Science of whiskey

Teemu Strengell writes

“There are small amounts of sugar in all whiskies. Scottish whiskies have some sugars dissolved from the oak cask and often some from optional caramel colouring (E150a). Total amount of sugars is quite low, usually well below 1 g/l, but in certain cases it is quite possible to reach a few grams per liter. The sweet aromas of whisky matured in refill bourbon or new oak casks mostly come from sweet aromatic vanillin and fruity esters, not from the sugars. However, sugars can have a significant role in the case of casks previously used for sweet wine or sweetened spirit….”

Sugars in Whiskey: whiskyscience.blogspot.com

“Whiskey is among the most diet-friendly alcoholic drinks there is, at least in terms of a calorie-to-booze ratio. A one and a half ounce shot of 86 proof whiskey contains just 105 calories. One 12-ounce bottle of craft beer that contains about the same amount of alcohol? You’re looking at double the calories….”

How many calories are in whiskey? The Whiskey Wash, By Margarett Waterbury

The Chemistry of Whiskey

Andy Brunning is a chemistry teacher based in Cambridge, UK, who runs the amazing Compound Interest website.

Chemistry-of-Whisky Compound Chemistry

Here are the compounds that give whiskey its flavour.

Phenolic Compounds

Phenols primarily contribute bitterness and smokiness to a whisky’s flavour. They’re particularly noticeable in whisky produced from barley that was dried using peat fires, as is still the case in a number of Scottish distilleries. The burning produces the phenolic compounds in the smoke, which are then absorbed by the barley. Charring of the barrels in which the whisky is later matured can also lead to the presence of phenolic compounds in the spirit.

Phenol, cresols, xylenol and guaiacol are amongst the most important phenolic compounds in whisky, in terms of contribution to flavour. Guaiacol is also somewhat responsible for smokey flavours in coffee, and in smoked meats. Compounds called cresols are the culprits when it comes to the oft-mentioned similarity in aroma between Scotch whisky and band-aids. The particular compound responsible is meta-cresol, which has a medicinal aroma, and was also traditionally used in band-aids as an antiseptic. Eugenol is also present in many whiskies, a compound more commonly found in cloves, and partly responsible for their spicy aroma.

Whisky Lactones

A large number of compounds get into the whisky during the ageing process. Amongst these are two compounds that have actually taken on whisky’s name: the whisky lactones. These are, in fact, just isomers of each other, and chemically are named cis- and trans-3-methyl-4-octanolide. Both of these isomers originate from the oak barrels in which the whisky is aged, and both offer a coconut flavour. The cis isomer is the dominant of the two, and has a stronger, spicier flavour.

Aldehydes

Although not shown in the graphic, acetaldehyde is a feature in many whiskies, representing a large percentage of the total aldehyde content. It originates from the fermentation process, and though some is lost in the ‘foreshots’ during the distillation process, some remains, and adds a pungent, sharp note to the taste.

Other aldehydes originate, as with the whisky lactones, from the oak barrels in which the whisky is matured. Syringaldehyde gives a spicy, smoky note, with furfural providing an almond-like, grainy flavour. More familiar is vanillin, the compound that also gives vanilla its aroma. Bourbons are particularly noted for their vanillin content; new casks are used for the ageing of each batch of bourbon, as the vanillin content of the wood is much lower after one ageing cycle. Other aldehydes from the wood include coniferaldehyde and sinapaldehyde.

Additionally, some simpler aldehydes such as hexanal can contribute a grassy note in some whiskies, whilst a malty flavour is associated with 2- and 3-methylbutanal.

Esters

A large number of esters are produced during the fermentation process, resulting from the combination of alcohols and either fatty acids, or the acetates produced during fermentation. Many light esters with fruity flavours and aromas are formed, though these are removed in the ‘foreshots’ during the distillation process. These include isoamyl acetate, an ester with a banana-like aroma. The most abundant ester in the ‘heart’ is commonly ethyl hexanoate, which has an aroma described as apple-like. There are a whole range of other esters that can be formed (check out this handy guide to their aromas), and these can be influenced by fermentation conditions.

Many whiskies undergo chill filtration to remove much of the ester concentration from the final product. The reasons for this are purely aesthetic, as their presence can contribute to the development of cloudiness in the final product.

Other Compounds

Other compounds in whiskey, outside of the aforementioned categories, can further contribute to its character. For example, two compounds commonly found in roses, beta-damascenone and phenylethyl alcohol, can also be found in some whiskies, and contribute a floral note. Diacetyl, a compound commonly associated with off-flavours in beers, is also found in whiskies, and has a buttery aroma.

The presence of some compounds is less than desirable. Sulfur-containing compounds, from the simple hydrogen sulfide to the more complex sulfur-containing aromatic compounds, are the primary cause of off-flavours in whisky. Their presence is reduced by the use of copper stills, as the copper is capable of binding the sulfur compounds, and preventing them from making their way into the final spirit.

Whilst we’ve mentioned a fair number of compounds here, really, we’ve barely scratched the surface as far as the assortment of compounds in whisky are concerned. Whilst it would be an impossibility to discuss them all, you can read a little more about some of them via the further reading links below. In the meantime, if you’re a whisky drinker, you’ll hopefully have a new-found appreciation for the chemical complexity of the spirit the next time you pour yourself a dram!

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Also see The Chemistry of rum. The Chemistry of Gin.

and from James K Monash see Infographic: Table of Esters and Their Smells v2 (200+ smells!)

Compound Interest: The Chemistry of Whiskey

____________________

Many people notice that adding some water improves the taste; adding water is said to “open up” the whiskey. But what is the science behind this? When one mixes alcohol and water, a minor exothermic reaction occurs, releasing a tiny amount of heat. This could allow more volatile aromatic compounds to escape. Adam Whisnant writes:

What other heat sources besides enthalpy of mixture are at play in the mixture, given the following: solutions of equal temperature to the environment, minimal transfer of heat from your hand to the glass in the seconds after pouring, and relatively small concentrations of other compounds including aromatics and methanol (as is required to be potable)?

The heat release is actually quite significant when diluting alcohol with water. Assuming an 80 proof whisky, 40% ethanol in water would be a molar fraction of roughly 0.21. Diluting the just surface with a splash of water, or the entire dram with a lot of water to say 20% ethanol, would give a molar fraction of roughly 0.077 (remember pure room-temperature water is roughly 55.6M). Ignoring -which I admit is improper- the changes in entropy, the enthalpy change alone is on the scale of kJ/mol.

Raoult’s law refers to vapor pressures of a mixture equaling the molar fractions of the solution, but of course ethanol/water solutions are known to not be ideal mixtures due to the nature of inter-molecular interactions.

Henry’s law is much more relevant given the small concentrations of compounds we actually sense – but both laws follow the same principles. In regards to the relevant volatile organic compounds that are more hydrophobic, reducing the ethanol concentration would indeed make them less soluble, but forcing compounds from liquid to gaseous states in the short timescale after dilution cannot be explained without an input of heat minimally equal to the respective enthalpies of vaporization. Otherwise they would precipitate or form another liquid phase separated by density.

As water has a considerable heat capacity, the overall temperature increase isn’t going to seem large to your 37°C hands without a calorimeter. However, our olfactory and gustatory neurons can detect some compounds at just a few parts per billion. I welcome correction or clarification.

Enthalpies of mixture of ethanol and water, by Boyne and Williamson

Endothermicity or exothermicity of water/alcohol mixtures, By Peeters and Huyskens

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