For Everything There Is A Season
In a previous article we discussed the flavour compounds found in green oak. Green oak provides the building blocks for the flavour compounds ultimately found in cask aged spirits. The first process in preparing the green oak for the spirit is seasoning.
Seasoning is usually done under natural conditions, which simply involves leaving the wood outdoors to dry slowly. Seasoning allows the high humidity in the wood to reduce slowly until it is in balance with the ambient humidity. This process causes the wood fibres to contract slowly making the wood staves more stable, thus reducing the risk of the wood splitting or the cask leaking. During natural seasoning the wood also matures. The chemical composition of the wood changes, reducing bitterness and astringency, increasing aromatic properties, and reducing ‘off-flavours’. The main chemical change reducing astringency is reduction in Ellagitannins. This occurs through diffusion of ellagitannins across the cell wall matrix, but as the wood remains damp for a long time and is exposed to oxygen in the air, hydrolysis, oxidation reactions, and fungal enzymatic degradations also occur, reducing the consntration ellagitannins. Leaching due to rain may also play a significant role. Changes in the aromatic profile of the wood occur due to changes the concentrations of volatile compounds. Phenolic aldehydes (vanilline, syringal- dehyde, coniferaldehyde, and sinapaldehyde) usually increase their concentrations during natural seasoning, but other volatile compounds ,such as eugenol or beta-methyl-γ-octalactones, show variable changes, possibly dependant on the environmental conditions. Disagreeable oak wood aromas, such as sawdust or herbaceous aromas are caused by volatiles such as 1-hexanal, 1-hexanol, 1-non- enal, (E)-2-octenal, 2-ethyl-1-hexanol, and (E)-2-nonenal.
Natural seasoning is very effective at reducing these volatile compounds
The optimal duration of natural seasoning is variable. Thicker staves, for instance, require longer seasoning. The optimum amount of time for natural seasoning of European oak is probably around 24-36 months. The optimum time for natural seasoning of American oak is probably shorter, 12-16 months, as it has lower levels of ellagitannins.
Because natural seasoning requires many months and vast areas to store the wood, there has been some attention paid to artificial seasoning. Artificial seasoning involves drying the wood in a temperature and humidity controlled kiln for 6-8 weeks. Because the process is much faster than natural seasoning the risk of the wood splitting or cask leaking is higher. Unlike natural seasoning, kiln drying does not reduce ellagitanins significantly, if at all. The only probable mechanism by which Ellagitanins could be reduced is through diffusion through the cell matrix. No hydrolysis, oxidative reactions or enzymatic reaction can occur. The aromatic profile may also be adversely affected. Beta-methyl-γ-octalactone, or the ‘Oak lactone’ is sensitive to heat, and during kiln drying the concentrations of both the cis and trans isomers can decrease significantly, sometime to the point of being undetectable.
Natural seasoning is likely superior to artificial seasoning because whilst both methods reduce off-flavours, with natural seasoning the staves are more stable, there is a reduction in the aggressive ellagitannins and there is no decrease in the ‘oak lactone’. To address the short-comings of artificial seasoning a hybrid of the two processes may be used, but invariably this ‘mixed method’ results in a similar chemical profile as artificially seasoned wood.
Having understood how seasoning changes the flavour compounds within the wood, it is also worth considering how much of an impact seasoning will have on the final spirit. Early fillings are influenced largely by solubilized material from toasting and charring, while subsequently fillings are more dependent upon reaction products created during seasoning. Therefore, seasoning will have less effect on the flavour of a young spirit ageing in a new cask than on a young spirit ageing in a cask that has previously been filled, such as a first fill ex-bourbon cask.
In the next article we will examine the next step in preparing the wood for the spirit – toasting.