Time
Maturation of whisky in oak is a three-pronged action that requires three different processes to occur. These three processes may be summarised as being additive, subtractive, and interactive. They are worth elaborating on:
Additive: The cask will add flavour and character to the spirit. Wood sugars, oaky notes, tannins, the influence of the previous filling (e.g. sherry), and colour will all be added to the spirit.
Subtractive: The cask will subtract certain volatiles and compounds from the spirit, making it more mellow, softer, and approachable. Casks are typically charred before being employed in the drinks industry; the charred wood acts as a natural filter that removes undesirable compounds such as sulphur. (As an aside, this is why the distilleries that produce a more sulphury new-make often benefit from longer years in the wood). Other volatiles in the spirit, some of them undesirable, will be lost through evaporation. The “angels’ share” is the lighthearted term given to evaporation, but it’s a serious matter for whisky producers in warmer climate countries: Filling a 200 litre cask with your precious spirit and only having 100 litres of it left in the cask after five years is a challenging issue.
Interactive: This is where the magic happens. Certain chemical and molecular reactions occur between the wood, the spirit, the residuals left by previous fillings, and – most importantly – oxygen, to create new flavours/aromas. The casks breathe; the spirit oxidises; and the oak and spirit transform one another. This interactive process can be where the different oak species produce different results, as they vary in the nature of their chemical and biological make-up: The differing characteristics of the wood’s density, permeability, hemi-cellulose, lignins, tannins, oils, lipids, and so on, will all impact the spirit differently. Despite the modern Scotch whisky industry being more than 250 years old, commentators and industry professionals such as Dr Bill Lumsden of Glenmorangie believe that this third interactive process is only just now starting to be understood from a scientific perspective.
Time is therefore of the essence, and due time is needed for all three of these processes to play out. Three years is the minimum period of maturation stipulated by law in Scotland, which was set as a minimum benchmark for quality….although the climate and production regimens in Scotland generally dictate that most whisky will take 8-15 years to peak. Climate and environmental conditions (namely heat and humidity) obviously have a large influence on maturation – particularly evaporation – and so different countries factor this into their production methodologies and timescales. There’s a truism that whisky matures faster in hotter countries, but it’s important to understand the difference between true, three-pronged maturation and simply base evaporation and taking on colour.
Evaporation over time is a complex problem for distillers, and it differs the world over, subject to the local climate – particularly the local humidity. The conditions in Scotland lead to the alcohol evaporating faster than the water within the spirit, meaning that the alcohol content of the whisky decreases with time. In contrast to this, the conditions in Kentucky, USA, lead to the water evaporating faster than the alcohol, meaning that the alcohol content of the spirit actually increases over time!