Vacuum Distillation and Spirits
Distilling alcoholic spirits from fermented products has evolved tremendously since its earliest roots, presumed to date as far back as the first or second century AD or perhaps even further. Some form or another of the alembic still, consisting of a pot where the liquid to be distilled is placed and heat applied, a long gooseneck to allow time for the vapors to cool back to liquid, and a receiving vessel, was used for hundreds of years until the invention of the pot still in its various incarnations and, later, the invention of the Coffey continuous column still (consisting of stacked plates in each column and two columns in series).
Nowadays, craft distillers typically employ some combination of the pot still and column stills, and large producers tend to use continuous column type stills. The benefits of the pot stills include a more complex and variable product, whereas a continuous still provides for much more uniform and consistent flavor. Liquors produced through continuous distillation will typically get much of their differentiation during the barrel aging process, if aged at all.
Vacuum distillation began to be used in the early 20th century for industrial processes where decomposition or degradation of the distillates was a concern, or where the boiling points were too high. By applying a vacuum, the boiling points of every compound within the system could be introduced to more manageable temperatures, or those below which thermal degradation could occur in certain compounds. In fact, vacuum distillation is critical to the oil industry, to prevent "cracking" of certain fractions of the oil mixture during refining.
So the question becomes, how can one apply this technology to distilled spirits in a manner that is effective and economical? And then again, are there any benefits to doing it this way versus the traditional methods? The answer to the first question is that it's a little harder than it seems (the picture at the top of the article is of the most BASIC vacuum set up one can do in the lab). The answer to the second question is decidedly YES!
Adapting vacuum distillation to fermented beverages is not easy. There are a lot of factors to consider in the still design, supporting equipment and how it will operate. First, it depends on the feedstock. In most whiskey distillations, the mash consists of grains and yeast and a whole host of mucky, sticky things. These can be introduced to a vacuum still and successfully distilled, but it's easier said than done. For example, the stirring motors for the mash during distillation on pot stills are a critical leak point where the shafts come through the still wall, and any leak points reduce the ability to pull a sufficient vacuum. Furthermore, they will result in the removal of a significant amount of what would otherwise be the finished product. On the other hand, using a filtered liquid feed is cleaner, but filtration is an issue unto itself with potential plugging, unexpected flowrate changes, etc.
Another issue that must be addressed is the presence of dissolved gases in the mash or beer. As soon as these enter a state of vacuum, they will rapidly volatilize and can create a foamy mess in the still that could boil over throughout the entire system, forcing a shut down and clean up process before resuming. There are several ways to potentially address this, each with its own drawbacks. An anti-foaming agent could be added to the feed, but this must be added continuously in the case of a continuous still, versus just once in a pot still setup. Another method could be applying a low level vacuum to the beer before it enters the still in order to remove dissolved gas in advance. Other methods can be employed as well, or some combination of these, but it presents another difficulty over atmospheric distillation.
Introducing beer and removing distillate can also present issues. While feeding liquid into a vacuum still is easy (the vacuum draws it in on its own), controlling the flow requires some finer control than just slugging it in. In a pot still, this is not an issue, but in a continuous still, the feed rate must be balanced with the withdrawal rate of distillate as well as heads and tails cuts. Additionally, at scale, removal of liquids from vacuum is difficult in and of itself. This is because most conventional pumps are not designed to maintain a vacuum while moving liquid, and smaller pumps, such as peristaltic pumps, have their limitations on what vacuum level the tubing used can handle and therefore a maximum diameter. So flow rates will be limited. Special, expensive pumps that move product via a rotary screw method in engineered housings are required for steady and high flow conditions, which will be necessary for scaling up a vacuum still operation.
As mentioned before, in a continuous still process under vacuum, all of the flow rates must be balanced to maintain a steady state of distillation in the system. No cuts are required because these occur automatically and continuously, unlike in a pot still, where cuts must be made via trial and error at first, and later refined until the final product character is achieved. Once a continuous still is set up and running, it just hums along. That being said, there are a million things that can go wrong in any vacuum system that can foul a distillation. Atmospheric distillation is far more forgiving and less expensive or technical, and these are certainly some major reasons why it remains popular. It also has a cache that vacuum distilling equipment does not, the beautiful warm glow of a gleaming all copper still and columns. Vacuum equipment is stainless steel, cold and complicated-looking. Well, it is complicated, actually!
But vacuum distillation has many benefits over atmospheric distillation, particularly for spirits. Because pure ethanol distills at 173F and water at 212F, depending on the fraction of alcohol to water, a standard distillation will occur at some range of temperatures in between these two, and cuts are made prior to reaching a certain temperature on the low end, and before reaching a certain temperature on the high end. The cuts remove the heads (low boiling, volatile compounds like methanol, acetone, etc.) and tails (high boiling compounds that exist in higher amounts there such as fatty acids and oils). Both of these are undesirable in large quantities, but may be desirable in some low level amounts along with the mid boiling compounds (the hearts). In a vacuum distillation though, the temperature and vacuum can be controlled and directly correlated with the proof. For this reason a vacuum distillation will create a very uniform product at whatever the composition is at the targeted proof. And since the temperatures of distilling are far lower (below room temperature), the character of the feedstock beer or beer wash carry into the final spirit in a manner that is impossible to achieve at standard distillation temperatures.
The resulting spirits are far cleaner tasting overall. One might say less complex, but in contrast, they can be every bit as complex, or more, because delicate compounds that would otherwise be bludgeoned away by the brute force and temperature of a standard distillation, will carry through to the final spirit. Similarly, if the vacuum distilled spirit is barrel aged for extended periods, the barrel will bludgeon away much of this delicate character in the same manner it would mute the harsh character of a traditionally distilled spirit. This begs the question, how to balance the desire for barrel character with the desire to maintain the character of the spirit? That's a discussion for a future article...
If the same base beer is distilled under vacuum and at atmospheric pressure, distinct differences will be obvious. Only in the vacuum distilled version will the distillate smell and taste almost exactly like a boozy version of the beer. Otherwise it will smell mainly of the grain or any other higher boiling compounds that can stand up to the heat of distillation, and any more volatile, reactive or delicate compounds like hop oils will be permanently altered, perhaps even giving off an undesirable cooked vegetable aroma.
Distilling under vacuum can be complicated, frustrating, technical and expensive. But it can also create a product vastly different, if not superior, to that which is already being created by traditional methods. While I've discussed many of the potential issues and solutions to some of the problems that might be encountered, I've just scratched the surface here. In future articles, I'll go into more detail based on my own experiences building and running one of the few, if not the only, vacuum distilleries in the US.