Proper fermentation is key to making a good spirit, and the levers you choose to pull here can have a profound impact on your product, steering it in any number of directions.
Whatever your choices, this much holds true: A healthy, vigorous fermentation is essential.
Ensure a Healthy Ferment
Slow fermentation can lead to off-flavors because of contamination, as the yeast fail to outcompete other populations or to digest sugars quickly enough to crowd them out.
That allows undesirable yeast and bacteria to get a foothold, producing flavors outside your preferred profile. It can also lead to loss of yield if microbes that don’t produce ethanol take hold, digesting sugars before your preferred yeast can consume them.
The solution is simple, though: Add enough yeast—whether dried or liquid culture—based on the volume and sugar concentration of the wash. Most yeast suppliers can offer suggestions based on their particular offerings.
Grain-based fermentations typically provide enough nutrient—particularly nitrogen—for a quick, thorough fermentation when using enough healthy yeast. However, for fermentations that rely mostly or entirely on sugar or fruit, you may need to add one or more doses of yeast nutrient.
There are some traditional methods to help ensure a healthy fermentation. Some distillers reuse liquid from a previous distillation to help lower the pH of a new batch, creating an environment that’s more hostile to microbes and less prone to infection. They also add the nutrients from a the previous batch’s yeast, which have autolyzed and been boiled, leaving their constituent nutrients behind. This addition is known as backset, and it’s the defining practice for sour mash whiskeys.
Temperature and sugar concentration can also impact fermentation health.
Different yeasts have different optimal temperature ranges, and an uncontrolled fermentation can easily rise past the upper end of that range. Temperature control can involve an active cooling system—usually with a glycol chiller—to counteract the heat produced by fermentation and to keep the temperature within the yeast’s optimal window. Other approaches include starting fermentation at a lower temperature—so that its rise stays within the window—or using open-top fermentation, which allows easier venting of the heat produced by fermentation.
The concentration of sugars can also impact fermentation flavor and efficiency. Yeast that face a relatively higher concentration of sugars also experience higher osmotic pressure, creating more stress than the same amount of yeast trying to ferment less sugar.
That puts you in the position of having to balance flavor with yield: A higher concentration of sugars provides a stronger solution going into the still and, all other things being equal, more efficient use of space and time. However, stressed yeast can produce higher levels of unpleasant congeners. Those lead to higher concentrations of undesirable flavors, which you’ll need to remove with larger heads and tails cuts, or age for longer, to mitigate. Yeast that are highly stressed also can fail to completely attenuate a fermentation, nullifying gains on yield.
Choose Your Vessel
Beyond the need for a strong fermentation, approaches to fermenting wash or mash for spirits diverge widely depending on the distiller’s preferred schedule and flavor profile. That divergence begins with fermentor choices—including closed versus open-top, stainless versus wood, and tall versus wide.
Open fermentation, allowing the ambient atmosphere to make contact with the fermenting media, encourages wild yeasts and bacteria in the air to inoculate the fermentation. Distillers can use this option to develop a “house” character, based on the unique microbes that may live in their facility and surrounding environment. A lot of that flavor development comes from the production of complex organic acids by bacteria—often varieties of Lactobacillus and Acetobacter—which can be catalyzed into desirable esters during distillation and aging.
The use of wooden foeders extends this dichotomy between “clean” and “wild” fermentations. These large, wooden vessels can be built in a closed or open style, with metal manway doors to allow for access to the interior of a closed vessel. (Besides fermentation, distilleries also can use foeders for blending and aging spirits; they allow for a slower diffusion of oxygen compared to barrels, which have a higher surface-to-volume ratio.)
When used for fermentation, foeders can help to develop and sustain a dependable house character. Because the oak used to build them is permeable, yeast and bacteria that contribute to the fermentation can create colonies in the wood, and these can be virtually impossible to remove. Because these populations stabilize over time and continue contributing to fermentations, they help to create a consistent fermentation profile.
Vessel geometry also plays a role in flavor. Wider, shorter fermentors—including but not limited to foeders—encourage greater ester development in the fermentation for a more flavorful final spirit. Taller, narrower fermentors, by comparison, suppress ester formation during fermentation because of higher pressure in the vessel, leaving the fermented media with a cleaner profile.
Choose Your Method
Process choices before, after, and during fermentation represent more ways to affect your spirit’s final character and, ideally, hit your flavor target.
Those choices include your preferred cleaning regimen. Many distillers eschew the rigorous approach to cleaning that brewers favor. Such a regimen commonly relies on strong chemical cleaners and a CIP (clean-in-place) cycle to remove organic residues, then a sanitizer to minimize any residual microbial presence, before adding a pure yeast culture.
In contrast, some distillers intentionally avoid that rigorous approach. They occasionally run cleaning cycles to keep wild fermentation populations in check, or they try to remove excess solids to prevent buildup. Otherwise, however, they allow wild yeast and bacteria to colonize the fermentor and carry over from batch to batch. Likewise, a distiller might steam their foeders or run a mild cleaning cycle to keep wild populations from growing unchecked.
Other distillers—particularly those from brewing backgrounds or those who focus on malt whiskey and its similar mash tuns and fermentors—lean into stringent cleaning protocols. These include the use of caustic soda and thorough CIP methods to set the stage for a cleaner fermentation, allowing their chosen yeast culture to drive fermentation flavor.
Yeast selection, of course, is another fundamentally important choice when it comes to fermentation. In the past, brewers and distillers within the same community would share yeast back and forth, depending on one or the other’s needs. However, as their industries and the science behind them have evolved, distillers and brewers have each selected and developed specific yeast strains to meet their different needs.
Dry active distillers yeast—so commonly used that it’s often called simply DADY—is fast and efficient and the default for many producers. DADY is available for different flavor profiles and fermentation media, from fruit to malt to bourbon and rye.
Distillers have selected these yeasts over the decades for fast and efficient fermentation, often prioritizing turnaround time, yield, and durability for high-throughput commercial operations. Straight out of fermentation, their flavor profiles may not be very enticing. However, they’re meant for spirits that age for years in a barrel, so those unpalatable notes can evolve or volatilize.
Brewers yeasts, by comparison, have evolved differently. Brewers have selected them over the years to provide a relatively slower fermentation with a more approachable flavor profile right out of the fermentor. That profile can include a variety of esters and phenols found in different beer styles—from funky, spicy Belgian ales to the banana-clove-bubblegum of German hefeweizen to the subtle fruity esters of a pale ale. Some craft distillers tap into these yeasts to create more unique expressions of spirits that otherwise follow standard production processes.
Finally, time is another dimension to consider. Some distillers allow for extended time in fermentation to encourage more secondary, wild fermentation. This approach embraces the previously noted house profile, giving wild yeast and bacteria more time to express their unique flavors and aromas. They could potentially even form a bacterial cap—a pellicle—atop the fermenting medium. A pellicle can compromise yield if the population includes a significant proportion of Acetobacter, which digests ethanol to produce acetic acid—i.e., vinegar.
In all, the wide variety of choices in fermentation equipment, materials, and methods gives you as the distiller countless avenues to create a unique product.