Step aside, whiskey connoisseurs. Scientists have a new way to discern quality among bourbons.

An analysis of residues from evaporated bourbons reveals that different types of American whiskey leave behind unique weblike patterns. Such signature evaporation marks, described online October 24 in Physical Review Fluids, could help identify counterfeit liquors or test new techniques to speed up whiskey aging.

Researchers at the University of Louisville in Kentucky discovered these “whiskey webs” by evaporating bourbon droplets diluted with different amounts of water and examining the dregs under a microscope. Bourbons with alcohol concentrations of at least 35 percent left uniform residue films previously seen in experiments on Scotch whisky, while bourbons with alcohol concentrations of about 10 percent left markings similar to coffee rings.

To the researchers’ surprise, almost every American whiskey diluted to around 20 percent alcohol left behind a unique, weblike microstructure. Fluid dynamics researcher Stuart Williams and colleagues suspect that compounds that leach into the whiskey while it ages in charred oak barrels create these webs. “A lot of [those compounds] do not like water,” he says, so diluting the bourbon forces those particles to flee toward the surface and form a skin over the droplet. As liquid evaporates away, that film contracts and buckles to create a network of wrinkles.

Only whiskeys diluted to around 20 percent alcohol form webs when evaporated (center). In whiskeys with lower (left) or higher (right) alcohol concentrations, flavor compounds in the liquor are likely either too diluted or too well mixed into the liquid to form webs. S.J. Williams, M.J. Brown, VI and A.D. Carrithers/Physical Review Fluids 2019

“We think each brand leaves a different pattern because each [surface film] has a different chemical composition,” Williams says. “They’re all going to bend and fold in different ways.” These webs probably don’t form in high-proof bourbon with little water because the compounds don’t migrate toward the droplet surface, he explains. And in extremely dilute droplets, there aren’t enough compounds to coat the surface.

Williams’ team couldn’t create similar webs using Canadian or Scotch whiskies, suggesting that whiskey webs are vestiges of flavor compounds specific to American whiskey distillation — where whiskey is aged in new, rather than reused, barrels. That process may allow more web-forming compounds to leach from barrels into the whiskey, Williams says.