Ask Jack Daniel’s enthusiasts what they like most about their preferred whiskey and the term “consistency” comes up time and time again. According to Kevin Smith, a microbiologist who serves as the Distillery Manager of Reliability & Technical Services for the brand, “The character and consistency of our spirits are the result of several different factors, and that is what defines our terroir.”

The concept of terroir expression in distilled spirits didn’t gain prominence until fairly recently, a shift driven both by research and best practices that determine desired flavors and character.

While grain sourcing is proving to be a factor of this expression for single malts, the use of multiple grains – as seen in the Jack Daniel’s grain bill of 80% corn, 12% barley , and 8% rye – makes the influence of any one component more difficult to detect.

“At Jack Daniel’s, we find that sourcing the highest-quality grains is far more important than the location in which the grains are grown,” Smith says. A grain bill is destined for conversion and Smith makes good use of the naturally-occurring enzymes in malted barley; this results largely in maltose fermentation versus a full glucose fermentation employing commercial (biotech) enzymes.

In other words, glucose fermentation tends to produces a lighter distillate, while a predominately maltose fermentation by yeast produces a richer, more flavorful distillate that better retains the natural flavor contribution of the barley.

“This choice has a greater effect on whiskey character that the terroir of the grain,” says Smith, who professes that microbiological processes are never easy to wrangle. Orchestrating the microbiome of a whiskey is no exception.

Good Yeasting

Yeasts also play a major role in defining a given whiskey’s aroma and taste. According to Smith, the Jack Daniel’s Distillery uses a proprietary yeast strain, dubbed “101,” with a subtle but identifiable character attributed to an ester (iso-amyl acetate). It’s sweet, fruity, banana-and-pear-like aroma carry over to the whiskey and remains post-maturation.

According to Assistant Master Distiller Chris Fletcher, the strain dates back to the Prohibition era when it was “originally maintained as an old ‘jug yeast.'” Fletcher places its importance second only to that of the barrel regime in terms of house style. “Commericial yeast is the standard in Scotland and Ireland, but we know that our yeasts are contributing a lot of flavor,” he adds.

Responsible for maintaining the yeast archive, Smith has isolated and catalogued various substrains of yeast since he joined Jack Daniel’s in 1998. When evaluating yeasts, Smith purposefully selects strains that produce low levels of fusel oils (200-250 grams per 100 liters). “The amount of fusel oils produced by a yeast can range from 100 – 700 grams per 100 liters. Our goal is to isolate desired strains for flavor and consistency,” he explains.

At the distillery, sour mashing is a two part process that unfolds during the yeast and mashing stages. The best-known component of the sour-mash process is the use of “backset” or spent stillage from the distillation run. This backset is added to the mix of grain and water in the mashing process. The second and very traditional component is the use of a “lactic-soured” yeast mash made exclusively from rye and malt grains and fermented by Lactobacillus debrueckii.

During this process, a lactic culture is scaled up from nine liters and added to into the yeast mash cooker; this lactic-soured yeast mash is then used to grow and scale up the yeast cultures in progressively larger stages from 250 milliliters to 13,000 liters before it’s added to fermenters. The combination of backset in the mash and the lactic-soured yeast mash drops the pH of the mash down to 4.8 or lower.

The souring process serves as a vital step that inhibits spoilage microbes and adds additional character to the whiskey. “By optimizing the [distillation] process through good yeasting, we not only achieve the highest yield [but] also improve aromas and flavors,” Smith says, noting that all Brown Forman distilleries use the traditional practice of lactic souring.

Distillation equipment also has a significant influence on the organoleptic character of the spirit and can either enhance or minimize the influence of the mash bill. “During and average 12-hour shift, we can run 300,000 to 350,000 gallons of fermented mash through the still columns,” says Distillery Operator Drew Smith. “In the grain mill, we can grind 55.000 pounds of corn in an hour’s time.”

A display of 16 screens in the control room makes it possible for Drew and the other distillery operators to maintain quality at this scale of production. “During the fermentation process we use technology that monitors set points and, during every shift, I measure and record the temperatures of each active fermenter as a precaution,” he explains. This ability to closely monitor all aspects of production enable the still house to meet demand and correct inconsistencies in real time.

The whiskey’s final influences come from charcoal mellowing – also known as the Lincoln County Process – and barrel maturation which together determines the finished style of the spirit. Not only do they modulate the yeast esters and the effects of lactic souring, they act as essential steps in maintaining the aforementioned consistency of Jack Daniel’s Tennessee Whiskey.

“Craft is about process, not about size,” says Kevin Smith. Considering his commitment to a task as complex as constructing the microbiome of Jack Daniel’s, perhaps “craft” would be better described as high art.