The tannin changes occurring during barrel maturation, lead to the softening of wines as a result of the polymerisation of the tannins and tannin-polysaccharide combination, which soften the tannins and add more body to wines. Extraction from the barrels also gives complexity to the wine flavour, which can be particularly complimentary to full, complex red and white wines. Wood contains volatile and non-volatile wood extract, which can be modified by the ageing and toasting processes during barrel making. This modification continues during the development and ageing of wine.
Cellulose, hemicellulose, lignin and tannin are the most important components of oak wood. The long chain polymer, cellulose, is mainly responsible for the strength of the wood and changes minimally during coopering and also contributes little to wine flavour. Hemicellulose is smaller polymers of mainly xylose and arabinose and can be hydrolysed into individual sugars and acetic acid. Wood toasting causes the breakdown of hemicellulose which can form aromatic compounds likes maltol, cyclotene and ethyl lactate. Lignins are branched chain polymers, which play a significant role in wine flavour. The thermic degradation of it during wood toasting liberates flavour compounds like vanillin, eugenol, guaiacol and phenols. Oak wood also contains tannins, which are polymers of gallic acid and ellagic acid. It is liberated during barrel maturation when the tannins are hydrolysed as result of the wood contact. These compounds can as a result of their astringency and bitterness contribute to the mouthfeel of the wine. Oak lactones increase during toasting and can impart a pleasant, sweetish wood flavour, but in high concentrations can become overwhelming and unpleasant.
Different factors like botanical and geographic origin, wood structure, individual trees, stave seasoning, toasting degree, maturation period and the previous use of barrels can cause a variation in the eventual flavour profile of matured wines. The flavour and impact of oak barrels are influenced by the oak wood species and wood origin. In Limousine Quercus robur is for example dominant and the wood has a coarser grain, which will add more tannin and flavour to the wine; contrary to wood from Alliers and Nevers from Central France with a tighter grain. The seasoning and drying of the wood are also important. Green wood has low concentrations volatile compounds, but oxidation during drying increases the aromatic aldehyde concentrations. Longer drying of wood is usually more acceptable, because shorter drying will add more smoky flavours and astringency to wines.
The degree of toasting also plays an important role. More toasting causes higher phenolic aldehyde concentrations and other favourable sensory compounds, but decreases the tannin concentrations.
If abovementioned factors are considered, it is obvious that wood can play a significant role in wine character. It is essential that barrels are coopered properly from high quality wood and winemakers must consider all the facts, before barrels are bought. The style of the wine to be matured and what is expected after maturation, will determine the applicable barrels that will be chosen. Full, tannic Cabernet Sauvignon will for example be complemented by a high percentage new tight-grain French oak, while Rhone blends will need less prominent wood character and consequently also a lower percentage new oak. In both cases the toasting degree of the barrels, maturation period and wood origin will play another role, because the flavour profile and intensity thus imparted will be noticeable in the final wine. Barrels are however not a commodity bought from the shelf and choice of cooper will determine the eventual barrel. This choice is mainly influenced by reliability and consistent quality, although knowledgeable sales staff, adjustable payment and delivery terms also play a role. It is consequently a sound practice to buy barrels from different coopers and also to apply an evaluation process continuously (Cohen, 2015).
Cohen, Remi, 2015. Know your oak. Many factors should be considered when putting together a barrel program. Vineyard & Winery Management, May/June 2015: 31 – 36.