Oxygen transfer during the oak aging of wine

According to Del Alamo-Sanza and Nevares*, the oak barrel behaves as an interactive vessel with wine, allowing the transfer of substances from the wood to the wine (covered here) and the dynamic transfer of oxygen from the air to the wine.

I cover the transfer of oxygen from air to wine in this post.

Oxygen Transfer Rate
The annual rate of oxygen entry into barrels is the amount of oxygen that enters a barrel full of wine over the course of a year; the unit of measure is mg/L.year. A number of studies over the years (Ribereau-Gayon, 1933; Amerine and Joslyn, 1970; Prillinger, 1965, for example) have defined and refined this measure, but it was Singleton's efforts which sought to establish the oxygen entry point. According to Singleton, oxygen entered the barrel through the dry wood at the top where it is in contact with the headspace.

Vivas and Glories, in a subsequent study, measured oxygen entry into barrels (sourced from Limousin and Centro) full of wine -- wet-wood barrels -- and found that (i) oxygen entry into the wine varied between 20 and 45 g/L for new barrels and 10 mg/L for the 5-year-old barrels. These values have become the accepted oxygen transfer rates for French oak barrels.

In the same study it was found that the oxygen transfer rate was 45 mg/L with a silicone bung and 28 mg/L when the barrel was tightly sealed.

Other later studies have shown that sealing the joints between the staves of full barrels did not prevent oxygen entry into the wine. The conclusion, then, was that oxygen entered the barrel through the wood, joints between the staves, and through the bung; not through the dry wood at the top of the headspace (as had been posited by Singleton). In more recent times, the bung as a source of oxygen entry has seemed to lose prominence with the widespread use of food-grade silicone bungs that allow a tight seal.

The entry of oxygen into the barrel, and its relationship with wine flow into the oakwood, is illustrated in the figure below.

It has been shown that French oak allows more oxygen entry than does American oak. While 50% of American oak's oxygen ingress was through the wood, fully 75% of French wood's ingress was through the same channel. The finer the grain, the greater the oxygen permeation in both species. An oak stave that is 27-mm thick would allow a maximum oxygen transfer rate of 26 ml/L.year.

Oxygen entry through the joints between staves is not uniform along the length of the joint. Rather, it is greater at the middle of the stave, where the pressure is between 0 and 3 bars, and less so at the ends where the pressure ranges between 25 and 30 bars.

The process of topping up the wine barrel has been seen as a source of air entry into the barrel. According to Del Alamo-Sanza and Nevares, however, good management of the barrel-topping process "does not necessarily involve an increase in wine oxygenation."

Oxygen reaching the wine does so both via the wood and the joints between the staves. The oxygen transfer rate of the barrel, then, depends on the wood from which it is constructed as well as the construction process.

Oxygen introduced into the wine reacts with pigments and tannins such that:

• The red color in wine is stabilized and enhanced
• Tannins are softened
• Complex aromas develop
• Improvement in the mouthfeel and body of the wine is evident.

*This post draws heavily on the work of Del Alamo-Sanza and Nevares (Maria del Alamo-Sanza & Ignacio Nevares (2018) Oak wine barrel as an active vessel: A critical review of past and current knowledge, Critical Reviews in Food Science and Nutrition, 58:16, 2711-2726, DOI: 10.1080/10408398.2017.1330250)

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