Distillation transforms a fermented liquid, known as the wash, into a concentrated spirit. This ancient technique, applied to a base liquid similar to beer or wine, is a physical separation method. It serves to purify and concentrate desirable compounds while removing less desirable ones created during fermentation. This process turns a low-alcohol beverage into a high-proof spirit.
The Scientific Principle of Separation
Distillation relies on differential volatility, the difference in boiling points between the various liquids in the fermented wash. Water boils at 100°C (212°F), and ethanol, the potable alcohol, boils at a lower temperature of about 78.2°C (173°F). When the wash is heated in a still, the ethanol vaporizes more readily than the water, along with other volatile compounds.
This difference in volatility allows the alcohol to separate from the bulk of the water and non-volatile solids left over from fermentation. The resulting vapor, which is highly enriched in ethanol, is channeled away and cooled in a condenser, returning it to a liquid state as the concentrated distillate. The separation is never absolute in a single run, but the distillate’s composition is significantly altered.
Boosting Alcohol Concentration
The most immediate outcome of distillation is the substantial increase in the spirit’s alcohol concentration, expressed as alcohol by volume (ABV). A typical fermented wash, like a beer or wine base, usually contains a low ABV, often 5% to 15%. By contrast, the final spirit, such as whiskey or vodka, is commonly bottled at 40% ABV or higher.
The initial liquid collected from the still, often called low wines, may contain 20% to 65% ABV, depending on the still design. To achieve the higher concentrations, the process often requires multiple distillation passes, especially when using a less efficient pot still. Highly efficient column stills can produce a spirit much closer to the maximum theoretical concentration of 95% ABV in a continuous process.
Removing Undesirable Compounds
Distillation functions as a purification step, separating the desirable ethanol from a host of other volatile compounds known collectively as congeners. These congeners are byproducts of fermentation and include undesirable substances such as methanol and fusel oils. Methanol, which can be toxic, is highly volatile and has a lower boiling point than ethanol.
Distillers manage this purification by “making cuts,” separating the run into three fractions: heads, hearts, and tails. The “heads” are the first distillate to emerge, containing highly volatile compounds like methanol and acetone, which are typically discarded. The “tails” come at the end of the run and contain heavier fusel oils, which can impart harsh flavors. The distiller isolates the “hearts” in the middle, which is the purest and most flavorful portion containing the bulk of the desirable ethanol.
Defining the Spirit’s Flavor and Character
The distillation process is deliberately manipulated to define the final spirit’s flavor profile. The type of still used has a significant influence; a pot still performs a batch distillation and is less efficient at separating compounds. This retention contributes to the complex flavors found in spirits like single malt scotch and pot-still rum.
Conversely, a highly efficient column still allows for greater separation, resulting in a cleaner, more neutral spirit with fewer congeners, typical for vodka. Distillers also intentionally retain certain desirable congeners, such as esters and aldehydes, which lend the characteristic aroma and taste. The distiller’s decisions regarding still type, material, and the precision of the “cuts” shape the unique final character of the spirit.