Spirits remain distinct under the microscope

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Fans of American whiskey now have the microscopic proof that there’s something special about it — just as long as there’s a “coaster” underneath their examination. Researchers recently found that when droplets of these spirits evaporate from a glass surface, the remaining solids leave an intricate web-like pattern behind. And interestingly, the droplets form unique patterns based on the brand of whiskey. That full-bodied flavor may have solid science behind it.

Stuart Williams, an associate professor in the Department of Mechanical Engineering at the University of Louisville in Kentucky, said he and fellow researchers extrapolated that it may be possible to tell the difference between various ages and brands of whiskey based on the droplets, and even to spot counterfeit whiskey. He noted similar research had previously been conducted on coffee, blood, and polymers, and that it had already been discovered that whiskey left a thin uniform film at high proof.

A drop of Jack Daniel’s evaporates to form a distinctive web-like pattern of solutes. Courtesy of Stuart Williams.

A drop of Jack Daniel’s evaporates to form a distinctive web-like pattern of solutes. Courtesy of Stuart Williams.

“I had various samples of bourbon whiskey,” Williams said. “I had them from a different study for Brown-Forman, a distillery in my city of Louisville, Ky., when I was on my sabbatical visiting Dr. Orlin Velev at NC State.”

It was in the lab of Velev, a chaired professor in the Department of Chemical and Biomolecular Engineering, that the discovery was made and one of Velev’s students helped draft the study, Williams said. “We also observed thin films at high proofs, but we unexpectedly came across these fascinating patterns when we diluted them further, an unusual phenomenon that we wanted to pursue further.”

Williams’ team of researchers used time-lapse microscopy to study droplets of the whiskey as they evaporated. A particular array of compounds — including phenols, aromatics, and esters — had formed a layer at the surface of the droplet. When the liquid evaporated, strands were left behind. And those with a flavor for their liquor will be relieved to know the strands could be matched with impressive accuracy to particular types of American whiskey.

The researchers were unable to establish a similar association for Scottish whisky or other varieties. This may be because the American brands mature in charred oak barrels and contain a higher concentration of solids. Whiskey is composed of thousands of chemicals, and this time-lapse microscopy method, Williams said, would not replace techniques such as chromatography, which separates a mixture by passing it through a solution. But it could provide an initial test — or taste.

According to Williams, the droplets his team used were about 2 mm in diameter. This size can be seen with a glancing light. Images can be captured with a lens or USB microscope, which is readily available to consumers. He said that for his purposes, a standard light microscope is the most effective, with light coming in from the side, along with the reflection of light from the structures.

“We have not applied this to other studies — but we want to!” Williams said. “However, we notice that once you add sugar that the webs are drastically reduced or are eliminated. We will have to carefully look at the contents of the spirit to determine if it can be done.”

So like any good bartender, researchers will need to learn the proper mixture to set down in front of the scientific community seated before them.

The findings were published in ACS Nano ( 9b08984).

Published: April 2020
The chemical method of separating compounds dissolved in one phase (usually mobile) through its equilibration with a second phase (usually stationary). The mechanism of separation may involve partition, adsorption, permeation or exclusion, or ion exchange.
whiskey dropletsUniversity of LouisvilleNC StateTime-lapse microscopyphenolsaromaticsesters clusterschromatographyPostscripts

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