The Paradox of Boiling Points: Why High-Boiling Aromas End up in Whisky

There's a fascinating phenomenon in whisky production: the distillate contains aroma compounds (congeners) that actually have a higher boiling point than ethanol – some even higher than water. So, they should technically remain in the still. But chemistry has its own ways.

Carrying over instead of evaporating

In fact, many of these low-volatility substances still enter the gas phase – not on their own, but "carried over" by more volatile substances like ethanol (boiling point: 78.4 °C). This so-called co-distillation phenomenon ensures that even aromas with high boiling points end up in the distillate – and thus in the taste of the whisky.

Not just the boiling point counts

It's not just the boiling point that matters, but the vapor pressure of a liquid. In a mixture of substances, the components influence each other: even at moderate temperatures, low-volatility molecules can evaporate if their vapor pressure is increased by other components – such as ethanol. This is explained by Raoult's Law. This is how high-boiling aroma compounds also get into the distillate.

Molecular tricks: Hydrogen bonds and azeotropes

Some congeners literally "stick" to ethanol, for example, through hydrogen bonds – similar to tiny magnets. They then evaporate together. Azeotropes also play a role: these are liquid mixtures that can no longer be separated during distillation and boil together – often at a temperature below their individual values.

Technology meets chemistry

How many and which aromas transfer into the distillate also depends on the technology: the shape and size of the still, temperature profile, distillation speed, and even the condenser used play a role. Copper, from which many stills and condensers are made, also contributes to the aroma profile – by reacting with undesirable sulfur compounds. In addition, new, more volatile compounds can form during distillation, further refining the whisky.

Conclusion

Although many aroma compounds have high boiling points, the interplay of chemistry and technology brings them into the distillate. The result: a complex whisky, in which every molecule tells a small story – and every drop is a true distillation marvel.