The source of whisky’s flavour

Vanilla. Christmas cake. Baking spice. Orchard fruit. Oak char. Homemade apple pie. An autumnal bonfire. Pot-pourri. Old bookshelf must. Nail polish remover. The smell of yesterday’s cabbage and the ghost of last year’s wife. 

The flavours that you perceive in a glass of whisky are personal to some degree because taste is subjective. But there’s also a reason why certain flavours appear in whisky, like the vanillin in oak that gives us vanilla. Throughout the production process, numerous factors influence the character of the eventual whisky we enjoy, so let’s explore the source of whisky’s flavour together. 

What makes whisky taste like whisky?

The grain: 

The selection of grain significantly influences the flavour profile of the whisky. Taste a whisky made purely of barley compared to corn or rye, and keep the production process exactly the same otherwise, you’ll sample three very different whiskies. Malted barley, surprise surprise, imparts a malty character. Corn brings layers of creamy sweetness. Rye adds spice and herbaceousness (sometimes menthol in quality), as well as a little citrus. 

If we want to get really nerdy, we examine the chemical compounds in the grains used and analyse what they contribute. Hexanal and furan, frequently found in barley, contribute citrusy, grassy elements and nutty caramel, respectively.  During the malting process, chemical reactions occur that lead to caramelisation that creates sweet, toasty flavours too.

Peat: 

When peat is used in the malting process to dry the barley, it imparts a distinctive smoky flavour to whisky, characterised by earthy, medicinal, and sometimes maritime notes. The intensity and character of the smokiness can vary depending on the peat’s origin and the duration of exposure, adding complexity and depth to the whisky’s profile.

Water:

The extent to which water has any effect on the final flavour of whisky is up for debate. But there’s a general consensus that the mineral content of water will have an impact. During fermentation, for example, different trace metals will influence yeast metabolism which then impacts the sensory properties of the wash.

Fermentation: 

During fermentation, yeast converts the sugars present in the mash into alcohol, but while that’s happening congeners and esters develop. These infuse the whisky with several different flavours. Short-chain esters, such as ethyl acetate, impart a green apple taste, while ethyl lactate brings a buttery flavour. Longer-chain esters primarily influence the texture rather than the taste. For instance, ethyl palmitate can lend a whisky a waxy or creamy texture. Variations in yeast strains, fermentation durations, and temperatures can significantly alter the levels of these compounds, thereby affecting the final character of the whisky.

Distillation: 

The distillation process concentrates the alcohol and flavour compounds, while also purifying the liquid and removing some flavours. This includes dangerous alcohols like methanol and undesirable aldehydes and esters, eliminated when the spirit is cut, removing the first and last liquid off the still, the head and tails. 

The design and dimensions of the still are also pivotal in shaping the whisky’s flavour profile. The shape of the still determines the surface area available for interaction between the vapour and the copper. Copper acts as a catalyst that removes sulphur compounds and other impurities from the alcohol vapour. A larger surface area allows for more interaction, leading to a smoother spirit. 

The shape and height of the still’s neck play crucial roles in determining which compounds are condensed back into liquid form and which return to the boiler for further distillation. Taller stills with long, slender necks tend to promote the condensation of lighter, more volatile compounds, leading to a more refined and delicate spirit. Conversely, shorter, wider stills allow more heavy compounds to pass through, resulting in a richer, fuller-flavoured whisky. The shape affects the degree to which lytic reactions (breakdown of larger, non-volatile molecules into smaller, volatile ones that contribute to flavour) occur, based on the temperature and interaction time with the copper.

The chosen distillation technique (pot still versus column still), is critical as well. Pot stills generally yield a spirit with greater richness and complexity, whereas column stills produce a spirit that is lighter and more refined. The frequency of distillation also influences the final character of the whisky. This article on triple distillation explains the effect it has. The rate of distillation, the speed at which distillation occurs, is important too. A slower distillation process allows for greater control over which compounds are collected, enhancing the complexity of the flavour.

Ageing and cask influence: 

Whisky is aged in wooden barrels, usually made of oak. This process plays a vital role in shaping its flavour profile, arguably the most significant. Oak barrels release compounds such as vanillin (imparting vanilla notes), cyclotene (offering caramel/maple nuances), and eugenol (adding clove flavours). Additionally, the ageing process facilitates the development or decomposition of other compounds under the influence of oxygen. An alcohol molecule may oxidise into an aldehyde, introducing a new flavour, which could further oxidise into an acid, leading to another flavour. This might then react with another alcohol molecule to form an ester, thus creating yet another layer of flavour. This continuous evolution, combined with the evaporation of more volatile compounds known as the angels’ share, contributes to the whisky’s maturation and mellowing over time.

The duration of ageing is vital. Whisky is aged to mature, to refine the spirit and smooth out its rougher edges, but excessively long ageing periods might result in an overpowering woody taste. Then you have to consider the barrel’s history. The most common cask used in whisky is one that previously held bourbon. Some whiskies are also aged in one style of cask and then “finished” (a final period of ageing before bottling) in another variety. Bourbon to sherry is the classic example, but all kinds of barrels are used to mature and/or finish whisky: Port, brandy, wine, Tequila, beer… All of these will introduce the characters of those drinks to the whisky with additional layers of dried fruit, nuts, and aromatic spice in the case of oloroso sherry. The cask’s treatment, including seasoning, toasting, and charring, further refines the flavour.

Even if the whiskey maker is oak ageing, that doesn’t mean one uniform set of woody flavours. There’s great variation depending on which variety of oak the cask is made of, namely European oak (often Quercus Robur) or American oak (mainly Quercus Alba), due to differences in oak species, growth climate, and cask processing methods. European oak lends a richer, spicier flavour profile with notes of dark or dried fruits and a nuttier essence, attributable to higher tannin levels that provide a drier finish and a structured, more robust mouthfeel. In contrast, American oak contributes sweeter, lighter flavours such as vanilla, coconut, and caramel, with its lower tannin levels yielding a softer, rounder whisky texture. The presence of lactones in American oak enhances creamy, coconut, and woody notes, distinguishing whiskies aged in these barrels.

Geographical Location: 

The geographical setting of a whisky distillery plays a pivotal role in defining the flavour profile of the whisky produced. The environmental conditions of a location – ranging from the climate to the humidity and even the altitude – exert significant influence over the whisky while it ages, accelerating or slowing down the maturation process, affecting how the whisky interacts with the oak barrels and absorbs flavours.

Human touch: 

The craft and skill of the distillers are also important to note. Where the cut points are made, for example, which flavours they choose to take forward and which they choose to remove, will have a big impact. You can also see the human impact in the art of blending. Marrying different single malts and grain whiskies can create a wide range of flavour profiles, from smooth and balanced to complex and multifaceted.

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