Whiskey's Complex Chemistry

By Jack Dini —— Bio and Archives February 4, 2018

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Whiskeys contain hundreds of compounds, including fatty acids, esters, alcohols and aldehydes, in a wide range of concentrations. The most important flavors in a whiskey come from the raw materials, the distillation process, and the maturation. ¹ Chemist Thomas Collins and his team have identified about 4,000 unique compounds in 70 American whiskeys. ² Whiskeys come in different variants (scotch, bourbon, rye, and so on) but are all essentially produced with just three simple ingredients; water, grain, and yeast. As part of the process, distillate is transferred into charred oak barrels for aging, where flavors such as vanilla, coconut and butterscotch are extracted into the whiskey. ³

The Aging Process

You might think that speeding up the aging process is simply a case of extracting the flavors quickly from the oak barrels, but the chemistry is more complex than that. Chemical reactions create new molecules, many of which are credited with the properties of the longest aged whiskeys. The barrel flavoring takes place in part because alcohol is a solvent that gradually breaks down elements in the wood over time. White oak in particular has an abundance of appealing flavors, including vanilla, nuts, and coconut, as well as butterscotch from sugars in the wood, which are caramelized during charring. ⁴ During the summer, heat increases pressure inside a barrel, and some liquor pushes itself through the char in the barrel's wooden pores, enabling the carbon to filter our impurities. During the winter, the liquor moves in the reverse direction. The process is repeated with less vigor during the heating and cooling cycles of day and night. So merely by sitting in a barrel, whiskey is slowly being filtered through the barrel. Wood-to-liquor ratio speeds the infusion of the wood flavors but that's only part of the aging process. The other part occurs during oxidation, when air gets in through the barrel's semi-porous wood and interacts with the spirit. ⁴ A new generation of innovators is using science to try and speed up the process. Different aging vessels, coupled with clever manipulations of light and temperature have been found to achieve authentic results in quick time. ³ Vibrations, movement, and pressure can also accelerate the interaction between liquor and wood. Bourbon in barrels subjected to a tornado and then exposed to rain, heat, and sun, were opened about five years later (the liquor had been aged for nine to eleven years). When the whiskey was tasted it was really good. ⁴

Blasting spirits with ultrasound has been shown to speed up the aging process

In another experiment one year of aging in warmer Kentucky was the equivalent of about three or four years of aging in cooler Scotland. One approach currently getting attention is the use of ultrasound. The underlying phenomenon driving this is called cavitation—the formation, growth and collapse of microscopic bubbles under the influence of a sound field. Blasting spirits with ultrasound has been shown to speed up the aging process of spirits and accelerate the formation of certain esters that give spirits their distinctive tastes. ³ Another distiller plays music to stir up the contents while another ages bourbon for almost four years on a boat. The idea being that on the water, getting sloshed around by the waves, exposes more of the liquid to the wood more rapidly. ⁵ A Taiwanese whiskey called Kavalan has been winning competitions against classic single malts despite just two years of age on its spirit. Its makers attribute the perceived maturity to the high heat and humidity in Taiwan. It makes sense theoretically—high heat drives the new-made spirit into the wood and accelerates all those chemical reactions. Distilleries across the Southern Hemisphere, in India, Australia, and South Africa, are trying the same approach. ⁶ Because small distilleries without stock on hand want to turn out product faster, they'll often simply use smaller barrels, just 2 or 3 gallons, compared to the 52 gallons in a typical whiskey case. The smaller barrels are more expensive, but the increased surface-to-volume ratio means faster extraction from the oak—three to five months instead of years and years ⁵