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Wood maturation of wine (Part II): The effect of various wood components on the colour, flavour and taste of wine
Wessel du Toit
Key words: Oak, barrels, wood maturation, wood toasting.
Introduction
Oaky, coconut, smokey, coffee, caramel, spicy, clove, vanilla etc. These are terms used to describe the extraction of flavour and taste compounds during the maturation process of wine in contact with oak. These compounds all play a role in changing the composition and quality of the wine. The colour and taste of red wine also change during this process as a result of the extraction of certain non-volatile components from the wood. This article focuses on the compounds extracted from oak during the maturation of wine and is the second in a series of three articles about wood maturation of wine.
Extraction of non-volatile compounds and the effect thereof on the colour and taste of the wine
During the maturation of red wine in wooden barrels the colour changes in due course of time. Usually an increase in colour intensity may be observed between three to nine months after storing the wine in the barrel, as seen in Fig. 1. Initially the modified colour intensity (where the bleaching effect of SO2 is negated) increases, but may later even decrease slightly. However, the difference between the colour intensity and the modified colour intensity decreases in time. This occurs due to the polymerisation of free anthocyanins with grape tannins (condensed tannins), making the colour less sensitive to changes in pH and SO2 decoloration. This in turn is reflected in an increase in the red pigment and a decrease in the free anthocyanin concentration during wood maturation. This reaction is catalysed by O2, which comes into contact with the wine and the extraction of wood tannins (hydrolysable tannins) during the barrel maturation period. The most important wood tannins are vescalagin, castalagin, granidine and roburins A to E and these consist mainly of esters of glucose with hexahydroxydiphene acid. These types of molecules are easily oxidised due to their multiple OH-groups and form H2O2 during their oxidation, which oxidises alcohol to form small quantities of acetaldehyde, which form a "bridge" between catechins (the monomers of condensed tannins) and anthocyanin molecules (the so-called Bayer reaction or acetaldehyde meditated polymerisation). Hydrolysable tannins and their hydrolyse products ellagic acid and gallic acid therefore protect the wine against excessive oxydation. This polymerisation reaction that they catalyse also plays a role in the polymerisation of grape tannins, however, which makes the wine softer and less astringent. These compounds also have an astringent and bitter taste, however, and may contribute, if the wine is overly wooded, to the harshness and bitterness of the wine. They are suspected to work synergistically with other compounds to influence the mouthfeel of the wine. Together with the increase in the number of barrel fills, the concentration of these compounds decreases however, therefore resulting in a decrease in the oxidative effect of the barrel on the wine. Small quantities of condensed tannins are also extracted from the wood to the wine, but the quantities are very small compared to the quantities coming from the grapes.
Extraction of volatile compounds and their effect on the flavour of the wine
Various aromatic compounds are also extracted from the wood, which may have a huge influence on the flavour of the wine. The extent of extraction is to a large extent influenced by the origin and the maturation and toasting processes of the wood. The effect of this was discussed in more detail in the previous article. One of the most important flavour components is the so-called oak lactones or whisky lactones. In fact it is the trans and cis isomers of ß-methyl-g-octalactone that impart the characteristic oak, or at higher concentrations, the coconut character to the wine. The oak lactones are characteristic of the Quercus genus. The cis form is more aromatic than the trans form and also has a lower threshold value in wine. These types of compounds also play a big role in the character of brandy and whisky. American oak in particular has high levels of oak lactones and the toasting intensity could also play a role. Vanillin (with a vanilla character) is formed by the degradation of lignin during toasting of the wood (Fig. 2). Maximum levels are usually found in medium and medium+ toasted wood. Chemical changes of vanillin may occur if the wine is matured for a long time and this may be converted to vanillin acid (which is less aromatic) if the wine is left on the lees. It therefore seems likely to play a more important role in the maturation of spirits. The threshold value of vanillin is 65 µg/L in white wine and 300 - 400 µg/L in red wine. It appears, however, that other compounds, such as oak lactones, may also contribute to the vanilla character of wine. Eugenol has a clove-like character and is more often encountered in young wood. During the maturation process this may be reduced in the wood, but it is not only generated in wood due to the toasting process. Toasting may cause it to increase, especially with medium and medium+ toasting, but this usually decreases with heavy toasting. Eugenol may also contribute to the spicy character in certain red wines. Other volatile phenols include guajacol and 4-methyl guajacol, which may impart a smokey character to the wine and increase at higher levels of toasting. There could likewise be an increase in the 4-ethylphenol (medicinal, horsey) and 4-ethylguajacol (smokey, medicinal) concentrations, but these are mostly formed by Brettanomyces spoilage. The toasting of the wood, which results in a breakdown of cellulose and hemicellulose, may release certain carbohydrate derivates in the wine. The most important is furfural, 5-methylfurfural and 5-hydroxymethylfurfural. These usually have a caramel-type character. Cycloten and maltol can also be formed by toasting, which may also contribute to the caramel character. Furfuraltiol may be formed from furfural by the yeast during alcoholic fermentation in a barrel with toasted dowels. Certain norisoprenoids, such as ß-ionone, may also be released into wine from wooden barrels, and have a violet-like character. This was found especially in American oak and Vosges wood, but it is not certain how it influences the flavour of the wine, since it also occurs naturally in certain red grape cultivars.
Trans-2-nonenal, trans-2-octanal and 1-decanal may contribute to plank-like or dusty flavours in certain wines and they are thought to contribute to the types of flavour that are sometimes observed in wine that has been matured on wooden chips. Acetic acid that is not bacterial in origin may also increase in wine that has been matured in new barrels. This occurs due to hydrolysis of hemicullose, of which acetic acid is a breakdown product. Fortunately the levels of this are low compared to the acetic acid of microbiological origin in the wine.
It also appears that oak flavour could hide the fruity flavour of the wine through the so-called masking effect. It is clear, however, that oak can have a big influence on the colour, flavour and taste profile of wine. The origin of the wood, type of wood (as may be seen in Fig. 2) and other factors all play a role and should be taken into account by the winemaker.
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