Congeners, the Rest of the Flavor

Read chemically, the contents of any spirit bottle are mostly ethanol and water. Yet vodka, whisky, and soju are distinct drinks, distinguishable blind. What tells them apart is the remainder, well under a few percent of the bottle, and that remainder has a collective name: congeners.

First, the name

Congeners are the minor compounds beyond ethanol and water, including higher alcohols, esters, and aldehydes, that shape a spirit's flavor.¹

The breadth of the definition is the useful part. Lovely aromas and harsh edges both live under this umbrella, so the interesting question about congeners is composition rather than quantity. Meet the two flagship residents.

On the glass's side: esters

Esters, formed when acids react with alcohols during fermentation and aging, give spirits fruity and floral aromas.²

When a spirit calls apples, pears, or flowers to mind, this family is usually responsible. No fruit needs to be anywhere near the recipe. Fermentation and aging manufacture the aroma on their own.

On the furrowed brow's side: fusel alcohols

Higher alcohols, called fusel alcohols, form during fermentation from yeast amino-acid metabolism and taste harsh and solvent-like.³

The same yeast household produces the opposite temperament. While esters perfume the glass, fusel alcohols handle the throat-gripping part. A finished spirit's impression is the balance of power between these two camps.

Fermentation composes, distillation amplifies

As for where that balance can be adjusted, there is a measurement showing which lever yeast choice pulls and which lever the still pulls.

Co-fermenting a soju wash with Saccharomyces cerevisiae and the non-Saccharomyces yeast Wickerhamomyces anomalus increases aroma-active esters, and the subsequent distillation markedly concentrates the low-boiling-point volatiles, those boiling below 200 degrees Celsius, such as ethyl hexanoate, isobutanol, and isoamyl alcohol.⁴

Which yeasts run the wash changes the ester total, and distillation then concentrates the low-boiling volatiles across the board. If fermentation writes the congener score, the still is the volume knob: whatever balance the wash carries arrives at the glass amplified.

The drinker's lever

Diluting a spirit with water lowers the ethanol concentration and softens the perception of harsh congeners.⁵

Composition was fixed at bottling, but perception is still adjustable. A few drops of water quiet the harsh camp's presence in the glass, and on the rocks, highballs, and proofing water are all variations on this one lever.

• In a glass of your usual spirit, hunt separately for fruit notes and solvent notes. It is practice in reading the ester camp and the fusel camp apart.
• Set a low-congener spirit like vodka beside a congener-rich one like whisky or pot-distilled soju and compare the density of aroma.
• With a spirit that drinks harsh, add water a few drops at a time and find the point where aroma and softness balance.
• Compare bottlings from different houses working the same raw material. Same grain, different yeast and process, different congener book.