Wine: Another Reason to be Thankful for Trees

by Nick Hellis on February 21, 2017

We all know that the majority of fine wines are aged in oak barrels. But why are they? What are the advantages? What are the disadvantages? Are there different kinds of oak? How are they different? What do these differences mean? What’s it all about?
 

Oak Aging and Wine

We all know that the majority of fine wines are aged in oak barrels. But why are they? What are the advantages? What are the disadvantages? Are there different kinds of oak? How are they different? What do these differences mean? What’s it all about?

Oak for Barrels

There are some 600 different species of oak but only three species are widely used to make barrels for wine, the Pedunculate oak (Quercus robur), the Sessile oak (Quercus petrea often considered to be superior to Q. robur) and the White oak (Quercus alba).

Ageing wine before it is bottled

After fermentation is complete the wine is racked to remove any remaining solids and to allow the young wine to settle for a period of time. This settling or ageing can be done in neutral containers such as stainless steel or it can be done in oak barrels which will influence how the wine develops.

Why do we use oak barrels?

We use oak barrels for two main reasons. Firstly, to gradually introduce oxygen into red wine to soften the astringency and to increase the colour and stability of the wine.

Secondly, oak wood is comprised of complex chemical compounds, each of which contributes its own flavour or textural note to both red and white wines.

Different barrels, different coopering, different flavours

Oak barrels can be large or small, old or new. The smaller the barrel, the newer the barrel, and the more time spent in the barrel, the more oak flavours will be imparted into the wine. The source of the wood is also very important. Barrels are made by cutting wood into long, narrow pieces called staves. After seasoning, the staves must be heated so they can be bent to form the barrel. Steaming is the cheapest method. The best method is to expose them to a naked flame. The longer the flame exposure, the more toasted or charred the wood becomes. This greatly affects the flavours imparted to the wine.

The composition of oak and its flavour chemistry

Tannin naturally occurs in oak as well as grape skins. As a characteristic of wine, tannin adds both bitterness and astringency as well as complexity. Wine tannins are most commonly found in red wine, although white wines have tannin from being aged in oak barrels, think Chardonny.

Vanillin is released when the lignin in oak starts to breakdown. It is most prominent and the principal flavour and aroma compound in vanilla and imparts a creamy aroma and smooth mouth-feel to both red and white wines in low concentrations.

Hemicellulose - Air drying oak initiates the breakdown of hemicellulose into simple sugars. When toasted barrels reach 150 degrees centigrade, further simple sugars are formed with 'sweet' aromas. When toasted barrels reach 215 degrees centigrade the ‘toasty’ aromas are formed.

The structure of oak and its contribution to flavor

In France, both the Pedunculate and Sessile oak are used for wine making, however, the latter is considered far superior for its finer grain and richer contribution of aromatic components like vanillin and its derivates, methyl-octalactone and tannins, as well as phenols and volatile aldehydes.

In America, the White oak is characterized by its relatively fast growth, wider grains and lower wood tannins. The structure of American oak’s hemicellulose and lignin results in more intense vanilla, wood sugars, and toasty aromas.

In Italy there is a long history of using Pedunculate oak from Hungary and Slovenia which has a tight grain, which gives a different spectrum of toasty aromas from the American oak and also medium levels of tannin.