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Oak ageing may sweeten the wine

Oak barrels are used widely in the winemaking process to increase color stabilization, soften the tannins, and add extra flavour compounds that increase the complexity of the wine. In addition to these effects, wine sometimes also gains in sweetness during oak ageing, although it has no residual sugar. This sweet taste does not always occur and is not caused by the typical oak aroma molecules. Apparently there are other compounds involved during oak ageing, but which, and under which circumstances?

The effect of oak ageing largely depends on the type of oak species used for the barrels, the production style of the cooper, the age of the tree, the part of the trunk that the staves were taken from, the seasoning of the wood, the amount of toasting, but also the size and age of the barrel. The choice of oak barrel for winemaking is important for it can have a major impact on the final wine. Oak ageing can result in aromas in the wine such as vanilla, oak, coconut, butterscotch, clove, smoke, nutmeg, chocolate and coffee. Interestingly, oak ageing can also increase the perceived sweetness of the wine, which is not caused by the volatile compounds – such as vanillin, β-methyl-γ-octalactone, eugenol, furfural and 2-furanmethanethiol – that are responsible for these typical oak aromas1. So, what other compounds contribute to the sweet taste after oak ageing?

Oak stamp on wood

Triterpenes

The most common natural sweeteners are the sugars glucose, fructose and sucrose. During the alcoholic fermentation these molecules are converted into alcohol. These sugars can therefore no longer have a sweetening effect on the wine. The wine is devoid of fermentable sugars and is ‘dry’. However, during oak ageing many compounds are released from the oak in the wine such as the volatile compounds mentioned above, but also ellagitannins and other non-volatile compounds such as coumarins and lignans. These compound contribute to the sensory properties of the wine and add for example bitterness and astringency.

In 2011, two new compounds, the triterpenes Quercotriterpenoside I and II (QTT I and II), have been reported in oak wood extract, of which QTTI was reported to have sweet properties with a sensory threshold of 590 µg/L in white wine2. For reference, the sensory threshold of glucose is at 4 g/L about 6700 times higher3, indicating that QTTI is a strong sweetener. According to this initial study, wines aged in oak barrels can have a concentration of up to 1.000 µg/L of all Quercotriterpenosides combined which shows that triterpenes can contribute to the sweet perception of the wine after oak ageing.

Follow up studies showed that QTTII also had sweetener properties, and that there was a whole family of triterpene molecules present in oak that could, depending on their isomere and galloyl and glycoside side-groups have a sweet or bitter taste4-6.

Differences in oak species

There are three commonly used oak species used for the manufacturing of barrels, the two European oak (Quercus) species Quercus robur and Quercus sessilis, and the American oak species Quercus alba. Each of these oak species have a slightly different effect on wine flavor. American oak has a low polyphenol content and the highest concentration of aromatic compounds, especially oak lactones that result in very bold oaky wines. The two European species are more commonly known as pedunculate and sessile oak. Pedunculate oak (Q. robur) has a high polyphenol content and adds a lot of structure, but less aromatic compounds to the wine. Sessile oak (Q. sessilis, or also named Quercus petraea) has a tighter grain and therefore contains less polyphenols, but is richer in aromatic compounds and as such generally regarded as the finest oak for wine making.

Pedunculate and sessile oak trees can be found in multiple geographical locations in Europe. France is often regarded as the best source for oak suitable for the production of barrels, but barrels are also manufactured from oak produced in e.g. Hungary, Romania, Russia, Poland and Croatia. In general, coopers regard the geographical origin of the oak as most important, and are not very concerned about the oak species7. But is this always justified? Interestingly, the amount of triterpenes in the oak are not so much dependent on the geographical origin, but on the type of oak species.

Triterpenes in pedunculate and sessile oak

When triterpenes are quantified in pedunculate and sessile oak, a clear difference can be observed. Pedunculate oak releases more bitter triterpenes, and sessile oak is richer in sweet triterpenes. A research group at INRAE, Bordeaux has determined the amount of four types of triterpenes in 46 oak samples (27 sessile and 19 pedunculate oak) derived from eight different French forests6. The average concentrations of the three sweet tasting triterpenes (QTTI, QTTII and QTTIII) were higher in sessile oak, while the bitter triterpene (Glu-AB) had a higher average concentration in pedunculate oak (See figure). This ratio between sweet and bitter triterpenes was noticed each time in sessile and pedunculate samples from the same forest, indicating that the species was more important than the geographical location of the forest for the triterpene composition of the oak. Nevertheless, the variation between the samples (from all forests) of the same oak species was high, i.e. the highest concentrations of the sweet triterpenes measured in pedunculate oak samples were higher than the lowest concentrations measured in sessile samples.

Concentration and structure of triterpenes in oak

Concentration (left) and structures (right) of triterpenes in oak
Adapted from Marchal, 2020 via CC BY 4.0

Selection of oak

If you have a bone dry wine in mind, you don’t want to accidentally use oak barrels that release sweeteners into your wine. Moreover, if a high concentration of bitters is a potential problem in the wine, ageing on pedunculate oak may be an unwise decision. 

Knowledge about the release of triterpenes from oak and their sensory properties on the wine would enable winemakers to more precisely tailor the cooperage oak used for ageing to their preferred style of wine. However, more sweet tasting triterpenes have been discovered recently5, and further sensory characterization of these molecules is necessary to give a more complete view on the effect of oak ageing on wine. It would be interesting if future research could elucidate the actual concentrations of these individual triterpenes in wine aged on oak, what their sensory thresholds are, and at what rate they are released to the wine. The natural variation of oak (within the same species) is large not only for triterpenes, but also for the other extractable compounds such as ellagitannins and lactones6,8. This variability makes it difficult to predict the exact effect of oak ageing on the wine. It would help tremendously if barrels are made from oak with similar properties – e.g. staves that are all low on sweet triterpenes – or made from a single tree and accompanied by an analysis that shows the concentrations of triterpenes, ellagitannins, lactones and other aromatic compounds of the wood. Nevertheless, the discovery of sweet (and bitter) triterpenes adds another variable to the already complicated selection process for the right oak barrel.

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Note: Most of these studies on triterpenes were funded and/or supported by tonnellerie Seguin-Moreau

References:
1. Marchal A, Pons A, Lavigne V, Dubourdieu D. Contribution of oak wood ageing to the sweet perception of dry wines. Australian Journal of Grape and Wine Research. 2013;19(1):11-19. https://doi.org/10.1111/ajgw.12013
2. Ponou BK, Teponno RB, Ricciutelli M, Quassinti  L, Bramucci M, et al. Phytochemistry. 2010;71:2108–2115.
3. Ribé reau-Gayon P, Glories Y, Maujean A, Dubourdieu D.. Handbook of Enology Vol. 2. The Chemistry of Wine Stabilization and Treatments; Handbook of Enology; John Wiley & Sons, Ltd, 2012; Vol. 2.
4. Marchal A, Génin E, Waffo-Téguo P, et al. Development of an analytical methodology using Fourier transform mass spectrometry to discover new structural analogs of wine natural sweeteners. Anal Chim Acta. 2015;853:425‐434. https://doi.org/10.1016/j.aca.2014.10.039
5. Gammacurta M, Waffo-Teguo P, Winstel D, et al. Triterpenoids from Quercus petraea: Identification in Wines and Spirits and Sensory Assessment. J Nat Prod. 2019;82(2):265‐275. https://doi.org/10.1021/acs.jnatprod.8b00682 
6. Marchal A, Waffo-Téguo P, Gammacurta M, Prida A, Dubourdieu D. Origins of the sweetness derived from the aging of dry wines: the role of oak triterpenoids. 2020 https://doi.org/10.20870/IVES-TR.2020.3286
7. Understanding wine technology. David Bird. Third edition. 2019. ISBN 978-0-9535802-2-4
8. Mosedale JR, Puech J-L, Feuillat F. The influence on wine flavor of the oak species and natural variation of heartwood components. 1999;50:503-512.

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