Science & Technology

Bourbon Whiskeys Are Like Snowflakes — They All Have a Unique Pattern

For some whiskey aficionados, enjoying the spirit may be more of an art than a science. A thoughtful swish, sniff, and sip can reveal nuanced colors, aromas, and flavors. But it turns out you can learn even more about whiskey using a microscope rather than your mouth. Not far from the bourbon capital of America, scientists are exploring the wonders of whiskey in the lab.

Pour One Out

It's pretty common to dilute whiskey with water — the combination is the basis for an old fashioned cocktail or the simpler classic, bourbon and water. This timeless pairing also played a central role in a recent experiment at the University of Louisville in Kentucky, where a team of researchers evaporated bourbon droplets at varying levels of dilution and looked at the residues under a microscope.

These researchers, led by fluid dynamics expert Stuart Williams, had their curiosity sparked by a past experiment involving Scotch whiskey. That experiment was a classic collaboration of art and science: Phoenix-based photographer Ernie Button reached out to engineer Howard Stone to investigate why whiskey dregs leave behind aesthetically pleasing patterns. They found that the unusual designs came from whiskey's composition of alcohol, water, and suspended compounds — likely from the barrel the whiskey was aged in.

Compared to water, alcohol has a lower evaporation point and lower surface tension, or the tendency for its molecules to stick to one another. When a mix of alcohol and water evaporates, the initial loss of alcohol creates a gradient in surface tension known as the Marangoni effect. This phenomenon is responsible for the "legs" you might see on a wine glass and should create a ring-like pattern of evaporation. But evaporated whiskey leaves a more uniform pattern, likely because the other compounds in it disrupt the surface tension and create a template for an artful design.

The resulting whiskey web patterns are unique to each sample of tested American whiskey. These images include diluted samples from (a) Four Roses at 22.5% ABV, (b) Heaven Hill at 22.5% ABV, (c) Maker's Mark Cask Strength at 22.5% ABV, (d) Jack Daniel's Single Barrel Barrel Proof at 25% ABV, (e) Pappy Van Winkle's Family Reserve 23 Year at 25% ABV, and (f) Woodford Reserve Double Oak at 25% ABV.

Barrels of Bourbon

In true Kentucky fashion, the Louisville team decided to test this effect with droplets of bourbon, a designation reserved for American whiskey made primarily of corn and aged in a new oak barrel. They had to experiment with a few different levels of dilution before observing anything out of the ordinary.

For starters, mixtures with high dilutions and alcohol concentrations around 10 percent left behind a clean ring pattern. The team also tried a low level of dilution, finding that bourbons with alcohol concentrations of 35 percent or more left uniform films similar to those observed in the Scotch whiskey experiment. But at an alcohol concentration around 20 percent, the whiskeys left behind unique, weblike patterns unlike anything observed before.

The team hypothesizes that the chemical compounds in whiskey migrated to the surface of the water droplets because they were repelled by the water. When the water evaporated, that surface film contracted and wrinkled to produce a pattern — one that was unique to each whiskey.

"We think each brand leaves a different pattern because each [surface film] has a different chemical composition," Williams, the lead researcher, told Science News. "They're all going to bend and fold in different ways."

Williams and his colleagues also tried the experiment with Canadian and Scotch whiskeys and couldn't replicate the weblike patterns created by American whiskey. Because American whiskey is distilled in new oak barrels, it's possible that a variety of compounds specific to those barrels leaches into the whiskey during the distillation process. The resulting webs could be used to test techniques to accelerate whiskey aging or to identify counterfeit spirits — almost like a whiskey fingerprint.

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Written by Andrea Michelson December 13, 2019

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