Time and again, wine descriptions refer to "aged in American or French oak barrels". In this blog, we would like to shed light on the questions of why wood is used to age wine, what the advantages of oak are and why a distinction is made between French and American oak, as well as the qualitative differences between the two types.
History of the use of wood in wine ageing
The first use of barrels made from staves dates back to the Celts and is first mentioned in Roman sources in 50 BC. The barrels were used to transport wine. It was discovered, first by the English, but also by German merchants and also by Louis-Gaspard Estoumel, who sold wine to India at the beginning of the 19th century, that storage in wooden barrels not only made wine more durable, but also significantly improved its quality, so that Louis-Gaspard Estoumel was able to sell his returned wine at significantly higher prices than before.
This method of winemaking has been established internationally since 1980, starting in Bordeaux.
Vinification
On contact with wood, wood compounds such as phenols, tannins and flavours such as vanilla are released, pass into the wine and change the aroma profile of the wine. The wood is heated over a fire so that it can be bent for barrel making. This process is also known as toasting. The oak wood can be given different degrees of toasting, from light to medium to strong. Depending on the degree of toasting, the heat on the surface of the wood develops different flavours, which are then reflected in the wine.
Toast degree | Flavours |
light | Vanillin, tannins |
medium | Vanillin, cocoa, caramel, coconut, chocolate |
dark | Coffee, caramel, coconut, smoked bacon, toast |
The extend of the additional flavours depends on several parameters: the duration of ageing, the degree of toasting, the barrel size, the stave thickness, how often the barrel has already been used, the grape variety of course, the type of wood and the way in which the wood was processed (sawing direction, for example).
However, the transport of substances is not the only aspect that has an influence on the flavour profile of the wine. Depending on the length of time the wine is stored in the barrel, air is also transported from the outside to the inside. The oxygen that is brought into the barrel with this air transport contributes to the oxidation of the wine, known as micro-oxidation. This micro-oxidation leads to the oxidation of tannins and other tannins, making the wine appear rounder and softer over time. This oxygen transport can be measured. It amounts to around 50mg/L per year [1] and decreases over time. This is probably due to the saturation of the wood with liquid, which then acts as an air transport barrier. The following figure illustrates this.
Fig. 1: Schematic representation of the liquid saturation of oak staves
Air exchange takes place not only through the staves, but also via the joints between the staves and the bunghole. However, there are clear differences in the use of different types of wood. This brings us to the differences between French and American oak.
Differences between American and French oak on wine quality
The pores of American oak are 10 times larger than those of European oak. However, they also have cell protrusions (tyloses) that block the pores, making them impermeable to liquids. The American oak has the largest number of tyloses, which means that staves can be sawn directly from the wood. The logs of French oak have to be split and then bent to minimise the leakage rate. This makes the use of American oak more economical due to the simpler processing procedure and the higher yield of usable wood [4]. It is possible to obtain 4 or more barrels from the trunk of an American oak tree, whereas the yield from European wood is 2-3 barrels. This makes barrels made from European oak more than twice as expensive as barrels made from American oak.
Wood compounds such as lactones, phenolic aldehydes, volatile phenols, carbohydrates, terpenes and tannins are released from the wood into the wine over time.
In this respect, American wood has more to offer than French wood, at least in terms of the concentration of lactones. This is why American wood tends to be used for the ageing of more robust, well-structured Spanish and southern French wines, as well as South African and Australian wines such as Syrah and Cabernet Sauvignon.
The fine flavours of high-quality, more elegant wines such as Burgundy or white wines such as Chardonnay would be overlaid by the wood compounds and no longer come into their own as well. These wines are preferably stored in European wood [2].
Alternatives to barriques
What alternatives are there to barriques?
Firstly, there is the steel tank, in which white wine is usually fermented and stored. Steel is chemically inert and therefore does not release any substances into the wine. The steel tank is also not permeable to air. Micro-oxidation as in barriques is therefore not to be expected. The wine in the steel tank is matured completely reductively.
Until a few years ago, concrete tanks were frequently used in wine ageing. These were large containers with a capacity of several thousand litres that were coated on the inside, either with a layer of plastic or lacquered. For the past 30 years, the use of steel tanks has continued to decline in favour of steel vats. Recently, however, they have experienced a renaissance. Some winegrowers have rediscovered the concrete tank and age their wines in egg-shaped tanks with a capacity of up to 900 litres. However, these tanks are not coated on the inside, which results in a number of disadvantages. The shape of the egg is based on mathematics and is reminiscent of the golden ratio. However, this shape is not harmless for work safety reasons, as it is difficult to get into the tank for cleaning. There is no ladder to lean against and a ladder for access is often not provided. Platforms must be used if the tank is to be cleaned from the inside. Another disadvantage is that the concrete egg is not inert like the steel tank. The concrete is not resistant to organic and inorganic acids and so, after a while, minerals that have been dissolved out of the concrete surface can be found in the wine. The pH value also changes towards an alkaline environment, which is not always desirable.
There is another variant that is similar to the barrique, but has a different design. It is a rectangular construction made of wood and metal, which is said to offer the advantages of the barrique but is far more sustainable in its use of materials. The frame of the barrel is made of metal, the walls of toasted oak. This means that the same materials are used as in conventional barrel construction, but in a more sustainable way. The individual wooden panels of the cubic barrel can be toasted to different degrees and combined so that the winemaker can determine the overall degree of toasting according to taste. After a barrel has been used, the top layer can be removed so that it should be as good as new. I can also imagine that new and used barrel parts can be combined so that the winemaker is not forced to age part of his wine in new barrels and the other part in used barrels. Theoretically, it should also be possible to use different types of wood (French, American), which would allow the advantages of each wood to be utilised (see above) [6]. However, it may also have disadvantages, as the removal of the surface of the wood also reduces its thickness, which will certainly have an influence on the OTR (Oxygen Transfer Rate) and thus on the quality of the wine. However, experience will have to show this. Although this is not a scientific study, Fallstaff commented favourably on it as part of the 2019 Pinot Gris Trophy.
References
[1] | Oak Wine Barrel as an Active Vessel: A Critical Review of Past and Current Knowledge; Critical Reviews in "Food Science and Nutrition", May 2017 |
[2] | Oak (wine); Wikipedia: https://en.wikipedia.org/wiki/Oak_(wine) |
[3] | Evolution of red wine in oak barrels with different oxygen transmission rates. Phenolic compounds and colour; LWT - Food Science and Techniology 158 (2022) |
[4] | Das Oxford Weinlexikon, Jancis Robinson; Hallweg Verlag; 2. Auflage (2003) |
[5] | Wine ageing in a concrete egg, article by the Bavarian State Institute for Viticulture and Horticulture, https://www.lwg.bayern.de/analytik/wein_getraenke/085502/index.php |
[6] | ReBarrique, Homepage of the manufacturer: https://www.rebarrique.de/ |
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