The Role of Oxygenation in Barrel Aged Spirits – Part I
Flavor development in barrel aged spirits is a complex process. As coopers, our focus is on oak extractives and their role in flavor development. We study the perception of those oak extractives in spirits and what we can do to heighten or mask their perception.
Oxygenation plays an important role in the maturation process which is why we devote a portion of our research efforts to further our understanding of this process.
Oxygen can enter the barrel the following ways:
The physiological makeup of oak allows gases to pass directly through the wood while remaining liquid tight.
Oxygen permeating through the joints and bung will not only get introduced to the spirit directly, but will also replenish the constantly depleting supply of oxygen in the head space.
Determining which pathway provides the most oxygen to the spirit is continually being tested. One experiment showed that most of the oxygen entered through joints within the first two years.*
The rate of oxygenation is not the same for all barrels. As coopers, we can influence the rate of oxygenation in several ways. For example:
- Utilizing different oak species (American oak vs French oak)
- Altering barrel construction (size, shape, etc)
- Using oak trees with different growth patterns (Coarse, fine, extra-fine grain)
- Season the wood for long periods of time which will leach out tannic acid (a consumer of oxygen) and decrease the density of the staves
There are multiple variables that influence the rate of oxygenation that we cannot control, such as temperature. Figure 1 below shows oxygen solubility in water as a function of temperature and pressure (showing that pressure has an effect as well)
Figure 1. Solubility of Oxygen in Fresh Water**
To add further complication, the effects of these variables are not static throughout the aging process. As the barrel ages, the wood becomes increasingly more saturated with alcohol. As this happens, the rate at which the oxygen can permeate through the wood changes. The diffusion of oxygen in air is multitudes greater than that of its diffusion through water. This suggests that as the barrel becomes more saturated over time, the rate at which oxygen transfers through the wood slowly decreases. Furthermore, the diffusion coefficient of oxygen in ethanol differs from that of water which would suggest that entry proof has an effect as well.
Oxygenation is arguably the most complex process taking place during maturation and involves many variables. Because of this, its effects are not completely understood. In the next post, we will move away from the mechanical aspect of oxygenation and focus on the chemistry by looking at some of the different reactions that take place within the barrel.
*Vivas, N. Modélisation et calcul du bilan des apports d’oxygène au cours de l’élevage des vins rouges. IV - Elevage des vins rouges en conditions d’oxydations ménagées controlées. Progrès Agric. Vitic. 1999, 116, 305−311
** “Oxygen – Solubility in Fresh Water and Sea Water.” The Engineering Toolbox, www.engineeringtoolbox.com/oxygen-solubility-water-d_841.html.
About the Author
Andrew joined the ISC team as the head of Research and Development in early 2016. His primary focus is working with ISC customers to develop and test new products. He will also design experiments to gather data on whiskey maturation, barrel performance, process and product innovation.
As a self-described, "bourbon fanatic" Andrew enjoys drinking and studying the history of bourbon. He also owns and operates a farm in Oldham County Kentucky that develops custom corn breeds for use in distilling.