Oaky flavours

Hundreds of different flavour compounds have been identified in whisky. The synthesis and degradation and synergistic properties of these compounds is still poorly understood as there are so many aspects contributing to the result of cask maturation.

Oak cask affects whisky by extracting wood compounds that influence the taste directly or together with the spirit compounds or by removing or changing some compounds from the raw spirit. Cask also allows evaporation and oxidization of spirit and the volatile flavour compunds through the headspace of the cask or by a lesser extent through the surrounding air through the wood or the bunghole.

Oak wood consists of cellulose (38-52%), hemicellulose (25-30%), lignin (22-25%) and extractives of the wood (5-10%). Oak cellulose is a linear chain of up to thousands of D-glucose-molecules and quite inactive in terms of flavour extraction during spirit maturation, extracting only some carbohydrates in high temperatures during toasting/charring.

Toffee

Hemicellulose contains several defferent sugars (xylose, mannose, galactose, rhamnose, arabinose, glucose) and forms branched chains of hundreds of molecules. Hemicellulose breaks easily when heated, producing a range of extractable aroma compounds, such as furfural (almond, walnut, grainy), hydroxymethylfurfural (butter, musty, waxy, caramel), maltol (malt, sweet) and cyclotene (maple, caramel, licorice).

Lignins are very complex macromolecules consisting of three monolignol units p-hydroxyphenol (not present in oak), guaiacyl (32% in oak lignin) and syringyl (68% in oak lignin) derived from dehydration and polymerization of cinnamyl alcohols. Usually natural lignin includes various other molecules joined in to the structure, including different sugars, acids and aldehydes. Heating breaks parts of lignin to soluble p-coumaryl-, coniferyl- and sinapyl-alcohols. They can transform into their respective aldehydes, acids and phenols including very aromatic compounds such as guaiacol (smoky), 4-vinylguaiacol (clove), phenyl ethanol (floral, rose), vanillin and vanillic acid. At higher temperatures a range of other volatile phenols are formed. Lignin breakdown continues at a slower rate during maturation by the effect of ethanol. Most of the lignin derivatives and extractibles decribed above are present also in the malted grains, peat and new make spirit.

Lignin synthesis, monolignols. From Nature Reviews

Oak wood extractives include two different natural isomers of oak lactones. The cis-oak lactone gives sweet coconut-vanillin aroma. The trans-oak lactone is spicier (coconut,cloves, celery, incence), but 2,5-20 times weaker if all the synergetic influences of other oak extractives are not taken into account. The trans-lactone is believed to promote the taste of cis-lactone and various other flavour compounds in whisky, producing heavy coconut amd incence aromas at high concentrations. Various other lactones are described as fruity, peach-like and vanillic.

Tannins can be divided into hydrolysable (gallo- and ellagitannins) and the non-hydrolysable condensed tannins, for example proanthocyanidins common in red wines. Oak tannins are more hydrolysable than the more stable wine tannins from grape skins and pips and thus more volatile and active during maturation. Ellagitannins consist of vescalagin, castalagin, their oligomers or their variations such as roburins or grandinin. Tannins impart astringent flavour at least in the early phase of maturation and take part in various oxidative reactions removing sulphury off-notes and promoting color stability, lignin breakdown and alcohol oxidation into acetals producing etheral top-notes.

Dried cloves (Eugenia aromatica)

Other important aromatic oak extractives include different eugenols (clove, cinnamon), β-damascenone (fruity, peach, cooked apple), cyclotene (toasty, caramel), hexanal (grass), trans-2-nonenal (saw dust, greasy), 2-octenal (green leaf, untoasted oak). 

Several extractives from oak wood show significant synergetic effects between each others and lower the odour treshold levels of aromas, for example vanillin and vanillic acid lowers the treshold for lignin-derived aldehydes.

Active carbon layer formed in charring has some filtering potential, removing especially sulphury aromas from the spirit. Some oak derived hydrophobic compounds also suppress the release of volatiles from spirit in room temperatures and mask some aromas especially when nosing whisky.

In conclusion, oak and especially the toasted/charred layer of the cask adds flavour compounds to the spirit and removes some undesired compounds by carbon filtration and oxidation reactions.

Vanilla planifoli

Different oak casks impart different amounts of flavour compounds. The species is the most important factor explaining the differences, but also the origin, seasoning and toasting of staves are significant. Three most common oaks used in whisky cooperage are Quercus alba, Q.robur and Q.petraea. Q.alba grows in northeast America and the latter two in Europe (see previous blogs).

Main differences between species are most likely in the concentrations of oak lactones, eugenol, tannins and other polyphenols. The variation inside the species is most distinct in Q.petraea, as Q.alba and especially Q.robur tend to be more predictable in terms of whisky maturation.

Coconut (Cocos nucifera)

Oak lactones are important flavour extractives in oak wood. The cis-isomer is usually dominant and imparts sweet vanillin and coconut aroma. The trans-oaklactone is more spicy and herbal in low concentrations, but in high concentrations produces heavy coconut and incence, part of this phenomenon is probably due to synergistic nature of trans-isomer with other lactones and polyphenols. The ratio of cis/trans-isomers differs between species: It is highest in Q.alba and almost non-existent in Q.robur. The ratio in Q.petraea varies, but it is usually less than in Q.alba. The only oak with greater proportion of trans-isomer is Japanese oak (reported as Q.mongolica, but it is more likely Q.crispula). Japanese Q.dentata is similar to American Q.alba and Japanese Q.serrata is similar to European Q.petraea in both oaklactone-concentrations and cis/trans-ratios. Total amount of oak lactones is highest in Q.alba and Q.crispula, usually less in Q.petraea (although some very high concentrations have been measured) and very low in Q.robur.

Grapes (Vitis vinifera)

Another significant difference between oak species is the amount of tannins and other polyphenols. Q.robur has most tannins, especially the more water-soluble vescalagin, castalagin and roburins. Q.petraea has about third of the tannins content of Q.robur, but the tannin contents in Q.alba and presumably in Q.crispula are very low. These differences are partly due to the growth speed of the species as older trees have less tannins than younger oaks, but the growth speed alone does not explain the differences between the concentrations.

The amount of lignin is quite similar in all the oaks, although it seems that Q.robur might be richer in lignin than Q.alba. Again there probably is more variation between different Q.petraea trees. It seems that Q.alba has little less soluble monolignols than the European species. Lignin- and hemicellulose- derived vanillin and furfural contents are usually highest in Q.robur. The differences in coopering and especially in toasting/charring practices influence the monolignol and vanillin contents apparently more than the variation between the species.

It was earlier believed in wine industry, that tight-grained (slow growth) oaks produced less tannins and more sweet notes, but the grain width is not significant if the species are taken into account. The observation is true in the sense that Q.petraea is usually tight-grained oak (about 1mm) and has less tannins than coarser Q.robur. Q.robur and Q.alba have usually coarser grains of about 3mm.

The origin of oak has some effects, too. East European oaks (not depended on species) have usually more lactones, eugenols and vanillin but less tannins than French or Spanish oaks.

It should be noted that although the sherry casks used in Scotch whisky industry are called Spanish oak casks, they most likely often are Spanish coopered American oak casks. Although made of the same Q.alba oak, these casks produce very different Scotch whiskies from the ex-bourbon casks due to the differences in cooperage practices, seasoning effects the sherry wine and the different sizes of casks.

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Garde-Cerdàn T et al. Effects of composition, storage time, geographic origin and oak type on the accumulation of some volatile oak compounds and ethylphenols in wines. Food Chem 2010; 122; 1076-1082

Günter Berger R. Flavours and Fragrances: Chemistry, Bioprocessing and Sustainability. Springer 2010
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