Wines are matured in oak barrel or in vats (stainless steel or concrete) for 3 main reasons :
- Modifying the phenolic structure and making tannins smoother
- Stabilizing the colour, limpidity and colloids
- Developping aromas
The length of maturation, the use of enological products, temperature, nature of wood affect the result. Mastering maturation in barrel (220 litres, 600 litres or more than 2000 litres or foudres) must be managed with care and knowledge.
Using oak barrels imply a wine more sensibility to acetic, lactic bacteria and bad yeast….Volatile acidity (like vinegar) may increase 0,1 g per litre with re-used barrels and 0,2 g per litre with new barrels.
In oder to avoid « brettanomyces » taint (a yeast) to the wine, bringing unpleasant flavours, the wine master has to be vigilant on cleanliness. He has to top up carefully each kegs, to keep the cellar at 18°C (64,4°F) maximum and to make sure that free hydrogen sulfite is 25 mg per litre minimum.
Maturation in oak leads to a micro oxygenation also named oxydoreduction. Air go through the staves and all tiny gaps in the barrel, that brings a regular oxygene incorporation all along the period of maturation, going to polymerisation improving the global quality.
Non volatile components (phenols) of the oak (there a great deal of variation according to the origine of wood and its drying process) are solvate in wine. That may sometimes bring an harshness to the wine, if its the quality and its tannic structure are not able to undergo oak.
Some volatile components that may be found in oak (as lactons, phenols, aldehyds…) or resulting from the drying process or burning (as phenylcetons…) may be extracted. Flavours as clove, vanilla, almond or grilled almond, caramel, toast and smoke can be found in the wine.
The way to dry the wood and the amount of burning may change the quality and quantity of volatile components, then bringing complexity or mask the natural charm of the wine, if this one is not able to resist maturation to improve.
- A strong burning gives : smoke, soot, coffee, caramel aromas as well as a light tanic extraction and a light sugar taste.
- A medium burning gives : complexity, grilled aromas, spices, vanilla, cocoa, and a light sugar taste.
- A medium burning gives : mineral aromas, fresh oaky flavours and tanins.
Filling the cask while the wine is still hot (just after the alcoholic fermentation and before the malo-lactic fermentation) may increase the quality potential of aok maturation, but that increases microbiological risks.
To take advantage of the oxydoreduction principle, the maturation must last at least 6 month. Only the tasting of all casks in parallel with chemical analysis can correctly assess the perfect evolution of each barrel, the duration of the maturation and the potential of each wine.