As any distiller will tell you: Boiling is a fundamental part of distillation and a required step in the spirit-making process.
Despite its importance, many distillers don’t fully understand why boiling is necessary for distilling, and fewer understand what goes into choosing the right speed at which to boil. The wrong boil speed can negatively affect the quality of your spirits—and it can be dangerous, too.
The Science of the Boil
Before you can determine the correct speed at which to boil your distilling media, you first should understand the science behind boiling and how that relates to distillation.
All liquids have what is known as a boiling point—the temperature at which the vapor pressure of the liquid becomes equal to its surroundings, so the liquid begins to rapidly change phases into a gas. Although many people think of boiling points as being set in stone, these temperatures are actually quite variable; they can change considerably along with small changes in altitude or atmospheric pressure.
The boiling points of liquids can also change when they’re mixed with one another, especially if the liquids have substantially different boiling points. The mixture’s new boiling point will be somewhere between the liquids’ previous boiling points.
To separate a mixture, one might assume that you need only heat it to the boiling point of the constituent that has the lowest one, then you’d be able to remove that compound in its gaseous state. Unfortunately, that method isn’t effective—the intermolecular forces that influence the mixture’s boiling point prevent phase change and effective separation.
Instead, you need to bring the mixture to its new, higher boiling point for the components to begin to effectively separate; the lower-boiling component will make up the bulk of the vapor phase. But then it gets more complicated: If these lower-boiling components are preferentially removed from the vapor phase, the composition of the mixture changes, further altering its boiling point and requiring more energy to maintain a boil. That’s why it takes an increasing amount of energy to maintain a boil and effectively distill during a distillation run.
Confused? Perhaps a real-world example will help.
Let’s consider a 50/50 mixture of pure water and pure ethanol; the first boils at 212°F (100°C), and the latter at 173°F (78°C). The calculated boiling point of this mixture should be about 183°F (84°C), assuming standard pressure and altitude.
If we heat the mixture up to 173°F (78°C), the mixture won’t begin to boil—and although we may have some ethanol-rich vapor evaporating off the top, it’s a relatively small amount. Also: If we were to capture that vapor, it wouldn’t be pure ethanol because it would also contain whatever the partial-pressure equivalent of water is at that temperature.
If, however, we continue to heat the mixture to boiling, 183°F (84°C), the entire mixture would begin to vaporize—with ethanol volatilizing preferentially—thus producing a large amount of concentrated vapor of about 80 percent ABV. As that ethanol-rich vapor escapes from the solution, the ethanol concentration in the remaining liquid phase decreases. That pushes the boiling point of the liquid higher, requiring more energy to boil and remove the ethanol. This process continues until the ethanol has effectively been removed from the mixture and only water remains, with its boiling point becoming 212°F (100°C).
Note: Because of azeotropic forces, some water will always also be collected in the vapor phase, thus preventing the distillation of pure ethanol.
Why Speed Matters
Now that it’s clear why boiling is necessary for distillation, it’s important to understand its ramifications.
A still requires a large amount of energy transfer to reach and maintain a boil. That energy can come from anywhere, including steam, direct flame, or heated oil. What’s important for distillers is how fast and evenly that energy is transferred. A still that is boiled too fast, too slow, or unevenly has serious implications for the quality of distillate produced, as well as for the safety of the operating distiller.
Below are a few of the major factors that go into determining the correct speed at which to boil.
Safety
The first and most important factor to consider when determining boiling speed is safety.
Distillation, by definition, creates alcohol vapor, which is extremely flammable and dangerous. Luckily, all stills are outfitted with condensers that cool this vapor into its much safer liquid form, so that it can be collected. There are several different types of condensers, and each is limited by a maximum temperature differential and throughput at which it can operate. If a distiller exceeds that maximum by boiling their still too vigorously, there is a possibility of excess alcohol vapor escaping the still and entering the open atmosphere.
That is extremely dangerous because the possibility for an explosion is very high. So, we must always be very careful to never boil our media at a rate that our condensers can’t handle.
Scorching and Boilover
The second factor to consider when determining boiling speed is risk of boilover and burning or scorching of the liquid being distilled.
When you’re bringing a solution to a boil, the amount of energy transferred into that liquid is often very large. If the liquid is made up of various nonliquid components such as proteins and starches, they can set up stable foams that can overflow out of the still, causing what’s known as a fouling event. That is both dangerous for distillers and bad for the spirit because these compounds often don’t taste good and can make the spirit cloudy.
Nonliquid constituents also can stick to the sides of the still during the boil and begin to carbonize. That carbonization damages the still and produces an off-flavor, a burned taste in the distillate that is very difficult to remove. This is especially likely when the still’s heat source is concentrated and unevenly applied, as it is with a direct flame.
Although more evenly distributed heating methods such as steam can reduce this concern, it can still occur when distillers are handling liquids that have a high solids load, such as bourbon or rye mashes. That’s why it’s important to evaluate the liquid your put into your still and to determine an appropriate boil speed relative to the heating source, to reduce the risks of either foaming or scorching.
Reflux/Flavor
When it comes to boil speed, the third and perhaps most important factor to consider is your desired level of reflux.
We’ve covered it in greater detail here, but simply put, reflux is the amount of vapor that condenses on the sides of the still and re-boils during a distillation run.
Each still has its own potential for reflux, which you can increase or decrease based on the speed at which you boil. Traditionally, we associate lighter spirits with slower boiling stills and increased reflux, while we associate heavier spirits with faster boiling and less reflux, though that’s not a hard-and-fast rule.
When determining your boil speed, consider what rate encourages the appropriate amount of reflux for the product you’re producing.
Entrainment
Another factor to consider when determining boil speed is entrainment—that’s when saturated vapor entraps either solids or other liquids within it as it moves through the distillation column.
Entrainment is often responsible for heavy compounds such as oils or even grain particulates being carried over into the distillate. Some level of entrainment is natural, and it can even be an integral component of a distilled spirit—as in the case of compounds such as 4-vinly guaiacol. However, when the level of entrainment becomes excessive, it can damage the spirit and even cause a fouled distillation.
Entrainment is often caused by sporadic or overly vigorous boiling, which causes heavy particles and compounds to be splashed into the vapor column. For that reason, distillers often try to focus on consistent, even boiling speeds when distilling.
Time Management
Finally, the last factor to considered when determining an appropriate boil speed is time management.
Distilling is an economic venture, and running stills is both financially expensive and energy-intensive. One of the easiest ways to increase the profit margin is to speed up boiling and increase throughput. It’s important for us as distillers to recognize the needs of each spirit as well as the economic realities of running a business before determining an optimal boil speed.
Boiling is a necessary part of producing spirits; choosing the correct speed at which to boil a still is as much an art as it is a science. There are many elements that inform our choice, including safety and quality of spirit.
To be successful, you evaluate each of these factors individually, and then determine what is right for them and the spirit they are trying to produce.