The process of distillation is possible because different compounds have different boiling points. Distillers apply the fundamental principles of phase change, turning liquid to vapor and back again, concentrating certain compounds—usually ethanol, but also potentially other desirable flavor components.
Think of the distilling medium—whether that’s a fermented wash, mash slurry, or distillate that’s already passed through the still one or more times—as a mixture of compounds at different volumes and with different boiling points. That array of variables creates a more complex system than simply dealing with a pure solution with a set boiling point. In a simple, single-compound system, the boil temperature is an easily defined data point. At sea level, the boiling point of water is 212°F (100°C), for example, while pure ethanol boils at 173°F (78°C).
No matter how hard you boil it, a pure liquid solution will never get hotter than its boiling point. When you boil a pot of water, for example, applying more heat results in a more vigorous boil and higher rate of evaporation—but the water molecules cannot get hotter than the boiling point until they change phase from liquid to gas. Once in its gas phase, naturally, water (steam) can get hotter than 212°F (100°C); a liquid can carry more heat energy once it moves to a more energetic phase.
That’s with a pure solution. When dealing with a complex mixture, however, the boiling point can be difficult to calculate—and in distillation, it’s a moving target.
Why It’s Not So Simple
The mash or wash to be distilled, or the spirit to be redistilled, generally contains water, ethanol, and a variety of compounds. These compounds include trace congeners that are lighter and boil more easily than ethanol, heavier compounds—which boil less easily than ethanol, or even water—and a range of other compounds that fall in between.
However, these compounds won’t evaporate, and thus be distilled, in a sequential manner—with the lightest congener carrying entirely over, followed by the next lightest, and so on down the line. This is because evaporation can occur at a temperature lower than a compound’s boiling point, albeit at a slower rate. So, the distillate will be a mix of different compounds that may overlap or have similar volatility.
The lightest compounds that remain in solution will volatilize at the highest rate, though their presence in the vapor is also proportional to their concentration in the distilling media—particularly with respect to other compounds that exist in high levels. High-presence compounds such as ethanol and water will collect through the entirety of the distillation, at varying levels, while trace congeners will see brief increases in their presence as vapor, occurring around their boiling points, and less presence before or after.
As a result, the mix of vapors coming off the distillation medium evolves over the course of the distillation; the lightest compounds become relatively depleted, while the heavier ones become relatively concentrated. Hand-in-hand with the shifting vapor composition, the mixture in the still also evolves toward heavier, oilier compounds. As the overall mixture shifts toward a higher concentration of higher-boiling fractions, the distiller will see a higher temperature in the still—i.e., a higher boiling point in the mixture.
Why That Matters for the Cuts
The elements that initially come off at the highest concentration in a distillation are the most volatile and have the lowest boiling points. These include compounds such as methanol, acetaldehyde, and ethyl acetate.
These compounds are collectively known as the heads, and generally distillers remove them for later redistillation or other purposes. (Large distilleries produce heads in enough volume to sell them for further separation or for industrial use.)
The central part of the distillation run, the hearts, is primarily ethanol, though there are other potential flavor-active compounds such as esters, some phenols, and diacetyl that overlap with the boiling point of ethanol.
These compounds highlight the importance of starting with quality raw materials and ensuring good conversion and healthy fermentation. A contaminated fermentation can lend buttery diacetyl notes to the distillate, while poor raw materials such as smoke-tainted grapes would impart smoky phenols. In either case, the distiller faces an uphill battle to remove the off-flavor because the boiling points overlap while collecting ethanol.
As the distillation continues, the concentration of ethanol continues to drop, and the temperature in the still rises. The vapor mix starts to lean toward heavier compounds, including higher alcohols. Also known as fusel alcohols or fusel oils, this group of higher alcohols contains compounds such as isoamyl alcohol, isobutyl alcohol, isopropanol, 1-propanol, 1-butanol, 1-pentanol, and numerous others.
These compounds with higher molecular weights than ethyl alcohol can lend spirits a variety of characteristics depending on their type and concentration. Different compounds among these late-boiling elements—which begin to run off later in the distillation and provide the defining character of the tails fraction—can be bitter, sweet, grainy, fruity, smoky, and solventy, among other things. Depending on the spirit being produced, their appropriate inclusion can range from zero for neutral spirit to noticeable for whiskeys meant for extended aging in the barrel.
Notably, heavier compounds take more energy to vaporize than lighter ones, so distillers may find themselves having to increase the amount of steam or other heat to maintain a flow of distillate coming out of the still.
The Role of Reflux
The same principles involved in the liquid phase are also at work in the vapor phase. This discussion applies to batch stills (excluding continuous stills) designed with at least some headspace—often with extra room to encourage reflux.
Reflux is the phenomenon where volatile compounds vaporize, condense, and then vaporize again. In an environment with a mixture of compounds, such as a still, reflux acts as a redistillation and an additional, gradual layer of purification. As the mixture of compounds in the vapor phase travels upward, the heavier elements in the mixture become more prone to condensing and dropping back down—again, that’s because they require more energy to remain as vapor.
Reflux allows the lightest fractions to concentrate more than they would through a simple process of boiling and vapor collection; the additional space in the still encourages greater separation of the various components into purer, more distinct fractions of distillate. That effect increases with the amount of space that’s allowed for reflux—whether that’s a little bit of headspace in a pot still, a helmet on top of a pot, or a batch column still. The more space allowed for reflux, the more refined and fractionated the distillate will be.
Condensing the Know-How
Understanding the process of distillation and how to use it to get the product you want is crucial to making the best decisions when choosing equipment and process.
With a strong understanding of these essentials, a good distiller can approach a new product or tweak an existing product with confidence.