Ageing in wood is a highly complex phenomenon that has yet to reveal all its secrets. Spirits professionals are increasingly concerned about the potential consequences of climate change on evaporation and the quality of spirits aged in barrels. Can we anticipate and prevent the effects of global warming?
After exploring the origin and control of volatile compounds in spirits through fermentation and distillation, this final part focuses on the physico-chemical mechanisms involved in wood ageing. It discusses the impact of rising temperatures and presents adaptation strategies to limit evaporation while preserving the quality of the spirit.

The acceleration of global warming is a reality

Evolution of global average temperatures from 1850 to 2022

Données du Ministère de la Transition Ecologique et de la Cohésion des Territoires

Among the potential impacts of climate change is a likely rise in the average annual temperature within maturation warehouses, often accompanied by a drop in relative humidity. Both factors have a significant influence on the physical and chemical dynamics of barrel ageing: they affect the rate and nature of evaporation, the respective losses of water and ethanol, the chemical evolution of the spirit, its sensory profile, and its physical stability.

Accurately characterizing the combined effects of temperature and humidity on barrel-aged spirits remains a complex scientific challenge.

Many parameters related to the choice of barrels and their environment come into play:

• The size and shape of the barrel.
• The type of wood (origin, drying process, thickness of staves, fine or coarse grain, level of toasting).
• The storage method (upright or horizontal, in racks or rows).
• The age of the barrel and its previous contents.
• Its position in the cellar (near a door, at the end or middle of a rack, depending on the height, and other positioning factors….).
• The cellar environment and its construction method – see the dedicated paragraph on this topic.

In addition to these parameters, certain practices also influence the ageing process:

• Refilling barrels with eau-de-vie from the same batch or with similar characteristics (designation, age, etc.).
• Temporarily transferring a batch to a tank for homogenization and to perform certain measurements (alcohol content, tannin levels, etc.) before returning it to barrels.
• Transferring the spirit into larger barrels or casks.

At the same time, various physico-chemical mechanisms take place:

• Direct extraction, by the alcohol, of certain low molecular weight compounds from the wood.
• Breakdown, by the alcohol, of certain wood polymers (cellulose, hemicellulose, lignin).
• Chemical reactions between the extracted wood compounds.
• Chemical reactions between wood compounds and those inherent to the eau-de-vie.
• Chemical reactions between compounds present in the eau-de-vie.
• Evaporation of the most volatile compounds.
• Concentration of non-volatile compounds.
• Precipitation of compounds that become insoluble.

To better visualize this complexity—better than lengthy explanations—here are three diagrams illustrating these interactions.
They are the result of an analysis based on various bibliographic sources, which you will find at the end of the article.

In addition to the reactions illustrated in the previous diagrams, wood compounds extracted during ageing also interact significantly with the volatile compounds already present in the spirit (aldehydes, esters, sulfur compounds, phenols, etc.).
Various reactions such as oxidation, condensation, transesterification, and others contribute to the gradual development of balance between fruity, woody, spicy, and oxidative notes over the years.
These transformations also affect the aromatic perception — leading to a reduction in bright, fruity notes and the emergence of more complex aromas, such as rancio.

What do we know about the effects of humidity and temperature on ageing?

• When humidity is too low, barrel wood dries out, encouraging evaporation—often with more water than alcohol being lost. In tropical aging cellars, it is common to observe significant evaporation rates (6 to 10%), where warm air (25–30°C) becomes too dry, despite high relative humidity levels (85–95%). Under these conditions, the alcohol strength may remain stable or even increase.
• Conversely, in cool environments with high humidity, total evaporation (water + alcohol) can be limited. Because the air is saturated with water vapor, it is the alcohol that tends to evaporate more.
• The rate and nature of evaporation also depend on the type of wood used (its porosity), as well as on lesser-known phenomena such as osmotic effects around the barrel. Inadequate ventilation or tightly packed barrels can saturate the air with alcohol vapors, slowing down alcohol loss.

As temperature rises:
– Certain wood compounds become more soluble and diffuse more readily into the spirit.
– The spirit penetrates deeper into the wood, accelerating lignin breakdown.

Phenolic acids from the wood are among the main contributors to this acidification, lowering the pH (which reflects the spirit’s acidity).

This speed up extraction → leads to quicker acidification → and makes the spirit more reactive toward the wood.

• When temperature rises and alcohol strength remains relatively high, combined with increasing acidity, evaporation intensifies. This accelerates oxidation and esterification reactions (alcohol + acid → ester + water). As a result, levels of certain light esters can rise significantly. For example, ethanol oxidizes to ethanal, which then forms acetic acid. Through esterification, this acid generates ethyl acetate—a key contributor to aromatic complexity, including notes like rancio. However, if the structure of the spirit is unbalanced, excessive ethyl acetate can become a flaw, making the spirit harsh. It’s all about balance—between volatile, aggressive compounds and heavier ones that soften their impact.

• In a private study on rum ageing, I observed that when alcohol content remained above 65% vol., the formation of ethyl acetate (along with ethyl formate) increased exponentially. Concentrations were multiplied tenfold over a 10-year period.

• Unlike wines, spirits actually benefit from storage under variable temperature conditions. The alternation between heat and cold promotes the precipitation of certain natural compounds, such as fatty acids. In tropical climates, where this temperature cycling is absent, products that haven’t been cold-filtered and are rich in fatty acids often show greater physical instability once bottled.

• If ageing conditions do not allow for a gradual reduction in alcohol strength, some compounds that are insoluble at the target final strength may precipitate later and cause stability issues.

Best practices to mitigate the effects of climate change on the quality of barrel-aged spirits

Before adapting practices to evolving climate conditions, it’s crucial to establish clear ageing objectives.
Beyond marketing constraints—such as the age stated on the label—the key is to define the desired organoleptic profile.
A useful approach may be to analyze and characterize commercial products that closely align with your desired profile, and analyze how spirits develop over time under specific practices and ageing durations.

Careful sensory monitoring is essential to understand how the spirit evolves and to adjust ageing conditions accordingly.

Effective ageing requires regular monitoring and logging of key environmental and product parameters.
This allows producers to detect trends early and adjust conditions as needed to maintain product quality.

📌 Key parameters to monitor:

The design and materials of an ageing cellar play a key role in the evolution of spirits, especially through their effects on humidity, temperature, and airflow.

• Flooring: Earth or gravel floors help maintain higher humidity levels, reducing evaporation and preventing alcohol overconcentration. Conversely, concrete or dry stone floors create a drier environment, speeding up evaporation and aromatic concentration.
• Wall materials: The choice of stone, brick, concrete, or wood impacts thermal insulation and humidity retention. Thick stone walls offer natural temperature buffering, while concrete may dry out the space and cause greater thermal variation.
• Ceiling height: A low ceiling traps alcohol vapors and concentrates aromas in a confined space. Higher ceilings allow for better air circulation and more consistent ventilation.
• Openings and ventilation: The presence of windows, vents, or grills helps regulate oxygen flow and dissipate alcohol vapors. Good ventilation avoids excess humidity and mold, supporting balanced ageing conditions.
• Orientation and sunlight exposure: A north-facing cellar enjoys more stable temperatures, while south-facing ones are more affected by seasonal changes—potentially speeding up chemical reactions and intensifying wood compound extraction.

Each of these factors plays a role in shaping the aromatic profile of spirits, by modulating oxidation, evaporation, and wood compound extraction.

This is why the ageing environment and cellar architecture should be carefully assessed to ensure optimal control over maturation.