R – like maturation and flavour Part 1
Why is whisky matured 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 cask is prescribed. According to the Spirits Regulation 2019/787 of 17 April 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 cask always have to be made of oak?
No. In some countries, the exclusive use of oak wood for casks is mandatory (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 maturation in casks affect the whisky?
But yes, and how! During maturation in wooden casks, the colourless new make changes its aroma and taste and takes on a colour that can range from straw yellow to amber to mahogany. Leading experts estimate that around 60 to 80 per cent of the aroma and flavour of a whisky comes from 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 casks 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 litres.
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 cask, the barrel, and charred inside with an open flame before filling until a thin, black layer of charcoal is formed. This thermal treatment of the inside of the drum 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. During this process, various colour and aroma compounds are formed, which are released into the distillate during the subsequent cask 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 flavours.
Which wood constituents are involved?
Oak wood, like all other plants on our planet, is made up of three main components: cellulose, hemicellulose and lignin. The exact percentage composition of these varies between the different oak species. All three main components are odourless and tasteless per se and are only chemically altered by heat to produce different aromatic degradation products.
What is cellulose made of?
Cellulose is a long-chain and unbranched giant molecule that is composed of many thousands of interlinked building blocks to form long sugar strands. The individual sugar building blocks are exclusively glucose (= grape sugar). Through certain interactions, the individual sugar strands form a very stable fibre structure. Accounting for around 40 per cent 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, the hemicellulose makes up approx. 20 to 30 per cent 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 per cent of the dry weight of wood.
Which flavour compounds are formed in the wood during heat treatment?
The thermal decomposition products of hemicellulose are various sugars, which partially caramelise due to the heat and form small ring-shaped compounds that have aromas of toffee, almonds, nuts, fruit and solvents as well as colour. 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, which can have aromas of cloves, chocolate, smoke and fruit.
How is the vanilla flavour created in whisky?
The chemical compound vanillin is responsible for the vanilla flavour. Vanillin is naturally contained in oak wood. However, a large proportion of vanillin is formed by the thermal degradation of lignin. The vanilla flavour, for example, is very characteristic of American bourbon whiskey, which has to mature in fresh, charred white oak barrels.
How do the wood flavours get into the whisky?
The aroma, flavour and colouring substances formed from cellulose, hemicellulose and lignin by carbonisation of the casks have different dissolving behaviour. 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 enters the cask with an average ethanol content of 63.5% vol., the ethanol-soluble flavour compounds are preferentially released from the staves by the distillate. In Germany, the alcohol content of the whisky decreases during cask maturation. As a result, the water content in the maturing whisky increases over time and the water-soluble flavour compounds in the wooden staves are now preferentially released. Cask maturation is therefore a constant dynamic process.
