In Parts One and Two of this series I attempted to describe the nuances of malt whisky character that arise from the raw materials (Part One) and from the distilling process (Part Two) respectively. The variables available to the maltster and the distiller lead to a wonderful diversity of new-make spirits ready to be filled into oak casks. However, anyone who has nosed or tasted newly distilled malt spirits might be excused some amazement that these raw, sharp flavoured, immature products should be capable of maturing into the complex, rounded and smooth-drinking whiskies which we know and love.I mentioned in Part Two that an accident of history resulted in stills being made of copper – a substance now known to be essential to our distillation chemistry. A second and equally essential point is that our forbears would quite naturally have turned to wooden casks for storing and transporting their wines, beers and spirits. In most of Europe wooden casks superseded animal skins many centuries ago, providing strong, durable and easily handled containers which had a much more attractive effect on flavour than animal skins, and whose basic physical design has not been improved upon in almost three millennia.Casks have, of course, been made of many types of wood over the years; beech, elm, chestnut, ash, teak, sycamore, pine, fir and even yew have all been tried. However, oak was favoured for most alcoholic beverages. The Scotch Whisky Order of 1990 specifies a minimum of three years’ maturation in oak casks, which should not exceed 700 litres capacity. If this definition had been widened to include casks of any wood type (as is the case with most other whiskeys) then one can only imagine the number of ‘wood finish’ malts which would be on the shelves today.Even within the oak species Quercus there are a number of options, although in Scotch whisky these are generally either American white oak, Quercus alba, or European (usually meaning Spanish) oak. The latter is mainly Quercus sessilis or Quercus robur. Newly made casks are rarely used, although there is a brisk trade in ex-bourbon barrels from the US. These have been used only once before being shipped to Scotland, either as American standing barrels (ASBs at 42 gallons/190 litres), or broken down into stave bundles (shooks) to be erected in the UK into dump hogsheads at about 56 gallons/250 litres. Another historical source of casks has been sherry butts (at about 112 gallons/500 litres) which were transported from Spain to the UK full of sherry until casks were superseded for shipping purposes by road and rail tankers from the late 1960s. Since then Scotch distillers have continued to buy ex-sherry casks in Spain. Somewhat confusingly, they can be made from either American (usually for fino sherries) or European oak (for olorosos). The main characteristics of American white oak are a hard, tight grain from which the spirit can extract relatively little in the way of flavour compounds. European oak has a more open grain and higher available extract. If you ask why British oak has not been used the answer often given is one of quality – knots in the wood and high acidity are cited – but I suspect that the real reason has been the ready availability of second-hand imported casks at competitive prices. Admiral Nelson’s navy provided the evidence that good straight oak could be grown in the UK. Given the economical Scottish practice of repairing and re-using casks several times it is obvious that a great variety of cask effects will be possible; and indeed two adjacent casks in a distillery warehouse filled with the same spirit on the same day can yield quite different whiskies when mature. Most casks are, of course, refills, and at each filling the cask will have less virtue – less extract or flavour to contribute during maturation. There comes a time to withdraw worn-out casks from service, and one major distiller uses a rule of thumb of four consecutive fillings or 20 years, whichever is sooner. Another approach is simply to ensure that a proportion, say 20 per cent, of all casks are withdrawn from use for repair, rejuvenation or scrapping at each emptying. Used casks can be rejuvenated by scraping the inside surface and recharring to give a fresh layer of newly charred oak. For the rejuvenation of sherry casks, those made of European oak are selected, subjected to a special scraping and light charring process, and then stored after being filled with an appropriate blend of Jerez or Montilla wines. There is, unsurprisingly, some lively debate as to whether a Scottish ‘bodega’ or the original Spanish version produces the better casks. Thus the consistency of flavour produced in a blended Scotch whisky, or indeed in a single malt vatting, will depend upon a judicious choice of casks and a careful averaging process across the blenders’ inventory. Turning now to the complex process of maturation itself, we know that mature whisky contains over 500 identifiable molecular components, although it is likely that fewer than a hundred of these have a primary influence on organoleptic character. These compounds, or congeners, include higher alcohols, aldehydes, esters, nitrogen and sulphur compounds, carbonyls, phenols and lactones. The long slow chemical reactions occurring in a cask during maturation can be grouped into three general categories:1. Removal of undesirable or immature components such as heavy sulphur compounds by absrption on tothe oak/char surface, and to a lesser extent by evaporation (light carbonyls) through the porous wood.2. Addition of sexirable components such as lactones by migration formm the oak to the spirit.3. Chemical interaction within the spirit between organic compounds, and with oxygen which diffuses through the oak to produce desirable flavour congeners such as acetals.A useful graphical description of these processes as an overlay to basic distillery character was devised by my colleague John Philp, one of the world’s few experts on the chemistry of Scotch whisky maturation. If the basic distillery spirit character is very robust, then this might dominate the final whisky even in high extract oak – a heavily peated Islay malt could be in this category. Conversely a gentle distillery character, such as a delicate Lowland or even a moderately heavy Speyside, can be swamped by the use of, say, a high percentage of first-fill sherry casks. The heavy sweetness derived from 100 per cent European oak sherry casks appeals to many consumers, particularly those who are just discovering Scotch malt whiskies. However, with the exception of the heavily peated Islays, the use of 100 per cent sherry casks usually masks the distillery character, producing a one-dimensional flavour totally dependent upon the casks.The age at which a whisky is bottled is obviously a significant variable. The really great malts are those which have achieved a near-perfect balance between distillery character, choice of cask type and optimum age. On this point of optimum age I have had the pleasure of sampling a single commemorative cask on each anniversary of its filling up to ten years old. I found that although the character improved steadily, this changing balance of concurrent effects caused some fluctuation in quality. I preferred the even years – six, eight and ten – over either seven or nine years. Perhaps it is no coincidence that malts are usually sold at eight, ten and 12 years old.There are three additional factors that come into play during maturation. They are the size of cask, the alcoholic strength (at filling) and the warehouse conditions. My views on each are that there is no doubt that smaller casks will accelerate and/or intensify the oak-spirit interactions, simply because of their higher surface-to-volume ratio. On the question of filling strength, a typical malt spirit collected after distillation has a strength between 65-75% alcohol by volume (abv). This is diluted, preferably using the distillery’s own water supply, before filling into casks. The ideal filling strength seems to be around 63.4% abv (111º proof) – arrived at originally by experience, but subsequently justified by science. Higher strength spirit matures more slowly (but gives better utilisation of warehouse space) and the converse also applies. However, given the great difficulty of forecasting future sales demand, more rapid maturation to peak quality could be a very mixed blessing.Finally, on the matter of warehouse conditions, nostalgia and instinct tend to favour the dark, dank and moist conditions on the earth floor of a traditional single storey warehouse, as against the modern racked version, but this has not been confirmed by objective tasting or by scientific analysis. What does seem important is warehouse temperature – a steady year-round coolness being preferred. Comparisons to warehouses in more extreme climates (US or Japan, for example) have shown that large seasonal fluctuations in temperature cause casks to breathe, drawing in excess oxygen and, of course, losing more to the ‘Angels’ Share’. A proficient maturation manager in any climate will aim to keep his warehouses as full as possible, thus ensuring less susceptibility to temperature cycles. This practice, combined with the comparatively moderate temperature fluctuations of the Scottish climate, limits evaporation losses of maturing Scotch to about two per cent each year – still a large penalty but thankfully duty-free at this stage. On a more subtle level, coastal warehouses (low altitude, salt air, steady ambient temperature and humidity) have been shown both analytically and organlopetically to have some slight differences to those at higher and drier
altitudes.Given all of these maturation variables of cask, age and warehousing, plus the wide choice of new-make spirits available in Scotland, it is hardly surprising that Scotch whisky has a unique breadth and unequalled depth of character. One can have a lifetime of exploration and experiment.