Distillation is the process of purifying and concentrating alcohol by separating it from a lower-proof fermented liquid, transforming a watery wash (essentially a weak beer or wine) into a high-proof spirit like whiskey, rum, or vodka. The entire operation relies on the fundamental principle that alcohol and water have different boiling temperatures. By harnessing this difference, a distiller selectively vaporizes the alcohol, condenses it back into a liquid, and drastically increases its concentration.
The Necessary Precursor Fermentation
Distillation is only the second stage in spirit production, as it requires a starting liquid that already contains alcohol. This precursor is called the wash or mash, and its creation is the result of fermentation, a biological process carried out by yeast. Yeast consumes fermentable sugars, such as glucose and fructose, and converts them into two primary by-products: ethanol (the desired alcohol) and carbon dioxide gas.
The starting material for the wash determines the initial flavor profile and can be anything containing starch or sugar, including grains for whiskey, grapes for brandy, or molasses for rum. This anaerobic process continues until the yeast either runs out of sugar or the concentration of ethanol becomes too high for the yeast to survive. The resulting wash is typically a low-proof liquid, often between 7% and 15% alcohol by volume, which is then ready for concentration.
The Science Behind Separation Boiling Points
Distillation is possible because of the differential boiling points between ethanol and water. Ethanol is significantly more volatile than water. Pure ethanol boils at approximately 173.1°F (78.3°C), while water boils at 212°F (100°C) at standard atmospheric pressure.
When the fermented wash is heated, the ethanol molecules begin to vaporize well before the water reaches its boiling point. By carefully controlling the temperature in the still, the distiller can cause a disproportionately high amount of alcohol to turn into vapor while leaving most of the water behind in the liquid state. This selective vaporization allows the alcohol to be separated and concentrated. The resulting vapor is therefore much richer in alcohol than the original wash, though some water vapor inevitably rises with the alcohol.
From Wash to Spirit The Distillation Process
The physical process of distillation takes place within a specialized apparatus called a still, which can be broadly categorized into two types: pot stills and column stills. Pot stills are traditional, batch-operated vessels that look like large kettles and are favored for producing spirits with a robust, complex flavor profile, such as single malt whisky. Column stills are tall, vertical structures that operate continuously and are far more efficient, capable of producing high-proof, neutral spirits like vodka.
Regardless of the still type, the process begins when the fermented wash is loaded into the still and heated. As the temperature rises, the alcohol and other volatile compounds vaporize and ascend through the still. This alcohol-rich vapor then travels into a condenser, which is a cooled area, often jacketed with cold water.
Inside the condenser, the rapid drop in temperature causes the vapor to revert back into a liquid state. This newly formed liquid is the distillate, a much more concentrated spirit that drips out of the apparatus. Multiple distillations or the use of a column still allows for repeated vaporization and condensation, leading to an even higher-proof and cleaner spirit.
Making the Cut Separating Heads, Hearts, and Tails
The liquid that emerges from the condenser does not instantly become the final, desired spirit; rather, it changes in composition as the distillation run progresses. Distillers must employ a technique known as “making the cut,” which involves separating the distillate stream into three distinct fractions based on their chemical makeup and volatility. This precise selection is crucial for both the quality and the safety of the final product.
The first fraction to emerge is the “heads,” sometimes called the foreshots, which contain compounds with the lowest boiling points, notably volatile and toxic methanol and acetone. These compounds have harsh flavors and must be discarded to ensure consumer safety. As the temperature stabilizes, the distiller switches collection to the “hearts,” which is the central, most desirable portion of the run.
The hearts contain the purest concentration of ethanol and the flavorful compounds that define the character of the spirit. This fraction is retained, as it will become the final bottled product after any aging or blending. Finally, as the run nears its end, the “tails” begin to emerge, characterized by lower alcohol content and the presence of heavier compounds like fusel oils. The tails are oily and impart off-flavors, so they are separated and either discarded or added to the next batch for redistillation.