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Analytical Chemistry

What’s that Stuff

What’s whisky and how do you best enjoy its hundreds of flavor compounds?

The controversial act of adding a little water might help drinkers savor the complex taste of this spirit

by Bethany Halford
January 24, 2017 | A version of this story appeared in Volume 95, Issue 5

Image of a whisky glass.
Credit: Shutterstock

With regard to whisky, the comedian W. C. Fields once offered the following advice: “Always carry a flagon of whisky in case of snakebite, and furthermore, always carry a small snake.” Although whisky never caught on as an antivenom, it’s thought that the spirit’s original purpose was medicinal. And while whisky was undoubtedly invented by Celts, whether it was first made in Ireland or Scotland is a matter of some dispute. Even St. Patrick himself is sometimes credited with the drink’s first distillation.

Origins aside, whisky is a spirit made from fermented grain, such as barley, corn, rye, or wheat, that’s often malted, distilled, and then aged in a barrel. Because the starting cereals and production processes vary tremendously from distiller to distiller, there’s an incredible variety in the hundreds of flavor molecules present in each wee dram of the spirit. That’s why, even to a casual drinker, a glass of Scotch whisky tastes very different from a glass of Irish whiskey.

A quick aside about spelling: “To e or not to e” is another controversial matter in the whisky world. Although there are exceptions to the rule, “whisky” is the favored spelling for spirits from Scotland, Canada, and Japan, while “whiskey” is usually reserved for potables from Irish and American distilleries. C&EN has chosen to omit the extra vowel in honor of Scottish poet Robert Burns, whose birthday on Jan. 25 inspires many a whisky toast and whose writing has inspired a spate of American authors, including J. D. Salinger, John Steinbeck, and the C&EN reporting staff.

The first step in making malt whisky is to malt the barley or other starting grain. Dry barley is steeped in water until the grains begin to sprout. Then they’re removed from the water and left to germinate. The barley’s maltase enzymes begin to break down the grain’s stored starches into the disaccharide maltose and other sugars.

At this stage, the grain is dried in a kiln to stop it from growing and using up all the sugars that will become the whisky’s alcohol. Some Scotch whiskies are renowned for their peaty taste, which they acquire when slow-burning boggy mud known as peat is used to dry the malted barley. This step adds flavorful phenols such as smoky guaiacol, spicy eugenol, and medicinal m-cresol, to the golden liquid.

The next stage is to prepare the mash. The whisky’s grains are finely ground and mixed with hot water to dissolve the sugars and other chemicals. The resulting slurry, called mash, is filtered to give a liquid known as wort.

Whisky makers then brew the wort, using yeast to ferment the liquid’s dissolved sugars into alcohol. The fermentation stage produces several flavor compounds, such as isoamyl acetate, which has a banana-like aroma, and apple-like ethyl hexanoate. But a large fraction of these chemicals are often removed via distillation or chill filtration because they are fatty acid esters and can cloud the end product.

After fermentation, a whisky distiller does as the name implies—distills the low-alcohol brew into a high-proof spirit. Even at this stage in the process, there are still plenty of chemical reactions going on in the whisky solution. That’s because the liquid is distilled in stills made of copper, which acts as a catalyst to further modify flavor compounds in the brew.

Depending on the whisky, this spirit may go through multiple distillations. While American whiskies are usually distilled only once, Scotch whiskies are typically double distilled, and Irish whiskies are distilled three times.

Regardless of how many stills the spirit goes through, the most important part of the distiller’s job is to collect the distillate at the right time. Collect too early, and low-boiling compounds make the spirit unfit to drink; collect too late, and stinky sulfurous compounds will foul the drink.

Finally, before the spirit can be called whisky, it must age in a wooden cask for a certain number of years, depending on what type of whisky it will be. Often, whisky makers let the drink mature for many more years than the law dictates. Naturally, part of the whisky evaporates during this time. Distillers call this loss the “angels’ share.”

“Maturation is easily the most important part of the whisky production process as regards to flavor,” notes Phillip Hills in his book “Appreciating Whisky.” A malt whisky, he points out, “acquires more than half of its flavor during maturation; some would say as much as 80% of the final flavor of the spirit comes from the cask.” These chemical flavorants come from the wood and from absorbed compounds left by the cask’s previous contents, such as sherry or port. These compounds include aldehydes, such as vanillin and the grainy furfural, as well as the eponymous whisky lactones, cis- and trans-3-methyl-4-octanolide.

Of course, there’s plenty of chemistry that can go wrong when making a brew as complex as whisky, says Paul Hughes, a chemist and expert on distilled spirits at Oregon State University. Acetaldehyde, for example, is produced during the fermentation stage of whisky making. Too much of it will give the spirit a sour-apple flavor, Hughes says. Another compound that used to plague whisky was the carcinogen ethyl carbamate, which forms from the reaction of urea and ethanol in the still. The urea originates from arginine or citrulline in yeast. Fortunately, Hughes says, it’s been possible to engineer yeast that generates less urea, thereby eliminating the carcinogen from whisky.

Although Hughes’s specialty is on the chemistry of distillation, he has some advice for appreciating whisky: Add some water to the spirit to increase your experience of the flavor.

Here’s his logic: When you add water to alcohol that’s at 40% concentration—as it is in whisky—the combination doesn’t mix into a homogeneous solution, Hughes says. It may appear that way by eye, but on a molecular level, you have clusters of alcohol in a background of water. Without added water, he says, those hydrophobic clusters hold onto flavor, and the drinker doesn’t get the full effect of whisky in the mouth. But, Hughes explains, when you start adding water, those hydrophobic clusters begin to break up, so more of the flavor reaches the tongue.

Hughes recommends adding a little bit of water to the spirit at a time until you find a flavor profile you like or you reach a 2:1 whisky-to-water ratio.

“Often people say you shouldn’t add water to whisky,” Hughes says. “But I’m of the view that it’s your drink and you can do what you like with it.”

UPDATE: This article was modified on January 24 to refresh its information and data.


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