Oak in Bordeaux wines
A visit to a barrel makers (tonnellerie) is always a pleasure, and I recently took a group of Ecole du Vin students to the oldest one in Bordeaux, located in the tiny village of Saint Caprais de Bordeaux. Called Cooperage Demptos, it is part of the bigger François Frères Group.
I can't recommend highly enough taking the time to visit a barrel makers if you are in a wine region. You will never experience cellar visits again in quite the same way once you realise just how much work goes into making a barrel.
First of all (for French oak), the trees have to be around 150 years old. That was the legal minimum until a few years ago, when the age was lowered slightly, sometimes to around 120 or 130 years old. And for one majestic 150 year old oak tree, you will be lucky to get three oak barrels - more usually two. Of course the off-cuts go into making furniture and other wood products, but that still means a lot of tree for a little barrel.
One of the very interesting thing about a visit here is seeing where the barrels end up. Demptos export between 60 and 70% of their production, and many of the final barrels were labelled LVMH - not for their local properties of Yquem and Cheval Blanc, but ready to be shipped over to New Zealand for Cloudy Bay (I imagine destined for its Te Koko label).
The life cycle of a barrel goes something like this: the government holds auctions where different tonnelleries (even individual properties for the really big guys) will make bids for their trees (this used to be where the price started high and got lower in the traditional manner with farming in France but today is more likely to be a straight-forward price-rising auction). The main forests used are the Allier, the Limousin and the Vosges. Your choice of forest and age is important: Oaks with a slow growth rate are finer grained and richer in aromatic compounds (eugenol and whisky lactones in particular). Older wood is also more porous which facilitates oxygen uptake.
So, you've successfully bid on your oak - you then are likely to have to wait another few years before it's cut, and then it's brought to the cooperage, where it spends a further few years air-drying outside. This is crucial and lasts usually from two to three years. The purpose of this is to dry out the sap, of course, to begin the process of softening the tannins. Bad weather is important aswell - rain is the physical medium that washes away the harshest tannins. With exposure to the open air, certain strains of fungi develop that influence the phenolic profile of the wood, converting lignin into vanillin far more effectively than kiln-drying.
Apparently, two years used to be the average, but as the fashion heads away from over-oaked wines, many wineries are asking for their oak to stay out for three years, to lessen some of the more obvious flavours.
Okay, so now it moves inside, and things speed up - in fact you can expect your barrel to be ready within a few days. Demptos makes around 150 barrels per day, but of course cooperages vary enormously in size.
The amazing thing to watch is what a tough phyiscal process it is - there are machines of course to cut the staves and cut out the bung hole etc, but a lot of work involves men (and it is men, I have yet to meet a female barrel maker - please correct me if you know one) physically hammering the thing together.
The most magical bit to watch is the toasting. This is done not only to make the wood pliable enough to move into 'barrel shape' but also to season the final product. Wineries can choose from light toast, medium toast, medium plus, high toast and a variety in between. The length over the fire doesn't alter (around one hour), but the intensity of the flames do.
According to Demptos, although more difficult to control, toasting over an oak chip brazier results in a more even "toast" of the barrel surface. This process releases the aromatic substances (vanillin, eugenol, syringol, etc) far more effectively than other toasting methods using electricity or gas burners or even infrared light.
A lot of great info (some of which I have reproduced above) is on the Demptos website.
I also wrote (in January 2013) a column for Decanter China on France's century old oaks, after a visit to the Boulogne forest with Seguin Moreau barrel makers.