Of all the steps in the whiskey-making process, fermentation may just be the most important. To begin with, it’s the first stage of alcohol development, boosting the distiller’s beer (also called wash) to around 8 to 10 percent ABV before it goes into the still. But more crucially, fermentation is when the complex flavors that make up a whiskey’s character start to develop.

In places like Scotland, most distillers use commercial yeasts that are bred for efficiency rather than specific flavors, so there often isn’t a ton of variance at the fermentation stage. But America’s whiskey tradition prizes unique fermentation character, often achieved through the use of proprietary yeasts that have been developed over decades.

Fermentation outcomes vary depending on the base grains, yeast type or types—some distillers use a cocktail of yeasts—temperature, and whether the distiller wants to promote additional flavor development through secondary, malolactic fermentation. But there are a few processes and tools that have big impacts.

Sour Mash

Though some brands highlight this term on their label, the fact is that the vast majority of American whiskey is made with sour mash. No, it doesn’t mean that the whiskey tastes sour. It refers to the process of retaining some of the grain solids left over after distillation and adding them into the next mash, or cook.

The technique was standardized in the mid-1800s; one Dr. James Crow gets the credit for applying scientific rigor to what was, even then, longstanding tradition. Sour mashing had, and still has, a few advantages. First, it’s a way to reuse some of the backset (also called setback or stillage) that’s left at the end of distillation. This slurry is rich with nutrients and often passed on to farmers for livestock feed. But those nutrients also make a nice feast for yeast.

In addition, sour mashing saves energy. Adding hot backset straight to a mash cooker raises the temperature, cutting down on the amount of external heat that needs to be applied as the grain cooks.

Most importantly, backset is highly acidic. Adding it to a fresh mash lowers the pH and creates an environment that’s, first, conducive to saccharification (converting starches to sugars) by enzymes and, second, friendly to yeast while keeping bacteria at bay. In an era before sanitization was widespread, that was critical in facilitating clean-ish fermentations and more predictable flavor development. Even though distilleries are meticulous about sanitization nowadays, they continue to sour mash because not doing so would change the inherent flavor of their whiskey.

Fun fact: Some distilleries’ house flavors actually do depend on a certain amount of bacterial contamination in the fermentation; blame it on those pre-sanitization olden times. So they add cultivated microbes alongside yeast and later pasteurize the mix to guarantee the development of those flavors without the risk of uncontrolled infection. Science!

Sweet Mash

The counterpart to sour mashing. Sweet mash is simply starting each mash and fermentation fresh, without the addition of backset. In the United States, sweet mashing is practiced mostly by small distilleries that don’t produce enough volume to make sour mash practices viable.

Despite losing the cost savings of sour mashing, and having a smaller margin for error, sweet mashing has a couple of distinct advantages. First and foremost, eliminating the possibility of bacterial contamination from the backset means flavors are more consistent.

And because the fermentation is “cleaner,” it’s easier to narrow in on desirable flavors at the distillation stage, with more of the pleasant congeners (see below) carrying over to maturation. Sour mashing tends to create certain off notes that have to be eliminated at the distillation or maturation stages. But sweet mashing gives the distiller more control, enabling them to distill to lower proofs and retain more delicate aromas and flavors. Sweet mash proponents also say that since the distiller’s beer has a higher pH, the whiskey itself ends up softer—or, dare we say, smoother.

Fermentation Tanks

The vessels in which fermentation takes place are called, well, fermenters, fermentation tanks, or, in Scotland and Ireland, washbacks.

Traditionally, fermenters were made of wood, which has the benefit of mild insulation: it doesn’t get too hot as the yeast activity ramps up. These days wood is far less popular because it’s expensive, and the textured grain makes a cozy home for microbes. Instead, most fermenters are made of stainless steel, which is easy to clean and much more durable. But it’s also a heat conductor, so most stainless-steel tanks have cold-water coils or jackets that can be adjusted to create the most favorable temperature for the yeast.

Regardless of the material, all fermenters must be cleaned and sanitized between every use. If anything stays behind, it can easily turn into the next batch’s infection.

Congeners

When yeast does its thing on a whiskey mash, it isn’t just spitting out pure ethyl alcohol—it’s creating congeners too. Think of them as aroma and flavor particles, expressing notes you might describe as fruity, herbal, sharp, or oily. The term congener incorporates a whole range of substances including acids, aldehydes, esters, ketones, and other types of alcohol like methanol and fusel oils.

Distillation not only concentrates ethanol; it allows distillers to highlight some congeners while reducing or eliminating others. The congeners that remain continue on to maturation. Wood, oxygen, and time can further diminish, enhance, or evolve various congeners, so that what ends up in the final whiskey may resemble what came out of fermentation, but will inevitably taste and smell at least somewhat different.

Dona and Yeast Tubs

Most distilleries no longer propagate their own yeast on site, instead relying on labs that precisely isolate their specific proprietary strains and ship them over on a regular basis. But once it’s arrived, the yeast needs time and space to grow until it can be added to a fermenter. (If you add just a small amount of yeast to a multi-thousand-gallon tank, it’s going to take forever to finish its work.)

The distillery might throw the yeast into a jug for a day or two, and then transfer it to a dona tub where it will grow to a couple hundred gallons or so. After that, it moves on to a larger yeast tub or tank where it gets fed malted barley and rye and continues growing until there’s a sufficient amount of “yeast mash” to kick-start fermentation of the main mash. Many distilleries like Four Roses, Wild Turkey, and Jack Daniel use this system, which promotes efficiency without sacrificing the integrity of the house yeast.

Interestingly, both the James B. Beam Distilling Co. and Maker’s Mark add an unusual ingredient during the dona phase: hops. This article provides more detail, but it boils down to the fact that hops help stave off bacterial growth, so some distillers historically used them when propagating their yeast or starting fermentation. In today’s era of meticulous sanitization, the practice is an anachronism. But given the high value bourbon distillers place on doing things exactly the same as they’ve always been done, lest a change impact the whiskey’s flavor, Beam and Maker’s will probably keep on hopping.

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