The Science of Spirits Aging

A Tales of the Cocktail panel dives into the processes—and molecules—that contribute flavor characteristics

Spirits under molecules and graphs
Illustration by Jeff Quinn.

The nerdiest of the nerds attending this year’s Tales of the Cocktail convention in New Orleans crowded into one room for a seminar named “Better Drinking Through Chemistry“ on Thursday. The panel focusing on spirit maturation featured two PhD scientists: Dr. Matthew Crow of Diageo and Dr. Don Livermore of Wiser’s/Hiram Walker, along with Jordan Bushell from Hennessy and moderator Ewan Morgan of Diageo.

Each speaker tackled different aspects of spirit maturation science and practice, showing audience members how analytical study allows producers to create specific flavor characteristics for their various brands and marques.

Livermore spoke first on “the most underappreciated molecule” of lignin, which is at a much higher concentration in rye than in other whisky grains. The lignin is broken down and transformed via cooking and distillation into compounds producing the flavor notes of clove and smoke, as well as the particularly distinctive spicy note we associate with rye whisky, from 4-ethyl guaiacol.

Following through to distillation, Livermore discussed how the yeast-derived flavors of a spirit (fruity, floral, green grass, soapy, and sulfur) are separated through pot distillation: the sulfur removed by the copper of the still; the green grass notes in the head cuts of distillation; the soapy notes in the tails cut.

In maturation inside an oak barrel, that magical lignin comes up again, in a role Livermore calls the “mortar to the bricks” of cellulose and hemicellulose that make up most of the wood. When burned in the barrel charring process, the broken down lignin products add to many of the smoky, phenolic components to the aging spirit, while the cellulose and hemicellulose impart many of the caramel-type flavors.

Livermore finished with some counter-intuitive experimental data showing that more char on a barrel doesn’t necessarily lead to more wood extractives in the spirit aging inside of it. He found that a new barrel charred to two millimeters depth gives more wood extractives than one charred to a four millimeters depth.

Next up, Diageo’s Crow spoke on three ways in which spirits change during barrel aging: subtractive (taking away the less desirable flavors), additive (color and flavors taken from the barrel), and interactive (the distillate reacting with the wood, with oxygen, and with the barrel char). The choice of brand-new versus many-times reused barrels for a whisky is based on how much and which type of wood influence is desired.

To demonstrate the point, attendees sampled a taste of a whisky (not commercially available) distilled at Cardhu and matured for 12 years in  “low activity, multi-fill American oak hogsheads.” The spirit had lost its characteristic immature notes of young Scotch, yet had picked up very little color or caramel flavor from the barrels.

The audience then tried a sample of Glen Ord whisky aged in a barrel with heavily charred new ends (i.e. the flat top and bottom) on an old barrel. Crow said they initially found the spirit to be too woody and spice dominant, masking the “distillery character” from the spirit itself. But after another eight to nine years of additional maturation, the distillery character resurfaced as the spirit matured.

To bring it all together, the crowd sampled Oban Little Bay, which was matured in a variety of barrels, then blended and rested in “low activity barrels” that Crow said made a significant positive impact on the character of final whisky.

An audience member asked how much spirit makers could rely on chemical analyses as opposed to human taste buds in creating spirits. Crow said: “The chemical analysis supports the sensory,” to mutual agreement by all panelists. Bushell of Hennessy, who was about to speak next, added that in producing Cognac the distillers have to make decisions on how to distill the wines before lab analyses could be completed. If they waited for test results, the wine would have changed enough that said results would no longer be useful.  

Bushell then spoke about the particular, meticulously planned aging process of Cognac. “The first flavor concentration is through distillation. The second concentration is through the angel’s share,” he said, referring to the evaporation of alcohol and water during barrel aging.  

At Hennessy, two types of aging warehouses are used to manipulate brandy in different ways. The company’s dry cellars have an annual angel’s share 4% on new barrels, while the angel’s share in its humid cellars can be as little as 0.5%. The humid cellars, therefore, are where the very old Cognacs are kept to mature at a slow rate without too much of the precious liquid evaporating away.

Additionally, Hennessy groups its barrels into categories depending on how long they’ve held maturing brandy (and thus how much impact they’ll have on spirits aging in them). Barrels used for up to one year are in Category A, while Category E indicates barrels that have already been in use for 20 to 35 years. Different marques of Cognac will age through different combinations of barrel categories and placement in different cellars. Further, Hennessy does not simply age a VSOP Cognac a little longer to get the XO marque; it ages the spirit in a different combination of barrels and cellars for each blend.

Overall, the seminar gave attendees an introduction to how complicated the chemistry of aging spirits can be, how advanced the scientific understanding of it is, and how much data can contribute to the decision-making in spirits production. But at the end of the day, someone with a trained palate—rather than someone trained in chemistry—will make the final decision on which products make it to market.


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Camper English is an international cocktails and spirits writer, speaker, and consultant, with a focus on the science of booze and big clear ice. His work has appeared in Popular Science, Cook’s Science, Whisky Advocate, Saveur, Details, the San Francisco Chronicle, and many other publications.

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