Micro-oxygenation in a wooden barrel - or why does my beer age in a maturing barrel?

For a trained brewer of the traditional school it is a big psychological hurdle to put his freshly brewed beer into a wooden barrel, of which we told him before: « Your beer matures in it because it gets air! »

That’s right, similar to whisky, precious fruit distillates, gin, wine, spices and whatever else is stored in barrels, the beer matures in the wood through whose staves air flows. And similar to a cheese matured in air, after some time a changed product appears, ideally this is a refinement of the raw material but often also a disappointment, namely when the matter is approached with a lot of verve but without knowledge and lack of advice and the maturing in the wooden barrel is simply underestimated or overestimated, what happens while a beer is lying in a former bourbon barrel or sherry barrel, to give two examples.


The atmospheric pressure outside the barrel is higher than inside the barrel because the wooden barrel is filled with beer containing alcohol. Consequently, air from the outside presses through the staves and takes the contents of the staves into the barrel, where they are again distributed by the thermal. We are also talking about the gradual marriage of the beer molecules with the macromolecules of the wooden barrel such as lignin, cellulose, hemicellulose and the aroma-giving substances such as sherry and bourbon in our examples.

This thermal – as long as you can take temperature variations with you during the maturing process – works similar to a heart, in summer when it is warmer the beer expands and presses itself into the inner wall of the stave, in winter when it is cool or cold it retracts. Admittedly, most beers are not stored in a storeroom where the seasons can be taken along, but what I want to express with the comparison is that a beer can only mature in a wooden barrel if the temperatures are high enough, otherwise you will only find an aroma in the barrel after a while, depending on the pre-occupancy of the wooden barrel.


The wood barrel aging or micro-oxygenation is therefore an interaction between the atmospheric pressure, the ambient air – clean air can be tasted in the product just as bad air or salty air – and the size of the wood barrel. The smaller a barrel is, the larger its inner surface is and the stronger the effect of this surface and the incoming air on the beer.

The thinner the barrel, the more speed the maturing process gains, the stronger the staves, the more balanced the process and the later taste – but the longer the barrel matures.

As a further aspect in the barrel aging time of the beer, the ambient humidity in the barrel storage plays a major role. We recommend an air humidity of approx. 80%, which ensures that no joints open up in the barrels and thus the barrel becomes leaky.

If the air humidity is higher, this makes the wood and barrel feel good; however, this humidity also occupies the wood pores and the associated breathing through the wood becomes more restrained. Conversely, air that is too dry will cause increased breathing but may also open the joints and cause technical problems.

Marriage in a barrel takes time

From experience, however, it must be said that the marriage in the barrel between the incoming air, the product maturing in it and the aromas of the barrel staves takes time and is only sustainable if we are talking about storage times of at least 6 – 8 months. Basically, the longer the better – exceptions are the rule, as everywhere in life.

It is exactly this process that distinguishes a wood barrel matured beer from a beer flavoured with chips or cubes.

By slowly maturing and combining the various aromas in the wooden barrel, a beer on the bottle retains its flavour almost indefinitely, and with the right alcohol it can be kept for years if not decades.
A beer flavoured with wood alternatives, on the other hand, does not last very long on the bottle, at the latest when opened and not drunk out immediately (which is certainly rare), the beer falls apart on the left side into its original form and on the right side into the wood alternatives.

Wood type

Last but not least, the nature or the typicality of the processed wood plays a further and not unimportant aspect in the barrel maturity and the breathing of the barrel.

Let us take the example of the most frequently used barrel wood: oak. We mainly use very fine pored wood of the species Quercus Sessilius, but also woods like the Limousin oak, which is much more open-pored, since it grows on very fertile soils, is used in barrel construction. An open-pored wood « breathes » more than a fine-grained wood.

Whether this is the case during the maturing of beers, where the effects of this can be seen over several weeks or months and not years as with distillate, is another issue, but physically it cannot be discussed in isolation.

All in all a complex topic for which there is no simple recipe or a generally binding solution – I think that’s a good thing, then the maturation in wooden barrels and the associated processes finally give the beer its uniqueness.


See you,

Markus Eder

The article also appeared in the summer 2020 issue (No. 46) of the magazine Bier&Brauhaus:
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