What Makes a Whisky a Whisky (Chemistry)

What Makes a Whisky a Whisky (Chemistry)

Whisky education from Whisk(e)y Advent 2025 (2025-12-18). Summary below; full write-up with sources and images: calendar/2025-12-18.qmd.

Whiskey is ~40% alcohol and ~60% water, yet flavor comes from trace compounds. A chromatogram of Kavalan single malt shows the defining character lives in a tiny fraction of esters, aldehydes, phenols and lactones drawn from grain, fermentation and - especially - the barrel.

Verbatim source text

Reproduced from calendar/2025-12-18.qmd (Whisk(e)y Advent 2025).

Whiskey is roughly ~40% alcohol, and ~60% water. But as anyone who has drank Hawkeye Vodka understands1, alcohol and water don’t taste like whiskey. So what are we tasting?

Functional group Peak area %
Alcohol 91.40
Acid 2.93
Ester 2.31
Aldehyde 0.61
Ketone 0.25
Lactone 0.14
Phenol 0.10
Alkane 0.09
Furan 0.08
Alkene 0.03

The table above from Oxford Indicies is a chromatogram measurement of a Taiwanese whisky (Kavalan single malt), which can give us an idea of relative abundance. Oxford Indices took the same measurement, and looked more closely at the ten compounds at lowest abundance, totalling only 0.019% of the compounds:

Compound Peak area % Functional group Flavour
Cyclohexanone 0.035 Ketone Minty
Phenol, 2-methoxy- 0.027 Phenol Woody, savoury, smoky
Acetic acid, 2-phenylethyl ester 0.025 Acid Sweet, floral, rosy
Butyrolactone 0.024 Lactone Milky, peachy
2-Furancarboxaldehyde, 5-methyl- 0.019 Aldehyde Caramellic, maple-like
Benzaldehyde 0.017 Aldehyde Cherry, nutty, woody
Butanedioic acid, hydroxy-, diethyl ester 0.014 Acid Sweet, fruity, herbal
1-Octanol 0.014 Alcohol Waxy, orange, fruity
3-Octen-1-ol, (Z)- 0.006 Alcohol Fruity, herbal, earthy
2-Methoxy-4-vinylphenol 0.006 Phenol Spicy, woody, balsamic

What makes a whisky a whisky is only a very small portion of what’s in the bottle. Some of this comes from the ingredients used, some come from the fermentation by yeast, and a lot comes from the barrel aging process2