Temperature plays a major role in the whiskey aging process. As with many natural and organic compounds, wooden barrels tend to expand and contract during hot and cold temperatures. During warm periods, such as hot summer days, the wood expands, pushing the whiskey deeper into the charred or toasted layers of the barrel. Conversely, during cold winter nights, the wood contracts, releasing the whiskey back into the barrel’s core. Many distillers refer to this process as “breathing”.

While each wooden barrel can vary, on average, a standard whiskey barrel can change in volume between 1-3 gallons per year because of temperature. Meaning, the surface area of the wooden barrel increases and allows the whiskey to interact with the wood in greater detail.

The type of wood used for the barrel can affect how much it expands and contracts. Oak, which is the most common wood for whiskey barrels, has specific characteristics that allow it to expand and contract efficiently in response to temperature changes.

It’s important to note that these volume changes don’t necessarily correspond directly to the amount of whiskey lost or absorbed by the wood during aging. The expansion and contraction of the wood affect the surface area contact between the whiskey and the barrel, which influences flavor extraction and concentration, but the overall liquid volume changes may be less pronounced.