Production

All in the mix

Ian Wisniewski explains how oxidation affects maturation
By Ian Wisniewski
With oak casks regarded as the key influence during maturation, it’s tempting to see oxidation (the interaction of air and spirit within the cask) as a secondary factor. However, as air (principally oxygen) is an essential enabler of maturation, oxidation is actually a vital element.But oxidation is also one of the most difficult influences to monitor and quantify, and consequently one of the least understood elements of maturation.Oxidation is the result of the cask being porous, and able to ‘breathe’ in air. As air within the cask dissolves into the spirit (oxygen being the key element), this sees esters, aldehydes and acids within the spirit continually reaching a new equilibrium. Beyond the science, this promotes a fruitier, more floral, balanced and mature spirit.One theory is that during the early stages of maturation, oxidation can help diminish less desirable sulphur compounds (which are primarily absorbed by the layer of char within a cask, while also passing through the pores of the cask as vapours).Moreover, a reduction in sulphur compounds, in the early stages of maturation, may help promote greater levels of oxidation, so it can be a selfperpetuating process.As sulphur compounds feature a distinctive line-up of notes, ranging from struck match, sulphurous, rubbery, meaty and sweaty socks, to cabbage and vegetal, they can ‘conceal’ other characteristics within the new make spirit. While a certain level of sulphur character can be desirable, depending on the house style, lowering the level of sulphur compounds allows the congeners, including esters, to show more readily.Oxidation rates may also be influenced by tannins, as higher tannin levels are thought to (possibly) promote greater oxidation. This suggests that spirit aged in European oak, which has higher tannin levels than American oak, will see higher levels of oxidation. Tannins also play a role that is beyond flavour development.“Tannins being extracted from the oak, and coming into solution, are being broken down by the interaction of oxygen, and that’s where you get some of your colour compounds,” says The Edrington Group’s John Ramsay.Air can pass through the pores of the cask and interface directly with the spirit, while also accumulating within the headspace (ie. the level of air above the surface of the liquid). So, an initial consideration is when does a head space begin to develop?This depends on the filling regime, and nature of the cask. While some distilleries fill casks to the brim with new make spirit, others don’t, which automatically creates an initial headspace under the bung.But then “even the act of filling an empty cask, which contains air, creates some mingling of air and spirit. Moreover, barrels are usually rolled out of filling stores, so the head space of oxygen is thoroughly mixed with spirit, so that could have an impact,” says Glenmorangie’s Dr Bill Lumsden.Meanwhile, a ‘dry’ cask will result in a high level of indrink, meaning the amount of spirit absorbed almost immediately by the staves after it has been filled with new make spirit. As this can be around two per cent or more of the total volume of spirit, equating to around four litres in a 200 litre barrel, a degree of headspace is created almost instantly. Correspondingly, a fresher cask with saturated staves can show far less indrink.Casks typically loose between two to two and a half per cent of the total volume (comprising alcohol and water) per annum through evaporation, with an additional loss of up to three per cent during year one covering indrink. As this totals a loss of three to five per cent, there is a significant headspace by the end of year one.The headspace increases due to continued evaporation, with the evaporation rate reflecting the temperature within the aging warehouse. This is more stable in a traditional (or dunnage) warehouse, where casks are stacked up to three high, compared to a racked warehouse where barrels are stacked eight or even 12 barrels high. Casks stored higher up in a racked warehouse experience the greatest temperature differential, promoting greater evaporation and headspace.The temperature also exerts another vital influence, with the spirit expanding within the cask when the temperature rises.Meanwhile, the gases (air being composed of various gases) in the head space expand at an even greater rate than the spirit. As the headspace can’t expand into the liquid, it expands by ‘exhaling’ through the cask’s pores, joints and bung.Correspondingly, when the temperature falls the spirit contracts, creating a larger area for the headspace to occupy within the cask. Meanwhile, the gases in the headspace contract at an even greater rate than the spirit, which results in fresh air being drawn into the headspace from outside the cask.In this sense the headspace acts like a ‘lung,’ inhaling fresh air and exhaling air that has become saturated with vapours from the spirit (which includes esters, sulphur compounds, alcohol and water). This cycle of fresh air being inhaled, and saturated air being exhaled, is a crucial factor, as fresh air creates a new equilibrium between the headspace and the spirit.As the Scottish climate can entail significant changes within the same day, ranging from warmer day-time to colder night-time temperatures, some distillers think the spirit can expand and contract (known as a ‘cycle’) even within 24 hours.Others believe it takes significant and sustained changes in temperature to see any movement within the spirit, and that daily fluctuations in temperature needn’t have an impact. Consequently, it takes months rather than days to see any movement, with a cycle more likely on a seasonal basis, reflecting temperature differentials between summer and winter.Apart from the temperature, changes in air pressure due to varying weather conditions are also influential. Moving from higher to lower pressure, and vice-versa, sees the spirit expanding and contracting, and thus influencing the rate at which the cask ‘breathes.’ Meanwhile, apart from the headspace providing an interface between air and spirit, a similar process occurs below the surface of the spirit.“Air is passing through the staves and being absorbed directly by the spirit, though quantifying the influence this has compared to air being absorbed through the headspace is another matter,” says Dr Bill Lumsden.Another consideration is whether there’s any difference in the movement of air through American and European oak.As European oak is more porous than more tightly-grained American oak, the movement of air could theoretically be greater. This theory is supported by American and European oak casks having different evaporation losses, which are attributed to the pore size.Meanwhile, some distillers believe the joints and bung are a more important ‘air way’ into the cask than the pores, while others say it’s principally down to the pores.Then again, another view is that it’s a combination of all the entry points into the cask that matters.Continued maturation leads to lower levels of liquid within the cask, and an increased headspace, “which starts to produce some components of really old whisky, which is an oxidation situation.“The French call it ‘rancio,’ it’s an indicator of a particularly old whisky, creating very complex, aromatic flavours,” says John Ramsay.“But you’re also getting a concentration of oak elements that have developed over the years, with a risk of the oak influence being very intense.” Gordon & MacPhail’s Ewen Mackintosh adds, “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.“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 high.” Whether the spirit is aged in a ‘first fill’ cask (ie. filled for the first time with new make spirit) or a refill cask, also influences how the effects of oxidation show in the resulting malt.“A first fill bourbon barrel has more natural sweetness and body, while a second fill contains less components such as vanillin, and allows more products of oxidation to shine through, which gives more of a floral top note,” says Dr Bill Lumsden. Apart from oxidation taking place in the cask, its influence within a bottle is another consideration.“As soon as you open a bottle you will start to get oxidation, and as the level reduces in the bottle the whisky can change dramatically, and tasting notes can be very different.“Occasionally we get a bottle back where the air-tight seal has gone, and comparing this to a control sample the nose tends to change from fresh to softened and blunted, from fruit to over-ripe, bruised apple fruitiness,” says Ewen Mackintosh.“We see remains of bottles which have been consumed over a long period slowly picking up the effects of oxidation. The head space keeps getting bigger, and replenished with fresh air, and in 18 months to two years there is a dramatic change. Eventually, the whisky basically falls to pieces becoming unpalateable, and almost sour.”