Most craft distillers are producing spirits via batch distillation, commonly—but not exclusively—using a pot still and a well-established process. In general, most craft whiskeys and a large portion of craft rums are products of a similar set of batch-distillation techniques—using two distillations: a stripping run and a finishing run.
Batch Distillation Basics
The function is in the name: As a distiller, you fill the still before each use, run it, then empty and possibly refill it. In other words, you run one batch at a time, in contrast to a continuous distillation that can go for weeks or months as long as there is feedstock to supply it.
You make the initial fill using fermentation media, whether that’s a grain-in mash or a liquid wash of fermented sugars. For flavor reasons, most distillers maintain a fermentation ABV lower than 10 percent, though some prefer to conduct their fermentations a bit higher than that to maximize their yield per batch.
You load the fermented media into the still and then heat it to begin the run. Using an agitator in the pot can help maintain homogenization while speeding up the heating process; agitation is especially important in grain-on distillations, where it can prevent or reduce grain solids sticking to the walls of the pot and scorching.
The goal of the stripping run is to collect as much of the volatile mass as possible—generally until the spirit being collected falls to somewhere between 0 and 10 percent ABV, depending on the dynamics of a given system. At a certain point, it’s no longer economical to continue heating the still for paltry, low-ABV returns. When the runoff drops to 10 percent ABV, there is about 1 percent ABV left in the pot. The value of that diminishing amount of ethanol is eventually less than the cost of heating the distillation media to a steadily increasing temperature.
The ABV of the collected distillate from the stripping run, known as low wines, generally comes in at about 30 percent ABV at a quarter to a third of the original volume. Although you make no cuts during a stripping run, the ABV of the low wines is important to making good cuts in the finishing run, or spirit run. If you see that the low wines are coming in much higher than 30 percent ABV, it’s best to add water to bring the strength down to 25 to 30 percent before running the second distillation.
That range is widely recognized as a sweet spot: It offers a high enough concentration of congeners to efficiently make heads and tails cuts in a spirit run, but not such a high concentration that the heads and tails bleed too much into the hearts fraction.
The Cuts
While a distillery might standardize cuts based on volume and ABV, flavor and aroma should be the ultimate factors in deciding when to make cuts.
The heads fraction includes compounds such as acetone (nail-polish remover), methanol, and other highly volatile compounds, some of which evaporate directly into the atmosphere. The size of the heads cut varies based on how long you aim to age the product, fermentation parameters, raw materials, and other factors. Fruit fermentations, in particular, can have high levels of methanol that result from pectin degradation, especially through the use of a pectin-reducing enzyme (pectinase).
Tails cuts, on the other hand, are richer and oilier than the hearts, composed of heavy fusel alcohols, fatty acids, and rich grainy flavors. Some of these are desirable and pleasant, but others are bitter and unpalatable.
You can opt to make tighter cuts—removing more heads and tails, resulting in a smaller hearts fraction—to obtain a cleaner base spirit that may be more appropriate for shorter aging in smaller barrels. On the other hand, looser cuts and a larger hearts fraction—one with more of the heads and tails—can lead to a more complex spirit when aged longer, allowing enough time for the congeners to react with oak compounds and trace oxygen that penetrates the barrel.
If you’re unsure, a demisting test can determine whether the oily, rich remnants of a previous run are still lingering in the system at the beginning of a new run. During the heads fraction, you can collect some of the early runnings and add an equal amount of water. If there are heavy, tails-like residues left in the system and not yet fully flushed out, the oils will come out of solution with the addition of water, and the mixture will become cloudy and milky.
Distillers often recycle heads and/or tails from previous runs into subsequent runs to increase yields and efficiency. That practice also helps to smooth out inconsistencies that can occur when making cuts. For example: If you make an unusually large cut on one batch, leading to a smaller hearts fraction, adding the removed heads or tails to the next batch can result in a slightly higher yield to average out the batch sizes.
Variations and Variables to Know
A distillery might use multiple stills for different purposes—to strip and finish, or to produce different types of spirits—or it might use the same still for stripping and finishing all its products. When using the same still, you might collect multiple stripping runs, clean the still, then put the low wines back into the still for a finishing run. Alternatively, if you’re using both a stripping still and a finishing still, you might distill a wash or mash one day and then finish it the next.
For a distillery using multiple stills of different sizes—lending slightly different character to the product that comes off of each—or one that triple-distills their products (as is often done in Irish whiskey), recycling heads and tails can become a complex undertaking with numerous variables. For a small craft distillery, however, it can be as simple as collecting low wines into the same receiving tank as heads and tails, to be ready for the next distillation.
Small tweaks in how you reuse heads and tails (or don’t) will impact the final product. The same is true of how you make those cuts initially. Broadly speaking, you should minimize the amount of congeners in white spirits, such as neutral spirit or unaged rum. However, there can be more congeners in a spirit destined for aging—but the amount should be in balance with the intended aging regimen.
Once you’re familiar with the major variables involved in making batch-distilled whiskey, rum, brandy, or other spirits, you can dive into additional variables.
For example, running an agitator—while important when stripping an on-grain mash—can increase the level of congeners in the vapor when used on a finishing run. Adjusting the level of heat to the still can speed up or slow down a distillation, as well as increase or decrease the amount of reflux that the vapor undergoes—and thus the congener level—in the collected spirit.
Running the still too fast, with too much heat, can result in entrainment, which occurs when the vapor flow increases so much that it actually carries liquid from the pot out into the distillate, harming the spirit’s flavor by adding undesirable characters that should be left behind. High heat can also increase scorching in the pot.
Finally, the fill level of the pot also affects the final spirit character. Because a lower fill level results in a higher level of empty space, and thus more reflux, it also leads to a more delicate spirit—meanwhile, a fuller pot will create a richer spirit.
Numerous factors have a subtle impact on the quality of the final distillate, but every distiller should have a tried-and-true roadmap for making batch-distilled products. Any practiced, competent distiller should be able to apply an understanding of stripping, finishing, and making cuts to produce quality spirits at the craft scale.