The Golden Standard

Art by Noora Said.

Whether you’re cooking a meal or mixing a drink, chances are that you taste your creation to figure out if it’s right or not. Maybe there’s too much sugar in the lemonade, or your tomato sauce isn’t cooked enough, or your stir-fry needs more salt. Our senses have always helped us decode the mysteries of what we eat and drink. But now, we can also use chemistry to figure out when our favorite refreshments are perfect to consume. Researchers from the University of Glasgow and the Scotch Whisky Research Institute have recently designed a way to determine the flavor maturity of whiskey by using gold.

After whiskey is distilled, it is stored for years in charred wooden casks to gain its characteristic flavor and amber color. The type of cask used for whiskey storage and the duration of aging can dramatically change its flavor profile. This flavor comes from chemicals called congeners that the whiskey absorbs from its wooden cask. Traditionally, casks of whiskey must be constantly sampled by a master blender, who determines if the flavor is just right. Since there are often hundreds or thousands of casks to sample, each taking so long to age, whiskey distillers are very interested in developing a quicker way to assess the maturity of their products. 

William Peveler, a chemist at the University of Glasgow in Scotland, first came across the science of whiskey when he saw a related infographic in Chemical & Engineering News magazine. He noticed that some of whiskey’s chemical structures looked similar to the chemicals he worked with during his doctoral studies, which focused on creating gold nanoparticles. He wondered if whiskey could also be used to make these nanoparticles, and his team tested the hypothesis by using a cheap supermarket-brand whiskey in their lab. “And it did work, which was surprising since things don’t always do that!” Peveler joked.

The researchers found that the qualities of gold nanoparticles that form in different types of whiskey can reveal how long it has been aged. Their analysis involves taking just fifty microliters of whiskey—the equivalent of one droplet—and mixing in the same amount of gold salt. The flavor congeners in the whiskey reduce the gold salt into gold nanoparticles and stabilize them against growing any larger than a few hundred nanometers. These particles are so tiny that they can’t be seen by the naked eye. However, they do give the whiskey a visibly different color because gold nanoparticles absorb light very strongly compared to many other materials. Gold nanoparticles typically absorb green wavelengths of light the strongest. This means that we don’t usually see green light from gold nanoparticles—rather, we perceive the gold nanoparticles as different tones of pink, red, or purple. After just fifteen minutes, the final color of the sample reveals how aged or flavorful the whiskey is. Whiskeys with more flavor congeners generally produce more nanoparticles, giving the sample a more intense color.

The researchers’ use of gold might strike some as strange. “Of course, you use gold, and everyone goes, ‘Well, gold’s really expensive. Why are you using gold?'” Peveler said. Peveler’s group also explored using silver for their study since silver is cheaper and its nanoparticles absorb light even more strongly than gold nanoparticles. However, the composition of silver is different, and the researchers found that the chemistry of whiskey wasn’t powerful enough to reduce silver into satisfactory amounts of nanoparticles. The color of the whiskey and silver mixture did change, but it took hours or days for the reaction to become visible, so Peveler’s team stuck with gold. And surprisingly, the gold they used wasn’t that expensive: the amount of gold needed for each whiskey test is much less than one cent.

In their research lab, Peveler and his team used a spectral photometer worth thousands of dollars to analyze the exact colors of these whiskey mixtures. This instrument looks at light on a wavelength-by-wavelength basis to see how much of each color the whiskey absorbs. However, it’s possible to create a much cheaper version of this device that whiskey distillers could use to quickly and inexpensively test their own samples in-house. This device would use a diffraction grating, a clear piece of plastic or glass that spreads out white light into the rainbow of colors that it is composed of. By shining a light through the whiskey mixture and looking at it through a diffraction grating, you can tell which specific colors of light are being absorbed or reflected by the whiskey, revealing its age. Such a setup could be strapped to a smartphone camera and would only cost tens of dollars to make.

In future studies, the researchers hope to use gold nanoparticles to measure more than just the age of whiskey. “What we saw tantalizing hints of in the paper but couldn’t necessarily pin down in the time frame that we were working with was that sometimes we measured a whiskey, and it gave a really different colored particle, or it was a much bigger particle, or a different shape,” Peveler said. “Sometimes we saw spheres. Sometimes we saw a sort of triangle plate-like thing. Sometimes we saw rods or a star-type shape. My hypothesis is that that is linked to the different chemistry that is coming out of the wood.” Ideally, the gold nanoparticles would not only allow whiskey distillers to determine how much flavor has infused the whiskey but also identify the specific flavors. For example, they might be able to correlate certain particle shapes or colors with buttery flavors or with smoky undertones. “That’s going to be a key challenge going forward,” Peveler said. 

Through chemistry, it’s becoming possible to dissect the flavors we encounter in our daily lives. “I’m fascinated by this kind of stuff: how we taste, how we smell, how we perceive flavor, and things like that,” Peveler said. Peveler has previously done similar sensing research that goes even beyond food and beverages, such as detecting explosives in wastewater and sensing liver disease in blood. But as a big whiskey fan—and a researcher based in Scotland, which is famous worldwide for its whiskey—he has particularly enjoyed working with it for this project. “It’s whiskey! It’s fun, right?”