Food & Drink: Little acorns make mighty malts: Whisky lovers may think they know about single malt, single cask and even cask strength; now they must learn about 'double wood', says Michael Jackson

With the high season of whisky upon us, lovers of the malt have ever more terms to ponder. As well as 'blended Scotch', 'single malt', 'single cask' (or barrel) and 'cask strength', we now have 'double wood'.

While a blend may contain anything from a dozen to 40 malt whiskies, a single malt must be the product of one distillery, sometimes of a single cask (perhaps at an undiluted strength of around 60 per cent alcohol). The newest designation describes a single malt successively matured in casks of two different origins (though both made from the traditional oak).

The phrase 'double wood' has been coined by William Grant & Sons, producer of the eponymous blended Scotch and of Glenfiddich, the world's biggest-selling single malt. In addition to the Glenfiddich distillery (established in 1886-87), the company also owns the lesser- known Balvenie (which celebrates its centenary this year). The two distilleries are neighbours in Dufftown, one of the Highlands' principal whisky-making centres. While Glenfiddich produces a lightly fruity malt whisky, Balvenie makes a more complex drop noted for its heather-honey character.

For decades Grant's has promoted Glenfiddich but left The Balvenie to be discovered by the more demanding malt-lover. As connoisseurship of whisky spreads, the company has decided to present The Balvenie with more flourish. Until now, The Balvenie has been available in two versions: Founder's Reserve (10 years old) and Classic (with no age statement but containing older batches). Now it is being sold in three forms: a Founder's Reserve, still at 10 years old; a Single Barrel edition, at 15; and the Double Wood, at 12.

Most of The Balvenie that goes into Founder's Reserve has been matured in barrels that had contained bourbon whiskey, but 10 per cent has been aged in butts previously used for sherry. The two are married and then bottled. The Single Barrel Balvenie has all been in bourbon wood. The Double Wood has spent most of its maturation in bourbon oak, but has six months' 'finishing' in butts that have held sweet oloroso sherry.

The Founder's Reserve is the lightest of the three, but very smooth, with the characteristic heather-honey note and a touch of sweetness: a perfect restorative after a walk in the country. The Single Barrel is a much more oaky (and, being at cask strength, more robust) winter warmer. The Double Wood is full of spicy cinnamon notes and fruity, orangey flavours: definitely an after-dinner malt.

The technique of maturing a malt whisky in one type of cask and finishing it in another is not new but, by labelling the result Double Wood, The Balvenie is highlighting the growing interest in this aspect of production.

The considered choice of the casks used in maturation is a relatively new element in whisky-making. For centuries, the cask was regarded as simply a container, and no thought was given to its influence on the flavours of the whisky.

The use of sherry casks arose simply because they were available. Spain exported a great deal of sherry to Bristol; the English did not want to ship back empty butts and the Scots cannily acquired them. It was only as the Spanish began to do their own bottling and England's sherry habit coincidentally diminished, leaving fewer casks in Bristol, that the Scots realised what they were missing.

Scottish distilleries that continue to insist on sherry casks have made a point of this in their publicity. Some, notably Macallan and Glengoyne, commission the production of casks in Spain and loan them to sherry houses for a couple of years to gain their wine character. Both those distilleries use dry oloroso, though Glengoyne also likes a proportion of palo cortado; Balvenie prefers the sweeter style. And I remember a wonderfully nutty batch of Springbank that had emerged from amontillado casks.

A hefty whisky such as The Macallan gains roundness from sherry casks, a lighter one such as Glengoyne acquires depth; the Springbank increases in complexity.

It was as sherry butts became harder to find that some distillers turned to bourbon barrels. The heavy corn-based whiskies of Kentucky are made in new barrels, where the sweet flavours of the spirit blend with the caramels and tannins in the oak. Having been used once, the barrels are shipped to Scotland. In their second use, the oak is gentler, which suits the more delicate character of a whisky made from barley malt.

Glenmorangie, for example, emphasises its use of bourbon wood in its 10-year-old, its principal product. Its 18-year-old, though, is a marriage of a large proportion of bourbon-aged whisky with a smaller proportion from sherry.

As whisky distillers work with wood in this way, they are learning that its variations have a greater influence than they ever thought. The whisky not only takes characteristics from the oak itself, and the sherry or bourbon, but also exhales spirity aromas and flavours, breathes in the air of the surrounding countryside and has a slow, gentle oxidation, in much the way that some great wines do. In whisky, oxidation seems to heighten flowery notes. Compounds that occur naturally in oak also seem to pull together the various aromas and flavours in the whisky.

Glenmorangie has twice asked perfumiers to analyse the aromas in its whiskies. Each time, the perfume houses found more than 20 aromas, ranging from almond, apple and cedar to rose, sandalwood and vanilla. On the more recent occasion, the distillery then asked the scientists at the Pentlands Scotch Whisky Research Centre, in Edinburgh, to try to trace the source of each aroma to a stage of the production process.

Whisky that came off the still with a suggestion of lemon in its aroma seemed to become orangey after a period in sherry wood. These casks also seemed to add raisin and plum notes.

When the whisky encountered the American white oak used in bourbon casks, a reaction took place which produced notes reminiscent of coconut. The bourbon wood also contributed hints of cloves, among other spicy suggestions.

Glenmorangie favours oak grown in the Ozark mountains of Missouri. It likes the trees to be as far west as possible, just before the Ozarks give way to the prairies. It wants trees that have grown from fallen acorns rather than in planted forests.

Perhaps we shall one day see a whisky label that tells us the location of the loch whence the water flowed, the variety and region of the barley that was malted, the site where the peat was cut to set the fire to dry the grains and, in addition to the whereabouts of the distillery, the age and origins of the oaks that made the casks. Bourbon or sherry, dry oloroso or sweet, amontillado, palo cortado . . . why not? At this time of year, I like a good read with my bedtime malt.

(Photograph omitted)