The chemistry behind the taste of whisky really is a complex one, even for something relatively simple as a single malt. The taste imparted to the whisky is given in several stages of its preparation. To fully understand these stages, it is important know at least some of those stages. Here are the basics:
- Barley is made wet in order to make the seed “think” it is time to start growing up into a plant. Germination starts and during this process, enzymes like Amylases transfer the starch into sugar. Then we stop the process since we are not interested in the barley plant but in the fermentable sugar. This is done by drying the malt, either by burning peat, blowing hot air from below or a mixture of both. Here the first part of the taste is introduced.
- Then the malt is ground and the sugars are extracted by hot water, yielding a liquid called wort. This wort is then subjected to yeast, turning the sugar into alcohol and carbon dioxide. The product then contains 8% abv (alcohol by volume) and tastes like a flat Belgian beer. Of course it tastes flat because there are no hops in there.
- The “beer” is then distilled for a first time into “low wines,” which can contain between 20 and 30% abv. The low wines are then distilled for a second (and occasionally a third) time to yield a strong liquor of 70-80% abv. During this distillation, volatile components in the wort will evaporate together with the alcohol, giving the liquid a further boost in taste. Interaction with the copper of the stills is essential here. Copper reacts with the sulfur-containing compounds that would destroy the delicate taste of the final product.
After thinning down to 63.5% abv (per road transport regulations), the liquor is stored in wooden casks. Interaction with the casks is by far the major factor contributing to the taste and colour of the whiskey. After three years and one day the liquor can be called whisky. In this period ethanol will extract taste from the cask, but also will evaporate due to porosity of the cask. This loss is called “the angels’ share” and is one of the reasons a 10-year-old is more expensive that a 3-year-old. Simply because there is less of it left.
The two main cask types used are the secondhand bourbon and sherry casks. The main difference is that bourbon casks are slightly burnt from the inside, whereas the sherry casks are not. During the burning of the inner 1-2 mm of the bourbon cask, of course some pyrolysis of the wood occurs. Here it becomes interesting for the chemist.
Wood is basically made up out of three components: cellulose, hemicellulose and lignin. Pyrolysis of cellulose and hemicellulose will yield Furane derivatives like the ones you find also in caramel:
Pyrolysis of the lignin will yield phenolic type compounds like Vanillin:
During one of my many distillery visits, this time to Ardbeg on the isle of Islay on the west coast of Scotland, the guide explained that the “peatyness” can be expressed in ppm (parts per million). So he was asked: “Ppms of what?” His reply started with an explanation of the unit ppm, until he noticed that there was one chemist talking to another. Immediately he unveiled a blackboard, showing the compounds that he referred to. A more elaborate overview of these compounds can be found in Ref. 1.This is the reason that bourbon cask-matured whisky often has a vanilla-like taste and a “campfirey” smell. It can easily be seen that most of the compounds that impart the strong taste to whisky indeed contain phenolic and furanoic moieties.
Sherry casks on the other hand are not charred. That means there is no activated carbon but rather just the bare wood. During aging in these casks, the ethanol extracts fruit flavours that were left there by the grapes of the sherry. Not all fruit flavor went into the sherry because of its relatively low alcohol content. The flavors prefer to be adsorbed into the wood. The higher alcohol content of the whisky especially makes the extraction of esters like ethyl hexanoate and isoamyl acetate more efficient. This makes Sherrycask-aged or -finished whisky taste more fruity.
An ideal combination is 12 year aging in a bourbon cask and six months finishing in a Doloroso sherry cask. This is sometimes known as the “distillers’ edition.”
Only very few taste components are the esters that originally were in the barley and co-distilled with the ethanol in the distillation range of 77–81˚ C. Interestingly, whereas a chemist would control this process by measuring the temperature at the top of the column, it is still done the traditional way by measuring the density of the distillate.
The peatyness usually ranges from 20 (Speyside area) to 120 ppm (Islay island), the strongest one often being referred to as “campfire in a glass.”
It is a mixture of these compounds that distinguishes one whisky from another, a Highland from a Lowland, an Islay from a Speyside. It is my firm belief than after reading all this, you will still like your whiskey. You should, we’re chemists and after all, it is only ppm levels that we’re talking about.
Slainté! (to your good health)
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