It’s a real irony. Excessive tannin levels are easy to recognise on the palate, but all the benefits of a desirable level are not. This is partly because they do a lot of important work without having a discernible presence, whereas you can’t miss the astringency and overt dryness of too much influence. Additionally, some advantages that tannins provide are described conceptually, rather than literally, such as ‘adding structure' and ‘backbone’ to a malt whisky, which is easier to appreciate than to define.
Tannins are acquired from the cask, with the level depending on the origin of the oak (in conjunction with the length of ageing). Levels in European oak can be up to several times higher than American oak. In terms of the two main cask types used, Bourbon barrels are American oak, while sherry casks used for maturing whisky are either American oak or European oak.
How casks are seasoned (ie. which type of alcohol they held, and for how long) before being filled in Scotland, also influences tannin levels. The higher strength of spirit filled into American oak Bourbon barrels in Kentucky, up to a maximum of 62.5% ABV, extracts more influence from the wood when compared to wine at around 15% ABV. Consequently, whiskies aged in casks that previously held lower-ABV liquid tend to acquire higher tannin levels.
Whatever the seasoning process or type of oak, the maturing malt whisky extracts tannins at the greatest rate during the first three years, after which the rate decreases significantly.
A key characteristic that tannins contribute is dryness. This provides a contrast, as well as a balance, to sweetness, and by delineating opposite ends of the spectrum. The duality of dryness and sweetness also create a sense of structure.
“I think of structure as the way in which the flavour of a malt whisky develops from the first initial taste through to the finish, and the dry base that tannins provide enhances the sense of evolution on the palate. The interaction of dryness with other flavours is also key,” says Michael Henry, master blender, Loch Lomond Group. Examples of this are the counterpoint that dryness provides to vanilla sweetness, with it also ‘underlining’ ripe fruit notes.
It follows that another aspect of interaction concerns the number of times a cask has been used. First fill casks (those being used to mature Scotch whisky for the first time) have higher tannin levels than second fill, and significantly more than third fill, as each time a cask is used its influence diminishes. However, quantifying this decrease depends on the length of each maturation period, which varies enormously. It’s not just about the level of tannin a cask provides on paper, it’s also about the context.
“First fill Bourbon barrels contribute a lot of sweetness and vanilla which hides the tannins, whereas in a second or third fill barrel the tannins are more perceptible as the levels of vanilla and sweetness diminish,” adds Michael. Similarly, depending on the fill, European oak sherry casks require careful monitoring, especially when approaching their teenage years.
“Around 12 years ageing in a first fill Spanish oak sherry butt (500 litre capacity) is usually the limit, as beyond this the whisky can become chewy, drying, and laden with tannins. It’s a style of whisky referred to as a 'sherry bomb'. Some people love them, others find the flavour too big and drying. If you want to age malt whisky for 20-30 years in a [European oak] sherry butt then second or third fill is better,” says Stuart Harvey, master blender, Inver House Distillers.
What's more, the character of a distillery's new make spirit also raises the question of how its mature whiskies will be delivered onto the palate, which in turn highlights mouthfeel. “Tannins contribute to Glenmorangie’s full, velvety texture, and also enhance the viscosity, creating a more syrupy consistency,” explains Andy MacDonald, Glenmorangie’s distillery manager.
Stuart Harvey agrees, “The mouthfeel of a malt depends on the interaction between tannins and other flavour compounds present, such as vanillin, as they also contribute to mouthfeel. As the level of tannins and flavour compounds varies among malts, this is one reason why mouthfeel is individual.” That’s the good news. But what maturation doesn’t go to plan?
“Astringency is the worst that tannins can do to a malt, on the palate and in the aftertaste, with real dryness that can overwhelm the flavours and ruin a whisky. Tannins require careful management, but if you know the history of your casks then it’s quite easy to manage,” Andy concludes.
Cask selection has become a key phrase, with a greater range of casks being used for finishing and full maturation. The way a cask has been ‘seasoned’ before arriving in Scotland can also supplement the tannin levels present in the oak.
“A red wine cask, for example, can provide additional tannins from the residue liquid remaining in the staves, so it’s a case of balancing the influence of the cask with the whisky, and the best way to control this is using the cask for finishing rather than for full maturation,” says Andy MacDonald.
Michael Henry provides a further example of this tailored approach: “Inchmurrin Madeira Finish is initially aged in Bourbon barrels, then finished in Madeira casks, which add dates, figs and sweetness, and at the end of the palate a nice, dry, tannin from the Madeira grapes comes through and balances the sweetness.”