Oak has long been used to make wooden barrels and casks for holding wine and other foods. This is due, among other things, to certain physical and chemical properties that make it unique compared to other tree species. One of these is the size of its medullary rays, which give the wood extraordinary strength when the barrel is built. Another is the almost complete absence of resins and other chemical compounds commonly found in other species such as pine, which makes oak a fairly pure wood from a chemical point of view.
Another advantage over other woods is the small size of its pores, which allows the wood to impart its aromas to the wine in a controlled way.
Chemical composition of oak wood
Like all woody plant tissue, oak wood is made up of cellulose, hemicellulose, lignin, tannins, some lipids and smaller amounts of other substances. One exception is American white oak, Quercus alba, which after the barrel- or cask-making process contains a certain amount of lactones (oak lactones) that give the wine considerably different flavors compared to other oak species. When assessing what the wood contributes to the wine during aging, it is very important to consider not only the chemical composition of the wood used but also every step involved in making the barrel, such as drying time and method, toasting times and methods, cutting, and so on. These factors will be covered in a later post.
Cellulose
Cellulose is the most abundant natural polymer on earth and consists essentially of a linear chain of glucose molecules. Cellulose plays no sensory role in the aging process but is important for maintaining the structure of the barrel. In some cases, acid hydrolysis can break the cellulose chain, releasing two glucose units joined together, a molecule called cellobiose, which can serve as a nutrient for spoilage yeasts of the genus Brettanomyces.
Hemicellulose
Hemicellulose is a polymer that, unlike cellulose, can be broken down into various simple sugars, the most abundant of which are xylose, mannose, rhamnose, arabinose and galactose. Another key difference from cellulose is that hemicellulose is less abundant and also less stable.
During barrel making, the wood is exposed to high temperatures (above 140 °C), which begins breaking hemicellulose down into its structural sugars. These sugars are in turn modified and become an essential part of what the wood contributes to the wine’s sensory profile during aging. These changes in hemicellulose are extremely complex, so the proportion of sugars released and the transformations they undergo are hard to predict and therefore hard to control. When hemicellulose goes through seasoning and then toasting, it changes the structure of the wine by contributing simple sugars, and the caramelization of those sugars produces aromatic compounds such as hydroxymethylfurfural, furfural and maltol, which ultimately play a role in changing the wine’s color during aging.
Lignin
Lignin is a three-dimensional polymer whose chemical composition varies by plant species. In the genus Quercus, it is made up mainly of two units, guaiacyl and syringyl. In aged wines, these two units give rise to two groups of compounds:
- Guaiacyl derivatives, which include vanillin and vanillic acid, responsible for the vanilla aromas of some wines.
- Syringyl derivatives, which include sinapaldehyde, syringaldehyde and syringic acid.
The barrel-making process has a strong influence on what happens to the lignin: toasting and high temperatures lead to the formation of the phenolic aldehydes mentioned above.
Oak tannins
Oak tannins are hydrolyzable tannins, which are esters of sugars with phenolic acids or their derivatives. The most common phenolic acids in these hydrolyzable tannins are:
- Ellagic acid: gives rise to ellagitannins.
- Gallic acid: comes from the hydrolysis of ellagic acid and gives rise to gallotannins.
It is very important to note that none of these tannins occur naturally in grapes or young wine; they are usually passed on to the wine by the barrels during aging, or added artificially. These tannins, especially ellagitannins, play a key role in stabilizing color by acting indirectly as antioxidants: they oxidize in preference to the anthocyanins, thereby protecting them from oxidation.