says Whyte & Mackay’s Tim Wood. Tannins often get the credit for smoothness, and while there is a correlation between tannins and smoothness, it’s still not certain whether they really deserve all the credit. Moreover, the bad news is that tannins can add bitterness, but as they are part of a package of extractives that includes desirable flavours, that’s the deal.Another significant change, which can occur within months, is a reduction in sulphur character. This is absorbed by the charred surface of the cask, with the level of sulphur character (variously manifested as struck match, rubbery or meaty notes) varying among distilleries.Being charred on the inside, bourbon barrels are equipped with a layer of char typically 2-4 mm deep, and perform this task far more effectively than sherry casks, which are merely toasted.As sulphur character masks other notes, reducing it can change the flavour profile significantly, by allowing floral and ester notes to come through. The char effect is more an amelioration rather than removal, with sulphur compounds oxidising into subtler, less flavour-active forms.“The char takes up the sulphurous, feinty, cereal quality, which also diminishes through evaporation, so you’re getting a dual effect at the same time,” says David Stewart of William Grant & Sons.Does this also mean that the various charring levels applied to bourbon barrels, graded from a lighter number one to a much heavier number four char, can absorb different levels of sulphur compounds ?“The real question is whether the char absorbs as much of the sulphur compounds in a subsequent fill,’ says Tim Wood.American and European oak casks offer a different flavour profile, and while this isn’t delivered like a set menu, there are some front-runners.“With a first fill Bourbon the aromas and flavours that come through first are sweetness, vanilla and fudge, spice develops thereafter. With a herry hogshead the sherry notes dominate initially, with nuttiness from the oak, a wee bit of vanilla, and a cream sherry note,” says Ewen Mackintosh of Gordon & MacPhail.David Stewart adds: “With a bourbon cask you would probably see vanilla first, then floral, slightly spicy notes. With sherry it’s a more intense, dry fruitiness coming through first with sherry and some sulphurous notes.”Another aspect of flavour development is oxidation. Being porous, the cask can breathe, which means an ingress of ‘fresh’ air into the cask, and the egress of ‘saturated’ air.The cask ‘breathes’ as a result of the headspace contracting and expanding, depending on changes in temperature and atmospheric pressure.As the temperature rises the liquid within the cask expands, though the gases (air being composed of various gases) in the headspace actually expand at an even greater rate. As the headspace can’t expand into the liquid, it expands by ‘exhaling’ through the cask.Correspondingly, a fall in temperature sees the liquid and headspace contract, drawing air in from outside the cask.Air can pass into the cask and enter the headspace, i.e. the area above the level of liquid in the cask, from where air dissolves into the spirit (oxygen being the key element). Alternatively, air can pass directly into the liquid through the staves. Comparing whether each route exerts a different influence is not the key issue.It’s the accessibility of liquid to air that really counts, with the headspace having a clear advantage.The process of oxidation sees esters, aldehydes and acids within the spirit continually reaching a new equilibrium. Beyond the science, the result is that new make spirit tends to become fruitier, more floral, balanced and mature.As continued maturation results in a lower level of liquid within the cask, and an increased headspace, opinions vary on how, or whether, this affects oxidation.“More head space doesn’t mean accelerated maturation as other equilibria regulate this. It’s a case of gradual changes” says Tim Wood.A larger headspace also raises another consideration.“After about 10 years we’ve lost around 50-60 litres from a hogshead, which is around one fifth of the cask, so it’s a significant headspace. As the headspace increases so does the influence of the oak,” says Ewen Mackintosh.“In casks with a low volume of liquid and a high volume of air space you run an extreme risk of over-maturing, because the volume to surface ratio of oak contact is higher than normal.”Whether American and European oak show any difference in the movement of air is another question.“European oak is more porous and has a higher rate of oxidation than tightly grained American oak. The evaporation rate of European oak is also higher, which in theory would mean a quicker though not forced maturation,” says Andrew Rankin.The rate of evaporation, typically two to two and a half per cent of a cask’s total contents per annum, including alcohol and water, leads to a decline in alcoholic strength and volume.Losing certain components from the spirit through evaporation can also effect a change in character.Temperature is one factor affecting evaporation, and as the temperature rises the rate of evaporation is likely to follow suit. Meanwhile, “the evaporation rate slows down as alcoholic strength declines, and after 20 years is down to about one per cent,” says David Boyd of Chivas Brothers. This means a malt barrelled at 63.5% abv may reach 58% abv after 12 years, 56% abv after 18 years, 54-55% abv after 25 years, and 46-50% abv after 50 years.The influence of a faster or slower evaporation rate on ageing is still not fully understood. However, a diminishing alcoholic strength, and consequent change in the proportion of water, affects the equilibrium within the cask.A higher strength can result in more alcohol-soluble flavour compounds being derived from the oak, while a lower strength (and relatively higher proportion of water in the spirit) may see more water-soluble extracts being derived. It’s something for me to think about, over the next dram.