then you should also accept that barrels would have been used - after all, it was the Celts who were the first coopers.Whatever its origins, the use of oak casks started in earnest when whisky began to be widely drunk. Distillers and innkeepers needed larger containers to store and transport it – and while Americans used barrels made from new oak, in Scotland casks previously used for wine, sherry, port and rum were pressed into service.At some point it was noticed that oak changed the character of whisky. In America, whiskey had to be shipped downriver from Bourbon County to New Orleans. By the time it reached its destination it had picked up extra flavour and a reddish colour (hence the term Red Eye) from the new wood.To understand why this is the case you need to understand what makes oak so special. It’s the perfect wood for storing liquids: hard, watertight, but pliable. It allows enough air in to produce a slow oxidation, but prevents rapid evaporation, thereby preserving aromatics. It also contains high levels of extracts that give the spirit colour and flavour.The whisky industry uses two very different types of oak – American white oak, Quercus alba, and European oak, Quercus robur. To see how each type behaves you need look no further than Glenmorangie and The Macallan. While Glenmorangie only uses ex-bourbon barrels made from American oak, Macallan uses ex-sherry butts made from European oak harvested in Spain. These, Macallan’s manager David Robertson argues, have higher tannins and give the malt a richer colour and character. ‘Macallan gets its resinous, spicy character and colour from Spanish oak. A light-coloured Macallan aged in American wood simply doesn’t have that character. It’s as fundamental as that. The driver in terms of flavour and colour is the wood.’Why this is the case is down to the host of compounds that lurk within the wood – vanillins, tannins, lactones (which give coconut/oaky flavours), spicy phenols and flavour-enhancing furfurals, which appear when the cask is toasted. ‘There are three separate reactions happening in the cask,’ explains Glenmorangie’s Dr Bill Lumsden. ‘The wood absorbs sulphur and certain feinty elements. It also adds compounds that give flavour and colour and allows cask-driven oxidation. This brings a myriad of other elements into play – the most important for us being acectaldehyde, which gives the floral top note to Glenmorangie.’The irony is that although the industry has used oak for centuries it has only just come to grips with the role it plays in a whisky’s flavour. ‘New make gives 40 per cent of a whisky’s flavour,’ says Lumsden. ‘The remaining 60 per cent is provided by wood and wood-driven chemistry. Wood is a major contributor to the spirit and to do it well you have to have good wood.’That may seem pretty obvious, but it’s not something the industry has given particular thought to until relatively recently. In the past, the quality criteria only extended as far as whether the cask leaked or not. That’s now changing.Malt distillers have always been at a slight disadvantage because traditionally they have only aged whisky in secondhand casks, leaving them at the mercy of the supplier. Surely these days it wouldn’t be difficult to source new wood and have control from the word go? The trouble is that malt whisky and new wood do not get along. ‘New make is actually quite a delicate spirit,’ Lumsden explains. ‘There’s so much flavour in new
wood that it would swamp the distillate.’Wood maturation involves the spirit being sucked into the oak, thus extracting flavour and colour. The newer the wood, the more there is to extract. A secondhand bourbon cask will therefore have had a lot of its initial flavours (in particular vanillins and lactones) already leached out. The more you use the cask, the less extract there is, which is why producers like Glenmorangie and Macallan only use first and second-fill casks for their malts. Third-fill casks are either sold on or used for grain whisky or fillings for blends.The question is whether there are ways to arrest this slow decline. Couldn’t a knackered cask be given the equivalent of Viagra and be perked up a bit – maybe by inserting a few staves of new wood? ‘Maybe in theory,’ said Lumsden. ‘but I have concerns about going down that route. You could end up with three-year-old whisky being decanted into a stainless steel tank filled with
oak chips.’That’s not to say that the industry isn’t investigating ways of refreshing casks other than by the traditional method of recharring. Two techniques are currently being tried – one involving microwaving the staves and then blitzing them with infrared light; or treating them with salts and heating them under an electric burner.Though the jury is still out, this indicates the seriousness with which the industry is now approaching wood. Distillers are exerting ever-greater control over their wood supply. Glenmorangie has set up a programme which involves casks being assembled from air-dried American oak that has been grown on the north-facing slopes of the Ozarks, while Macallan uses barrels made to its specifications from air-dried Galician oak.Kidology? When you look deeper it makes sense. The Ozarks have poor soil, while north-facing slopes get less sun, meaning the trees grow slowly and a slow growing tree gives the ideal pore size. Air-drying is equally important. It reduces the perception of tannin in European oak and increases it in American, while by air-drying American oak the lactones in the wood are reduced, thereby cutting down on aggressive coconut/oak aromas. The trouble is that air drying takes time. These days, most American (and many European) coopers kiln-dry their staves. The process is quicker, but it increases astringency and gives acrid notes.In Kentucky, Maker’s Mark and Labrot & Graham only use air-dried timber and it is widely used at Wild Turkey. Those who use kiln-dried wood admit, off the record, that they would prefer to use air-dried timber – but the accountants and the major coopers are calling the shots. It enforces Lumsden’s point that if you want quality, you have to pay a premium.That’s what Macallan is doing in Spain. As well as specifying the type of timber, it also insists that the cask must then have wine fermented in it and then be filled with sherry for two years. There are good reasons for this. ‘The sherry producers want minimal wood influence in their wine,’ said Robertson. ‘Fermenting in cask takes out flavours which otherwise would leach into the sherry. It cleans the cask up and coats it.’There are two further differences between ex-sherry and ex-bourbon wood - the first being the alcoholic strength of the liquid that is first put into the cask. Not surprisingly, new spirit penetrates deeper into the wood and sucks out more extract than a relatively weak wine. By the time a bourbon cask has arrived in Scotland, it has had most of its primary flavours leached out. Sherry however hasn’t extracted nearly as much, so there’s still a lot left in the wood.The final difference lies in the way in which the two types of cask are seasoned. Bourbon barrels must be made from new white oak which has been heavily charred. Charring leaches impurities out of the wood as well as releasing vanillins and lignins. It’s this combination of new wood and a heavy char which gives bourbon its distinctive aroma of caramel, honey and vanilla. Sherry producers however, toast rather than char their barrels meaning there are fewer caramel notes.Then there’s the issue of finishing which is based on the principle that what goes into a cask, must come out. Robertson however insists that it is the type of wood that makes the difference, not what the cask has previously held. ‘We've done studies on casks filled with different types of sherry and there was little difference between them,’’ he argues. ’If you use American oak and different sherries, you may get more wine-driven notes, but not with Spanish oak.’That leaves you baffled as to how Glenmorangie’s fino finish is more fino-accented than its sherry finish. Over to Lumsden. ‘Macallan is maturing in sherry butts, rather than just finishing. A heavily sherried malt needs the influence from the wood, but wine is more important in finishing.’The explosion of finishes is the most obvious indication of how the industry has got its act together on wood management. The long-term effects will be more subtle. Better wood policy won’t necessarily mean better (or different) malt whisky, but it will mean more consistent whisky. Let’s drink to that.