Distillation
How fermented liquid becomes spirit
Distillation is a physical separation process — a type of production process in the domain of spirit production — in which a fermented liquid is heated to exploit the difference in boiling points between ethanol (78.37°C at standard atmospheric pressure) and water (100°C), producing a vapour enriched in ethanol that is then condensed into a liquid of greater alcoholic strength than the original fermented base. Distillation does not create new flavour compounds; it selects and concentrates compounds already present in the fermented liquid, while simultaneously allowing the distiller to separate desirable congeners from undesirable ones through the practice of making cuts. Distillation is the production process that distinguishes spirits (whisky, rum, gin, vodka, brandy, tequila, baijiu, and all other distilled beverages) from fermented-only beverages (wine, beer, sake, cider, and mead). The maximum achievable ethanol concentration through simple distillation at atmospheric pressure is approximately 96.4% ABV — the ethanol-water azeotrope — beyond which further distillation produces no additional concentration.
What distillation does — simply
After fermentation, you have a liquid that contains alcohol — but also enormous amounts of water, and hundreds of other compounds. Wine is typically 12–14% alcohol. Beer is 4–8%. To make whisky, rum, or any other spirit, you need to concentrate that alcohol — and that is what distillation does.
The principle is simple: alcohol boils at a lower temperature than water (78°C versus 100°C). If you heat a fermented liquid, the alcohol-rich vapour rises first. You capture that vapour, cool it back into liquid, and you have a liquid that is much higher in alcohol than what you started with. That is distillation.
A single pass through a pot still takes something that was 7% alcohol and turns it into something that is 25–30% alcohol. A second pass takes that to 60–70%. The distiller then reduces that to drinking strength — typically 40–46% ABV — by adding pure water.
Two fundamentally different types of still
Every spirit in the world is made in one of two types of still — or a combination of both. The still type is not a stylistic choice. It is the primary determinant of the spirit's character.
A pot still is essentially a large copper kettle with a pipe coming out of the top. You put fermented liquid in, heat it, collect the vapour, condense it. Pot stills produce spirits with significant flavour complexity because they carry over a wide range of congeners — the compounds that create taste and aroma. Scotch single malt whisky, Irish pot still whiskey, Cognac, and traditional rum are all produced in pot stills. Pot stills operate in batches — you fill, distil, empty, and fill again.
A column still (also called a continuous still or Coffey still after its inventor Aeneas Coffey, who patented it in 1831) is a tall vertical cylinder through which fermented liquid falls while steam rises. The result is a spirit of much higher alcohol content and much less flavour complexity — a cleaner, more neutral spirit. Vodka, grain whisky, most rum, and industrial gin base spirits are produced in column stills. Column stills run continuously — feed goes in at the top, spirit comes out at the bottom, without stopping.
Why the cuts matter
Not everything that comes out of a still is the same. The first liquid to distil over — called the foreshots or heads — contains methanol, acetaldehyde, and other compounds that are either harmful or produce harsh off-flavours. The distiller discards this entirely.
The last fraction — called the feints or tails — contains heavier compounds: fusel alcohols, fatty acids, sulphur compounds. These are either discarded or redistilled.
The middle fraction — the hearts — is what the distiller keeps. The art of distillation is knowing precisely where to make the cut from foreshots to hearts, and from hearts to feints. These cuts define the character of every spirit. Two distilleries using identical stills and identical wash will produce different spirits if they make their cuts at different points.
Pot still versus column still — the complete comparison
- Operation: Batch — fill, heat, collect, empty, repeat
- Output ABV: 25–75% ABV depending on spirit type and number of distillations
- Flavour profile: Complex, congener-rich, character-forward
- Copper contact: High — reduces sulphur compounds
- Used in: Scotch single malt, Irish pot still, Cognac, Armagnac (alembic), Jamaican rum (pot), traditional mezcal, most artisan spirits
- Regulatory requirement: Mandatory for Scotch single malt (SWA regulations), Cognac AOC, many traditional spirit designations
- Operation: Continuous — runs without stopping
- Output ABV: Up to 96% ABV (azeotrope limit)
- Flavour profile: Clean, neutral, light
- Copper contact: Variable — depends on column construction
- Used in: Grain whisky, vodka, most industrial rum, gin base spirit, most bourbon (column + doubler), baijiu (liquid state fermentation category)
- Regulatory limit: TTB: spirits distilled above 190 proof (95% ABV) must be labelled "neutral spirits"
The distillation run — cuts in practice
| Fraction | Key compounds | Sensory character | Treatment |
|---|---|---|---|
| Foreshots | Methanol, acetaldehyde, ethyl acetate, low boiling point esters | Solvent, harsh, nail polish | Discarded entirely. Methanol content is the primary safety reason. Regulated: maximum methanol levels set by TTB, EU 2019/787, FSSAI. |
| Heads | Ethyl acetate, acetaldehyde, some ethanol | Harsh, solvent-forward | Usually discarded or returned to next distillation run (Scotch practice). The head-to-hearts cut point is a critical skill decision. |
| Hearts | Ethanol, desired esters, fatty acid ethyl esters, moderate higher alcohols | Clean, characteristic, the spirit's defining profile | Kept. Everything the distiller puts in the barrel or bottles as new make spirit comes from this fraction. |
| Tails | Higher alcohols (fusel), fatty acids, furfural, water | Oily, grainy, heavy | Partially retained in some rum styles for flavour contribution; returned to next distillation in Scotch production; discarded in vodka production. |
| Feints | Heavy congeners, residual ethanol | Pungent, sulphurous | Returned to the still for redistillation to recover residual ethanol. Standard practice in Scotch and Irish whisky production. |
How many distillations?
| Spirit | Distillations | Governing standard | ABV after distillation |
|---|---|---|---|
| Scotch single malt whisky | Minimum 2 (pot still) | SWA Scotch Whisky Regulations 2009 | Must not exceed 94.8% ABV |
| Irish pot still whiskey | Minimum 3 (traditionally) — no legal minimum beyond SWR equivalent | Irish Whiskey Technical File 2014 | Must not exceed 94.8% ABV |
| Cognac | Exactly 2 (double distillation in copper pot still) | AOC Cognac regulations, BNIC | Must not exceed 72% ABV |
| Armagnac | 1 (alembic armagnacais) or 2 (pot still Armagnac) | AOC Armagnac regulations, BNIA | 52–72.4% ABV |
| Bourbon | 1 (column still) or 2 (column + doubler/thumper) | TTB 27 CFR Part 5 | Must not exceed 160 proof (80% ABV) off the still |
| Tequila (100% agave) | Minimum 2 | NOM-006-SCFI-2012, CRT | Must not exceed 55% ABV for bottling |
| Vodka | Multiple column distillations — no fixed number | EU 2019/787; TTB | Minimum 37.5% ABV (EU) / 40% ABV (USA) for bottling |
| Feni (Goa, India) | 2 (traditional bhatti pot still) | GI Tag protection; FSSAI | 40–45% ABV |
The physics of vapour-liquid equilibrium
Distillation exploits vapour-liquid equilibrium (VLE) — the thermodynamic relationship between the composition of a liquid mixture and the composition of the vapour in equilibrium with it at a given temperature and pressure.
For the ethanol-water binary system, VLE is described by Raoult's Law for ideal solutions:
The ethanol-water system deviates positively from Raoult's Law (due to hydrogen bonding differences between ethanol-ethanol, water-water, and ethanol-water interactions), producing the well-known azeotrope at 95.63% w/w ethanol (96.4% ABV) at 78.15°C and 1 atm, where vapour and liquid compositions are identical and no further enrichment by distillation is possible. This is the physical basis for the regulatory maximum of 94.8% ABV for whisky new make spirit — not an arbitrary limit, but a practical one close to the azeotrope at normal operating conditions.
The McCabe-Thiele method and theoretical plates
Column still performance is characterised using the McCabe-Thiele graphical method, which plots the equilibrium curve for the ethanol-water system against the operating line to determine the number of theoretical plates required to achieve a given separation. Each theoretical plate represents one equilibrium stage — one theoretical distillation step. A column with 40 theoretical plates can achieve vodka-grade purity (96%+ ABV) from a 7–8% ABV wash in a single continuous operation. Real plates achieve approximately 50–80% of theoretical efficiency, documented in distillation engineering literature (Léauté, 1990; Smith, 2005).
Copper's role — the documented chemistry
Copper is not merely traditional in still construction. Its chemical role is documented in peer-reviewed literature. Copper acts as a catalyst for the following reactions during distillation:
| Compound | Reaction with copper | Effect on spirit | Source |
|---|---|---|---|
| Hydrogen sulphide (H₂S) | Cu + H₂S → CuS + H₂ (copper sulphide precipitate) | Removes sulphurous, struck-match off-aromas. Essential in Scotch malt whisky production. | Piggott et al. (1989), The Science and Technology of Whiskies |
| Dimethyl sulphide (DMS) | Oxidation catalysed by copper surface | Reduces cooked vegetable, sweetcorn off-aromas in spirit | Harrison et al. (2011), J. Inst. Brew. |
| Fatty acids (caprylic, capric, lauric) | Reacts to form copper salts that precipitate in the still | Reduces soapy, rancid notes; concentrations of fatty acid ethyl esters (fruity) are preserved | Nykänen and Suomalainen (1983), Aroma of Beer, Wine and Distilled Beverages |
| Acrolein | Polymerisation catalysed by copper | Removes pungent acrid notes from poorly fermented washes | Léauté (1990), AJEV |
The documented implication for still shape: taller stills with more copper contact surface (longer necks, more lyne arm curvature, worm tubs versus shell-and-tube condensers) produce lighter, fruitier spirits. Shorter, squatter stills produce heavier, more sulphurous spirits. The SWA's technical documentation confirms that Scotch malt whisky distilleries select still geometry specifically to achieve a defined new make spirit character — a documented house style decision with direct flavour consequences.
Methanol — the chemistry of the foreshots fraction
Methanol (CH₃OH) is produced during fermentation primarily from the demethylation of pectin — a structural polysaccharide in plant cell walls — by pectinase enzymes. Fruit-based fermentations (Calvados, Pisco, grappa, fruit brandies) contain higher methanol concentrations than grain-based fermentations because fruit cell walls are rich in pectin. This is the documented biochemical reason why fruit spirits require more careful foreshots removal than grain spirits.
Methanol boils at 64.7°C — lower than ethanol (78.37°C) — and therefore concentrates in the foreshots fraction. Commercial distilleries remove this fraction entirely. The maximum permitted methanol levels in finished spirits are regulated as follows (verified against official regulatory text, April 2026):
| Jurisdiction | Category | Maximum methanol | Source |
|---|---|---|---|
| European Union | All spirits | 10 g/hL pure alcohol for spirits; 200 g/hL for fruit marc spirits | EU Regulation 2019/787, Annex I |
| USA | All distilled spirits | 0.1% by volume | TTB 27 CFR §5.25 |
| India | All spirits | 30 mg/100 mL for IMFL | FSSAI Food Safety and Standards (Alcoholic Beverages) Regulations 2018, Schedule I |
| Codex Alimentarius | Spirit standards | No universal standard — defers to national regulations | FAO/WHO Codex Alimentarius Commission |