R - like ripening and aroma Part 1
Why does whisky mature in casks?
For a whisky to be officially called whisky, the fresh distillate - the new make or white dog - must be filled into a wooden cask. Depending on the country and the regulations that apply there, it usually has to mature for at least two or three years. In Scotland, for example, a three-year minimum maturation period in an oak barrel is prescribed. According to the Spirits Regulation 2019/787 of April 17, 2019, which applies to the European Union and therefore to Germany, a minimum maturation period of three years must also be observed for German whisky.
Does the maturing barrel always have to be made of oak?
No. In some countries, the exclusive use of oak wood for maturing casks is prescribed (see Scotland), but in Germany, for example, a wide variety of woods may be used for the construction of casks for maturing whisky in addition to oak. However, the majority of such casks used in Germany are made of oak.
Does maturing in casks have an effect on the whisky?
But yes, and how! During maturation in wooden casks, the colorless New Make changes its aroma and taste and takes on a color that can range from straw yellow to amber to mahogany. Leading experts estimate that around 60 to 80 percent of the aroma and taste of a whisky comes from the cask maturation.
What is the most common type of cask used in the whisky industry?
The most commonly used casks for maturing whisky are former (ex) bourbon casks. In other words, fresh oak barrels that are used once for the maturation of American bourbon whiskey and then exported to other countries. These are usually American Standard Barrels (ASB) with a capacity of 190 liters.
What wood are bourbon barrels made of?
Bourbon barrels are made from American white oak ("Quercus alba"). The staves of this type of oak are assembled into a barrel and charred on the inside with an open flame before filling until a thin, black layer of charcoal is formed. This thermal treatment of the inside of the barrel is known in specialist circles as "charring".
What is the purpose of charring oak barrels?
During the charring of the inner surface of the oak barrels, certain substances in the oak wood are chemically altered by the effect of the heat. This results in the formation of various color and aroma compounds, which are released into the distillate during subsequent barrel maturation. Charring also physically breaks up the structure of the oak wood, allowing the maturing spirit to penetrate the staves more easily and more deeply during the maturation process, thus releasing a wide range of aromatic substances.
Which wood constituents are involved?
Like all other plants on our planet, oak wood is made up of three main components: cellulose, hemicellulose and lignin. Their exact percentage composition varies from one oak species to another. All three main components are odorless and tasteless per se and are only chemically altered by heat to form different aromatic decomposition products.
What is cellulose made of?
Cellulose is a long-chain and unbranched giant molecule that is made up of many thousands of interlinked building blocks to form long sugar strands. The individual sugar building blocks are exclusively glucose (= dextrose). Due to certain interactions, the individual sugar strands form a very stable fiber structure. Accounting for around 40 percent of the dry weight, cellulose is the most common component of wood and therefore the most widespread organic compound on earth.
What is hemicellulose made of?
In contrast to cellulose, hemicellulose does not consist of a single sugar building block, but is made up of many different sugars (e.g. glucose, xylose, galactose, arabinose, mannose) to form a large molecule. Compared to cellulose, these sugar chains are significantly shorter, have additional branches and are partially esterified with acetic acid. In oak wood, hemicellulose makes up around 20 to 30 percent of the dry weight.
What is the composition of lignin?
Unlike cellulose and hemicellulose, lignin does not consist of sugars. It is a complex giant molecule which - to put it simply - is composed of different phenols to form a three-dimensional network. Lignin makes up around 20 to 30 percent of the dry weight of wood.
Which aromatic substances are formed in the wood during heat treatment?
The thermal degradation products of hemicellulose are various sugars that partially caramelize when heated and form small ring-shaped compounds that, in addition to color, have aromas of toffee, almonds, nuts, fruits and solvents. The more heat-stable cellulose only produces aromatic compounds at high temperatures, which give off notes of dried fruit and roasted almonds. Lignin, on the other hand, is converted by the influence of heat into a wide range of phenolic compounds that can have aromas of cloves, chocolate, smoke and fruit.
How is the vanilla scent created in whisky?
The chemical compound vanillin is responsible for the vanilla scent. Vanillin is naturally contained in oak wood. However, a large proportion of vanillin is formed through the thermal decomposition of lignin. The vanilla aroma is very characteristic of American bourbon whiskey, for example, which has to mature in fresh, charred white oak barrels.
How do the wood flavors get into the whisky?
The aromas, flavors and colorants formed from cellulose, hemicellulose and lignin by charring the barrels have different solubility characteristics. Some of these chemical compounds are only soluble in water, others are only soluble in ethanol and still others dissolve in mixtures of the two substances. When the New Make with an average ethanol content of 63.5% vol. enters the cask, the ethanol-soluble aroma compounds are preferentially dissolved from the staves by the distillate. In Germany, the alcohol content of the whisky decreases during the cask maturation process. As a result, the water content in the maturing whisky increases over time and the water-soluble flavorings in the wooden staves are now preferentially released. Cask maturation is therefore a constant dynamic process.