in the whisky world, they are essential for an attractive smell. Malt whisky has identified well over 100 different ester compounds that are mainly responsible for the fruit flavors. Since ethanol is the main constituent in whisky and acetic acid primarily the acid formed from it, ethyl acetate (also called ethyl acetate) is the most frequently found ester in whisky. Ethyl acetate as well as amyl acetate are also common organic solvents in the industry. They are mainly used as solvents in adhesives and nail varnishes. It is therefore not surprising that some whiskys have an intense smell of glue, caused by ethyl acetate. Amyl acetate is the chemical product of amyl alcohol and acetic acid and has the fruity smell of ripe pears.

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