The History and Science of the Barrel- Lesson 6: Curing the Wood

Lesson 6: Curing the Wood

The Importance of Wood Curing

A common misconception among industry neophytes is that rum barrels can be constructed using oak staves from a recently-milled tree. While it is possible to do so, it is not done for three main reasons:

• When an oak tree is first cut, over half of the weight of the wood obtained is water. As this water evaporates, the wood will undergo physical changes (i.e. shrinkage), which will result in severe leakage if the barrel is assembled with the wet wood.

• When the oak is naturally cured (exposed to naturally-occurring weather  patterns), light, humidity and fungal activity break down tannins into smaller fragments, releasing ellagic acid.

• During curing, undesirable extractives in the wood migrate to the surface and are naturally leached away.

And what is the importance of ellagic acid? Ellagic acid is converted by plants and trees into a form of tannin known as ellagitannins or ellagic tannins. Ellagic tannins are glucosides which are readily hydrolyzed by water to regenerate ellagic acid.

The first two (and the most common) ellagic tannins to be identified are vescalagin and castalagin. Subsequently, six additional water-soluble ellagic tannins (roburins A-E and grandinin) were also identified (Du Penhoat et al., 1991) as dimers of vescalagin or castalagin and/or characterised by the addition of a pentose residue to said molecules.

Many studies have suggested that ellagic tannins are involved in the oxidation process of wines. Similar studies have not confirmed this for spirits, but available evidence suggests a similar behavior. When ellagic tannins are present in a solution, they quickly absorb the dissolved oxygen and facilitate the hydroperoxidation of the constituents.

While sun and open air aid in the curing of the cut wood, they can be detrimental to the logs prior to milling. For this reason, many mills have installed sprinkler systems in special areas of their yards, to keep logs evenly wet until they are ready to be processed. The logs are kept this way, with their bark intact, until then.

The total length of the barrel-building process, starting with the storage of the logs, the curing time for the planks after milling the logs and the fabrication time of the barrel can be of up to three years. This puts a heavy financial burden on cooperages, which need to invest heavily in raw materials and storage space in order to stay in business.