Distillation is generally associated with heating a solution to drive off and collect the most volatile compounds in a mixture. However, another option bypasses the inherently destructive impact of heating and boiling, allowing distillers to capture some of the bright, fresh essences that can be both elusive and enticing.
In recent decades, vacuum distillation has made the leap from the lab to the distillery, offering distillers an avenue to preserve the most delicate, volatile flavors and aromas to be used in gins and botanical spirits. Consider the fresh flavor components that degrade with heat—the fresh versus cooked flavors of ingredients such as cucumbers, parsley, or rose petals.
How It Works
Vacuum distillation requires a basic understanding of the factors that go into phase changes: freezing, thawing, boiling, and condensation. These changes in physical state are commonly associated with specific temperatures; water freezes or thaws at 32°F (0°C), and it boils at 212°F (100°C). However, these well-known data points leave something unmentioned: the impact of air pressure.
Those temperatures are calibrated to sea level, where air pressure is roughly one bar, or about 15 PSI—though atmospheric conditions will vary slightly (as will the temperatures at which phase changes occur, hand-in-hand). Distillers and brewers who work at higher elevations may know that the boiling point of water drops by almost 1°F per 500 feet in elevation gain, or by 1°C per 300 meters. That’s because thinner air at higher elevation exerts less pressure on the liquid—the lowered pressure allows liquids to escape into the atmosphere more easily, requiring less heat energy to change phase.
While traditional distillation relies on heating to boil a solution and preferentially concentrate certain components—such as ethanol and desirable flavor/aroma compounds—vacuum distillation leverages the atmospheric pressure to lower the boiling point almost to room temperature.
A vacuum still consists of a glass or stainless-steel chamber with a pump on the still’s outflow arm; the pump pulls air out of the chamber, lowering the vapor pressure. With atmospheric pressure removed from the chamber, distillation can proceed at somewhere between room temperature (or even lower, in some cases) and about 100°F (38°C).
That design allows the distiller to capture flavors and aromas that would usually be altered or destroyed in a traditional, heated still. Many of those compounds are associated with fresh botanical and fruit flavors and aromas—the bright notes of fresh fruit versus cooked, jammy notes; or vibrant, floral notes that are too delicate to capture and translate into a traditional distillate.
From the Lab to the Distillery
Aside from flavor factors, the vacuum still has a couple of other advantages compared to a traditional still.
Not needing to boil the distillation media and then condense the hot vapor means that the system requires much less energy, even with the necessary vacuum pump. Also, the lack of high temperatures paired with ethanol vapor creates a much safer environment.
Laboratories have used vacuum distillation for years to isolate specific volatile compounds. A small number of gin distillers started to recognize the ability to effectively “cold distill” in the last half of the 20th century, but the approach has grown in popularity over the past 15 to 20 years.
One of the most common forms of vacuum still is the rotary evaporator, or rotovap (pictured at top). This design relies on a chamber with a neck that is tilted to one side, not straight up-and-down, with the bottom of the chamber in a temperature-controlled water bath, and with a motor that rotates the main chamber. That rotational aspect allows the liquid in the still to coat the flask as it turns, increasing the surface area and encouraging evaporation to shorten distillation time.
As with other vacuum-still designs, the rotovap’s collection apparatus is part of the same self-contained atmospheric environment. However, the distiller can use a peristaltic pump to remove distillate as it accumulates and select specific fractions without breaking the sealed environment. (Because this is an otherwise sealed environment, the distiller should be using a high-quality base distillate that won’t require heads and tails cuts.)
Behind the Bar
Besides gin and other botanical spirits, vacuum distillation has become a potent tool for mixologists trying to capture brighter, fresher notes and bring them into cocktails.
Because vacuum stills are available in a range of sizes—from one- or two-liter lab-scale stills up to 200 liters in capacity—they enable the production of small amounts of vibrant, intense, possibly concentrated components that can add special notes to bespoke house cocktails.
Notably, as with standard distillation, using a vacuum still for mixtures with ethanol is tightly regulated by the federal government and is restricted to legal, licensed distilleries. However, non-licensed users can still tap into the technology to produce hydrosols—water-based distilled botanical flavors.
Even besides their legality, hydrosols allow a beverage program to create bespoke zero-alcohol components for low- or no-alcohol cocktails. Traditionally distilled (hot-distilled) hydrosols are often used in cosmetics as well as in Middle Eastern cooking, in the form of rose water and orange-blossom water. Both traditional and vacuum-produced water distillations also have found a home in the growing market for mocktails.
Because water boils at a higher temperature than ethanol, the system has to run hotter, losing some of the benefits of distilling cold and saving energy. However, it opens the door to a wide range of unique alcohol-free flavors.
More Considerations
Because vacuum distillation preserves a range of more delicate flavors and aromas, the distiller needs to keep that level of delicacy in mind when handling vacuum-distilled components.
Oxidation and other reactions can be a greater factor with these flavors and aromas because they’re inherently more prone to degradation via exposure to light, ambient heat, and air.
Vacuum-distilled fractions also can be used with traditionally distilled flavors, allowing the distiller to add the vibrant, fresh notes of vacuum-distilled components atop a traditional profile of juniper, orange, and other classic gin notes that extract better—or just differently—at higher temperatures. A distiller could even potentially use the same botanical in both stills—for example, by pulling fresh orange notes out of the vacuum still and more traditional, heavier orange notes from a pot still.
Vacuum distillation can be a valuable part of a botanical distiller’s arsenal. Combined with atmospheric (higher-temperature) distillation, ethanol- and water-based vacuum distillation broadens the catalog of flavors available to help distinguish a gin, botanical spirit, or upscale tasting-room cocktail.