The Distillation Process

Phase Change, Vapor-Liquid Equilibrium, Copper Chemistry, and the Science of the Cut

Copper pot stills — the vessel where fermented wash becomes spirit.

Distillation is one of the most elegant applications of physics in all of food and beverage production. It takes a complex, fermented liquid — part water, part alcohol, part a thousand different flavor compounds — and uses a fundamental thermodynamic principle (phase change) to selectively separate and concentrate those components. Understanding the physics tells you everything about why distillers make the decisions they do.

By the time the fermented wash reaches the still, it has traveled a remarkable biochemical journey from grain field to mash tun to fermenter. Now it carries a rich cargo of ethanol (typically 7–10% ABV), water, and hundreds of congener compounds. The distiller's challenge is to concentrate the desirable components — primarily ethanol and specific flavor-active congeners — while separating out or reducing the undesirable ones. Distillation is how that separation is accomplished, using nothing more exotic than a heated vessel, a coiled pipe, and cold water.

The Physics of Distillation: Vapor-Liquid Equilibrium

Distillation exploits a fundamental thermodynamic property: different compounds have different vapor pressures — different tendencies to transition from the liquid phase to the vapor phase at a given temperature. At any temperature above absolute zero, molecules in a liquid are in constant motion, and some have enough kinetic energy to escape from the liquid surface into the vapor phase. The temperature at which a compound's vapor pressure equals the surrounding atmospheric pressure — the point at which it boils — is its boiling point.

Ethanol boils at 173.1°F (78.37°C) at atmospheric pressure. Water boils at 212°F (100°C). This 40°F difference is the entire physical basis of whiskey distillation: by heating the fermented wash to temperatures between these two boiling points, the distiller preferentially vaporizes ethanol along with other volatile flavor compounds, leaving behind the majority of the water and the non-volatile compounds.

In practice, ethanol and water don't behave as pure compounds in solution — they form hydrogen bonds with each other that modify their vapor pressures according to Raoult's Law. The result is a vapor that is always enriched in ethanol relative to the starting liquid, but never perfectly separated. A single pot still distillation of an 8% ABV wash might produce a distillate at 25–30% ABV (the "low wines"). A second distillation brings this to 65–75% ABV new make spirit. No number of distillations can exceed the ethanol/water azeotrope at 95.6% ABV — the point at which liquid and vapor have exactly the same composition and no further enrichment is possible by simple distillation.

Boiling Points of Key Distillate Compounds

Understanding the boiling points of the major compounds in a fermented wash explains why the distillation process can selectively concentrate certain compounds over others — and why the order in which they appear in the distillate follows a predictable sequence.

Pot Still vs. Column Still: Two Design Philosophies

The Role of Copper: Far More Than Tradition

Copper is the overwhelmingly dominant material used in whiskey still construction, and its use is not merely traditional — it has a profound and measurable impact on the flavor of the finished spirit through at least two distinct mechanisms.

Mechanism 1: Sulfur Compound Removal

During fermentation, yeast produces a range of sulfur-containing metabolic byproducts — including hydrogen sulfide (H₂S, which smells of rotten eggs), dimethyl sulfide (DMS, which smells of cooked corn or cabbage), and various thiols and mercaptans. Many of these compounds have perception thresholds in the parts per billion range — meaning they're detectable at trace concentrations that would be invisible to standard chemical analysis. Copper reacts readily with sulfur compounds when they contact the heated copper surface of the still:

Mechanism 2: Medium-Chain Fatty Acid Ester Profile

The degree of copper contact also influences the concentration of medium-chain fatty acid ethyl esters (MCFAEE) — specifically ethyl hexanoate (apple/anise), ethyl octanoate (waxy/fruity), ethyl decanoate (fatty/waxy), and ethyl dodecanoate (soapy) — in the finished distillate. These compounds are produced during fermentation when yeast reacts ethanol with fatty acids from cell membrane decomposition. At low concentrations, they contribute a complex, waxy richness and enhanced mouthfeel. Copper catalyzes reactions that modify these esters and their fatty acid precursors, reducing their concentration in the distillate. More copper exposure → fewer MCFA esters → lighter spirit. Less copper exposure → more MCFA esters → heavier, more viscous mouthfeel.

This is why the shape of the pot still profoundly impacts Scotch single malt character. A tall still with an upward-sloping lyne arm provides more copper contact (through greater surface area and increased reflux — the condensation and re-distillation of heavier compounds within the still neck) and produces a lighter, more delicate spirit. A short, squat still with a descending lyne arm provides less copper contact and reflux, allowing heavier fatty acid compounds through to produce the robust, heavy, waxy character of certain Islay malts or old-fashioned pot still Irish whiskeys.

The Cut: Separating Heads, Hearts, and Tails