Yeast is an amazing organism. It’s a fungus, one we encounter every day—not  only through the fermented beverages we drink and the bread we eat, but also in the very air we breath. For millennia, humankind collaborated with wild yeast to produce wine, beer and bread, yet with no understanding how it functioned on a molecular level. With the invention of  the microscope, we developed the skills and techniques to domesticate a few  strains of yeast suitable to our needs, but the vast majority remain wild, floating on the wind, ready to be discovered (or avoided).

In 2009, Rob Arnold moved from Tennessee to Dallas to begin a doctoral program in biochemistry. Having earned a bachelor’s degree in microbiology, Arnold was at home in the labs of the University of Texas. He spent many of his days isolating marine bacteria from samples of sea water. Yet, in his off time, Arnold began to dream of starting a distillery. His family had been in the alcohol industry for generations, and it seemed to him the right avenue for applying his skills and passion.

As Arnold began exploring the idea of distilling, he met Leonard Firestone and Troy Robertson, who were already in the process of starting a distillery in Fort Worth. As the three men talked, they realized they shared a common vision, and that each possessed skills valuable to a potential collaboration. Even from the beginning, Firestone and Robertson had known that they wanted a proprietary yeast strain for their wheated bourbon. And Arnold seemed to be the right man to make that a reality.

In September 2010, not long after the founding of the Firestone & Robertson Distilling Company, Arnold joined the  team and went to work looking for yeast. But to do it right,  he was going to need a lab. He reached out to local colleges, and Professor Dean Williams of Texas Christian University responded. Williams helped Arnold set up a lab, advised him on using equipment, and even helped him gain adjunct faculty status so he could come and go at the University as needed.

Arnold immediately began collecting samples from the distillery, which is located in a pre-Prohibition brick warehouse. He even took samples from his own home. Arnold applied the samples to petri dishes and waited to see what would grow. The petri dishes, or plates, contained a growing medium similar
to wort, intended to encourage the growth of yeast species suited to fermentation. However, when Arnold examined his first set of plates, none contained a desirable yeast strain.

Because not all yeast species are capable of fermenting the complex sugars within a bourbon mash into alcohol, the three focused specifically on finding Saccharomyces cerevisiae, a variety widely used by brewers and winemakers. Due to millennia  of selective pressure, S. cerevisiae has evolved into an effective fermenter of maltose, capable of surviving in an alcohol solution at concentrations deadly to most other bacteria and micro-organisms.

As the search for S. cerevisiae continued, Arnold headed out of Fort Worth to Rancho Hielo Brazos in Glen Rose, Texas. The ranch manager there, as it turned out, happened to have a background in botany. Arnold spent half a day with the ranch manager traversing the property, collecting samples from the dirt and a variety of plants. Back at the lab, the 20 samples he collected netted hundreds of different bacteria and fungi, including yeast.

Once again, Arnold went through the work of isolating each of the yeast strains by plating and re-plating them repeatedly. In the end, the 20 samples Arnold brought back from the ranch yielded 100 separate yeast strains. To whittle that number down to a manageable size, he used a technique  called Polymerase Chain Reaction to isolate the DNA of each strain. Upon analysis, it turned out 11 of the 100 strains were S. cerevisiae. Arnold checked his notes, and discovered that all 11 came from just three sources on the ranch: a cactus fruit,  an oak tree and a pecan tree.

Arnold then tested his 11 wild yeast strains against a couple of commercial strains in a series of fermentation trials. For a month, he pitched these yeasts into a standardized wort and recorded when fermentation began, when it stopped, and how well each strain consumed the available sugars. He noticed that almost all of the wild S. cerevisiae strains began fermenting right away, although most stalled after 36 hours. Out of the 11 wild yeast strains, only three were capable of fully fermenting the wort and able to hold up to alcohol without dying.

The team ran fermentation results from the final three wild strains, plus a commercial strain, through a blind sensory analysis. They nosed and tasted the distiller’s beer and the white  dog resulting from each yeast strain to see which they liked best. All three picked the same strain as their clear favorite, which they named Brazos.

Arnold checked his notes, and he and the team were delighted to find that all their effort had paid off—not only had they all picked the same wild yeast, but it also came from the same source: a pecan nut. Native to Texas and a number of other states, the pecan tree (Carya illinoinensis) has been the
official state tree of Texas since 1906. 

From the beginning, Firestone and Robertson had dreamed of creating a Texas bourbon that represents the home that they love. It was pure serendipity that, after all of Arnold’s hard work to find a wild yeast strain that could imbue their bourbon with the character of Texas, they found it waiting on a  Texas pecan nut. To protect all that they had accomplished, their Brazos yeast strain is now stored in deep freezers at the distillery as well as at a couple of universities in the area. If Firestone & Robertson ever needed to start from scratch, they have a number of ready samples stored for just such a purpose.

Once they settled on Brazos as their house yeast strain, they went into high gear distilling and laying down barrels of bourbon to age. Firestone & Robertson are making wheated bourbon. The wheat, corn and barley are milled, mashed, then fermented in open stainless steel fermenters using the
Brazos yeast strain. Once fermentation is complete, the wash is pumped into the stills grain and all. Firestone & Robertson stills are a pot-column hybrid, custom built by Vendome Copper & Brass Works, designed so they can distill bourbon in a single pass.

When enough hearts are collected, they pour the spirit into 53 gallon barrels from Independent Stave. Slowly but surely  the stacked barrels are filling up the open floor plan of their brick warehouse, with the oldest barrels in a loft overlooking the distillery floor. According to Arnold, the brick helps to moderate the hot summer temperatures, which can reach into the 100s, and also keeps temperatures stable through the  mild winters.

In the loft, a barrel marked “Brazos” has been quietly sitting  for three years, working its magic with the spirit inside. Arnold, using a copper whiskey thief, drew a small sample and poured it into a nosing glass. Three years in, their bourbon has a fantastic color of deep amber and burnished copper. A nice aroma of vanilla, cinnamon and caramel danced in concert with the oak just above the rim of the glass. Higher notes of floral and fruity esters lingered faintly in the background. At first taste there is an immediate impression of young wood mingled with walnuts, and a lingering maple syrup sweetness that floats on the tongue. Even at barrel strength the bourbon  was supple and smooth, without any harsh heat.

Firestone & Robertson is one of a very small group of distilleries to isolate a wild yeast strain for making spirits. The micro-distilling industry’s vast opportunities for artistry and creativity drive their passion. In the meantime, their bourbon is developing a uniquely Texas flavor born of the pecan trees
in the watershed of the Brazos River.

Originally published as part of the "Defining Craft" series in Distiller Magazine (Summer 2015): 90-92.