Casks for aging Spirits

Some wine-making shops sell little oak casks. These really do work well. Leave your finished liqueur in these for a while, and they mellow out even more.

See for a selection of the small casks available in Australia.

If you leave the bung-hole of the cask (once filled) open for several days, it is said to improve the flavour. Donald disputes this ...
... that taking the bung out of a barrel might help the whiskey age with a quicker maturation. It will not! This will only cause the alcohol to evaporate will not add improvement to the whiskey. This does bring up an important point of evaporation and the "angels share" the part of the alcohol that evaporated though the wood and is therefore "lost to the angels".

The truth is, we don't need any drunken angels! More to the point, the process of evaporation is distinct from the process of oxidation. Although they are often confused and used interchangeably. The whiskey flavour can be achieved with out any evaporation at all, because the flavour component comes from oxidation process alone. The alcohol solubilizes the lignins of the oak which then oxidize into vanillins. Usuing oak chips with an air tight container (stainless steel or glass) with a 1/3 to 1/4 air space at the top, so 100% evaporation is recaptured with 100% oxidation. This also eliminates the risk of a sour barrel, just like a screw cap prevents rotten cork spoilage (3%-4% average).

To price out full sized barrels for yourself, see
  • World Cooperage,or
  • Roll out the Barrel Cooperage : 299 MacDonnell Rd Clontarf, QLD 4019, ph (07) 3284-9490 . Email :
    Prices (AU$) as at Sept02 (please check - they may have increased): 2Lt Round $159, 5Lt Round $184, 9Lt Round $229, 14Lt Round $279, 22.5Lt Round $359, S/H 220 French oak wine Barrels $165, S/H 320Lt American Oak Wine Barrels $176, Shaved & Scorched $40 , Freight $7 up to 15Kg
If you would like to make your own barrels, has a book "The Cooper and His Trade" for $18.95 US and it is book number 482.

See for more information about oak casks. Its written regarding using them for brewing Lambic beers, but there's heaps of info that's still pertinent.

Kristofer D offers the following advice :

I have decided the best way to age alcohol is in a charred barrel. Sometimes prechared barrels can be found, but they are hard to find. I have charred my own barrels that were bought at homebrew stores. I also add about a cup or two of charcoal that has been made from hard wood. A five gallon barrel works best. Any smaller is too small, and any larger takes to long to age. When ageing in a barrel about a quart of alcohol is lost every month. To make up for this you must add to the barrel once a month. I stop the process when half of the original amount has evaporated. In a five gallon barrel that takes about eight months. After this period the alcohol should of mellowed greatly and also become a golden brown color from the charcoal. I have found the best why to char a barrel is by putting a small layer of hot coals in the barrel, and then using an air compressor to blow oxygen into the barrel. By rolling the barrel slowly a complete charing is possible. It takes about an hour. You should feel the hoops on the barrel get hot. Usually hot enough to boil a drop of water. If time is taken to age alcohol in this manner I guarantee it will be the best ever. I have done this with apples, peaches, choke cheeres, and malted barley all grain. Each one came out superb, and the charicteristics of the fruit wasn't even lost.

Mike P writes :
... Then I bought an oak keg and tried aging the stuff, well after six months it just wasn't ready and I was getting err... impatient. So I surmised that the surface area of the keg was too low compared to the amount of contents to produce much of a surface contact for the liquor. I added lots of charred oak chips!! Bout 4 qts to my little 4 gallon keg! My mom was beginning to wonder when she came home one night and I had the oven mitts on and was peering into the oven at a cookie sheet full of smoking oak chips!! However the increased surface area did just the trick! The now golden liquor was a great tasting and visually appealing product.

An old codger from down the road told me if I added a few (6-10) black peppercorns to my aging process I would get a new appreciation! I did and it worked very well, there was no taste whatso ever from the peppercorns but you got that warm feeling inside right away after drinking it! It was a nice touch. I have used it often since then with very pleasing results, careful though and don't fall into the same trap I did, if 10 was good, then 20 must be better and a 100 would be better yet!!! Ohhhhhh... the heartburn!!! :-)

How do casks work to mature the spirit ? Its a matter of the wood letting in a little oxygen, and letting out different proportions of the alcohols and cogeners present, as well as reacting with some of the ligins & other substance of the wood. An email from Jim Busch, to the HBD, offers:
As for the issue of casks and diffusion of ethanol/fusels:

In whiskey maturation one of the key reactions involves diffusion of products from within the cask, through the cask wall. Basically its called a subtractive reaction in this case. The key parameters that pass through the cask are water and ethanol. The lower the molecular weight of the compound the easier it is to evaporate through the cask. This means that it is much easier to reduce the concentration of ethanol in a cask than it is to reduce fusel alcohols, as the latter are longer carbon chained alcohols. This route is also the main factor in DMS reduction in whiskeys. Hasuo and Yoshizawa, '86, found that the rate of evaporation of a model whiskey ranged from 32% of the total present in the spirit for acetylaldehyde to 5% for isoamyl alcohols to 1% for ethyl hexanoate and acetic acid. So the permeability per compound is quite different. The texts seem to indicate that in whiskey maturation there is a net concentration of fusel alcohols and esters of fatty acids. Similar results apply to cognac production where Ive seen a reference that indicates a typical loss of 3% per year in both water and ethanol and a concentration of fusel alcohols.

The chemistry of spirit maturation in casks is quite involved and interesting. The key reactions are extractions from the cask, chemical reactions and oxidative reactions inside the vessal (which continue as the flavanols, tannins, hemicelloses and lignins leach) and concentration reactions.

Arthur Bell says ..
Over the decades OAK wood has proven itself supreme for ageing potable liquors, became it is semi permeable and allows the diffusion of alcohol and water through the thickness of the barrel STAVE promoting minimal evaporation on the wood surface to the extent of 2% per annum in Scotland. This is the slow process of maturation so essential to product quality. By tradition Scotch whisky is not matured in a NEW cask as with Brandies (Cognac and Spanish) and Bourbon, which historically have benefited from the new oak extracts. The associated CARAMEL bitterness of the new oak due to excess TANNINS, LIGNIS, PHENOLS and RESINS are rarely present and are not appreciated by the connoisseur of aged malt whisky.

Within the Scotch whisky COOPERAGE two distinctive types of white oak are employed to fashion barrens from America or from Spain. The AMERICAN oak (Quercus ALBA) contains less than half of the total wood extract found in the EUROPEAN oak (Quercus ROBUR). The Bourbon Whiskey industry uses solely American oak suitably charred to promote the carmelisation of the wood sugars, so essential to bourbon character. This pre-treatment of the American barrels with fire and spirit has become acceptable to the Scotch distiller. The alternative and much sought after source of pre used cask is JEREZ Spain which offers both American and European oak casks having previously contained SHERRY wines.

It will be appreciated, as with all things in life, the expected enhancement of spirit quality within the cask does not always materialize and those SULLIED barrels are discarded, no matter how slight the imperfection. A similarity exist between CORKS in wine or whisky and wood for casks, that the SOURISH MUSTY - DANK flavours of bad wood spoils the barrel content.

Charlie Maclean, writing "The Language of Whisky Tasting" at the Scotch Malt Whisky Society offers ..
...we were delighted to be able to help Colin Brown, a Manufacturing Scientist undertaking an Investigation by Analytical Chemistry of Compounds Affecting Flavour in Single Malt Whiskies, for Napier University, Edinburgh, with the support of United Distillers.

He writes: "Single malt whiskies are chemically complex and are known to contain several hundreds of individual components (congeners) including a variety of alcohols, aldehydes, acids, esters and phenols, as well as carbonyl-, sulphur- and nitrogen-containing compounds. Many of these contribute to the flavour of a whisky and the relative concentrations of each are dependent upon variations in raw materials and production methods. Consequently, each single malt whisky is discernable from any other, including those produced at a neighbouring distillery."

The investigation, which is continuing, began by identifying the relative concentrations of compounds known to be extracted from the cask during maturation ('cask-extractive congeners') and a group of those which arise from the raw materials and during production ('volatile phenolic congeners'). It went on to make a correlation between these and the perceived flavour of the finished product.

Four samples of whisky were considered, affording a regional spread. They were: Caol Ila (Islay), Scapa (Orkney), Rosebank (Lowland) and Balvenie (Speyside). "Cask-extractive congeners are of great importance to the overall flavour as well as the aroma and colour of the finished beverage.

Whisky casks are traditionally treated to thermally degrade the internal surface of the cask, by firing and charring, for three major reasons:
  • to generate a layer of active carbon which removes undesirable flavour substances;
  • to effect the release and dissolution of flavour compounds such as vanillin during maturation;
  • to yeild colour and phenolic substances which result in new flavour compounds by oxidative interactive reactions.
The results revealed that all four samples contained similar levels of the compound furfural, which is associated with a general 'whisky flavour'. The unmistakably woody flavour in the Scapa sample is attributable to syringealdehyde, which is also present in Caol Ila, although the dominating flavours of the Islay whisky are discussed later. The Balvenie chromatogram reveals a high level of ellagic acid, a compound known for its pungent odour and flavour. The number and relatively large abundance of lignin degradation products in the Caol Ila sample would suggest that a heavily charred oak cask was used for the maturation process. The presence of 5-hydroxymethyl furfural in Caol Ila would allow the taster to expect a slightly reddish tint to the whisky.

"Volatile phenolic congeners' primary source is from the peated malt used in whisky production. So it was not surprising to discover that the Caol Ila sample had a large number present and in concentrations greatly in excess of the other whiskies under investigation. "The presence of phenol at 2.1 parts per million (ppm), was almost five times that for Rosebank, 20 times that for Balvenie and 200 times greater than that found in Scapa. Phenol has a medicinal taste and therefore would be expected to be present in an Islay whisky. Guaiacol, known for its smoky and phenolic flavour, was detected in concentrations of 1.2 ppm in Caol Ila, but was barely detectable in the others. Further investigation showed that the levels of compounds responsible for 'tarry' and 'disinfectant' flavours (m-, p- and o-cresol) were evident in the Islay sample but rela tively low in Rosebank, very low in Balvenie and not detectable in Scapa. However, similar concentrations of the compound eugenol (perceived as having a flavour not dissimilar to that of cloves) were determined in both Scapa and Caol Ila. All of the whiskies investigated contained varying concentrations of 4-ethyl phenol and 2,5-xylenol, which co-eluted and could not be determined separately. Both of these substances have a powerful 'whisky' flavour."

Mr Brown concludes: "Many other compounds were determined analytically during this investigation, however their individual impact on flavour is considered less important than their contribution to the overall flavour of each whisky. These analyses are intended to give an insight into the complex chemistry of single malt whiskies and the impact which each of the many hundreds of compounds has on the characteristics of the finished product."

Charlie Maclean

Another article, "Whisky Dectectives" at the Scotch Malt Whisky Society describes how different techniques for drying the wood affects how well the spirits mature.

Yue Hung has forwarded some information from an article "The Production and Aging Of Wine in Small Oak Cooperage" published in the May 1969 edition of "Wine & Vines¨ ...
Mechanism of Aging

The mechanisms and processes which occur during the aging of wine in the barrel can be grouped roughly into three closely inter-related categories. viz. physical, physico-chemical, and biochemical.

Physical. The most important physical aspect of cooperage in the aging of wine is its size, because the surface-to-volume ratio affects the rapidity with which certain processes take place and the degree to which they influence the character of the wine. Since all of the aging mechanisms are influenced to a different degree by variations in barrel size, it follows that there must be one single optimum size, one ideal surface-to- volume ratio which establishes the best balance of the wine and which most perfectly favors and develops the character of the wine during aging. We are, fortunately, the beneficiaries of centuries of experience in this area, and we can state with some certainty that, in the great wine regions of France, at least, the barrel size of 225 to 230 liters (about 60 U.S. gallons or 50 Imperial gallons) has proven to be the most perfect. We have little reson to double that this size, or a size reasonably close to it, will produce the same good results in other areas. Indeed many of the best wines of California and Australia (to give two examples ) have been produced and/or aged in this size cooperage. Similarly, the optimum barrel size for Cognacs and fine brandies has been established to be 300 liters (about U.S.gallons or 66 Imperial gallons) It should be emphasized that half-quarter-eighth- etc.. size barrel are made for the purpose of storing odd amounts of wine or for breaking down a full barrel when topping.(Not that wine cant be successfully aged in these small sizes, its simply that the results are not quite so satisfactory, and that much greater care must be taken, through more frequent organoleptic examination, to avoid an imbalance in the finished product)

The most obvious physical process which occurs during aging is the slow imbibition some of the liquid by the wood and the subsequent evaporation into the surrounding atmosphere .There are five prince influences in this process : relative humidity, temperature, air movements in the area where the barrels are kept, the physical characteristics of the wood, and finally, the frequency of topping. This last is important because, where the topping is less frequent there is more opportunity for an area of wood to become dry and thus slightly more porous due to shrinkage of the wood tissue The important of the first three items first emphasized by Pasteur,has been forcefully demonstrated by Riberean-gayon(3) whose experiments with 60 gallon oak barrels gave the following results: in good oak barrels which are cold and humid, the evaporation lose can be kept to 1 or 2 per cent; in ordinary wineryies it is 4 or 5 percent;and in poorly designed wineries which are too warm,too dry,and/ or too much expose to air currents, the loss can be as high as 9 per cent. The evaporation loss is also greatly influence by the type and thickness of the wood..

Ian Wisniewski (whisky and spirits writer, and author of Classic Malt Whisky (Prion Books)) wrote about Oak (big thanks to Whisky World for letting me copy these extracts !)
    While oak ageing is an historic tradition, wood management is a far more recent discipline. The benefits of oak ageing have long been appreciated, rather than understood, but it.s only since the 1970s /80s that technical knowledge has supplanted anecdotal theories. With the continual advance of analytical techniques revealing the extent of oak.s influence during maturation, the growing focus on malts, as well as longer maturation terms, wood management has become key. The archetypal .maturation mandate. is to reach a balance between the beneficial characteristics gained from the oak, while also retaining and developing the individual .distillery character. embodied in new make spirit. As oak ageing can account for up to 60 to 70% of a malt.s eventual flavour, or around 40% in the case of heavy peating, it.s a variable balancing act. Because different malts have varying ageing potential, longer maturation does not guarantee a finer malt. It.s a case of variations on a theme: yielding different expressions at different ages, as the oak influence intensifies. Moreover, the appeal of a malt may be a case of .less is more,. or .more is more.. The .right age. depends on your own palate.

    .Younger whiskies can have great enthusiasm, then after about 10 years you start to get a real mellowing out and balance; for me the balance is complete within 15 to 20 years,. says Jim McEwan, Bruichladdich.s malt maestro. .At 20 years the oak starts to really exert its influence, and at 25 years has a different, really good character, being mellower, rounder, slightly drier, and the heat drops into the chest, not the palate. Between 25 to 30 years the oak, and where the malt is matured, start to play a very important role. Thirty years and onwards there is a huge concentration of oak in the spirit..

    Maturation can be divided into three essential elements. Subtractive maturation, like a .rites of passage. for the ingenu new make spirit, entails the loss of immaturity. Additive maturation sees the oak endowing the spirit with colour, aromas and flavours, while interactive maturation refers to reactions between the spirit and the oak. This is something of a .mystical union. that is not fully understood, yielding an additional range of characteristics that neither the spirit or oak possess individually.

    It would be very convenient if subtractive, additive and interactive maturation followed each other in an ordered, chronological sequence. However, these separate elements effectively occur simultaneously throughout maturation, albeit at differing rates.

    Oak-derived flavours, for example, can be evident within the spirit from around six months, though losing immaturity may take a couple of years, or even extend over several. Similarly, filling a barrel with spirit can be said to initiate a form of interactive maturation. This reflects the fact that 2 to 3% of the new make spirit is .drunk in. by the staves, possibly even within 48 hours, and subsequently begins to mingle with the .wood extractive liquid. of the cask.s previous contents (although the full effect will, of course, take years to complete). In fact, specific flavours can be attributed to interactive maturation, due for example to the formation of esters.

    .While The Macallan.s citrus and floral notes originate from the new make spirit, the dried fruit notes derive from esters that have been created during interaction with the cask, and also from oxidisation,. says David Robertson, The Macallan.s Master Distiller. An initial consideration is the difference between using bourbon barrels, fashioned from American oak, and sherry casks, which are usually produced from European oak, though American oak casks can also be seen in action at sherry bodegas.

    European oak, typically harvested from 60 to 150-year-old trees, comprises a looser, more open and porous grain than American oak. This enables the spirit to penetrate the oak more readily than American oak, usually harvested from 40 to 100-year-old trees, which comprises a straighter, tighter grain. The level of tannins, promoting astringency, balance and structure, is also far higher in European oak than in American oak. However, charring bourbon barrels on the inside, while sherry casks are merely toasted, makes the interior of a bourbon barrel more accessible to the spirit.

    These technical differences also promote a varying range of characteristics. In terms of colour co-ordination, sherry casks lend an orangey, amber hue, which is distinct from the lighter, golden, straw tint of bourbon barrels. The flavour profile also varies significantly with sherry casks contributing rich fruit (such as raisins, prunes, dates, figs, apricots), fruitcake, fortified wine, almond and walnut notes, spices such as nutmeg, ginger and cloves, not to mention creme caramel, chocolate, and a (positive) sulphurous note, delivered within a rich sweetness.

    Bourbon barrels lend a lighter, drier sweetness, with a palate thriving on a medley of flavours: vanilla, honey, various fruits, almonds, hazelnuts, coconut, creme brulee, sherbet, spices such as cinnamon, as well as mint and eucalyptus notes.

    The influence of bourbon barrels can be further sub-divided according to the degree of charring. The scale ranges from one to four, beginning with a .burnt toast. effect, and culminating in an .alligator char,. the popular term for a number four, as the surface resembles alligator hide. Various malt distilleries have an inventory of bourbon barrels with a range of char levels, with a heavier char typically giving greater amounts of vanilla, creme caramel, toastiness and hint of smoke, not to mention more intense colour. Alternatively, a milder char promotes greater sweetness, honey and vanilla, while also endowing the spirit with more body.

    The .fill. (referring to the number of times the cask has been filled with spirit) is another important consideration, with many distillers using a .recipe. of different fills to help achieve consistency, or a particular flavour profile. Each fill of a cask results in its degree of influence on the spirit diminishing, until the Master Distiller deems it to be exhausted. If a first-fill cask is said to contribute 100% of its available characteristics, a second-fill will contribute around 60%, with a third-fill (when relevant) dropping to around 35%. Consequently, a second- and third-fill require a longer time-frame than a first-fill to complete subtractive maturation. For example, spirit maturing in a second- or third-fill could require about 10 and 15 years respectively, in order to reach the same degree of maturity as a spirit aged for seven to eight years in a first-fill cask.

    However, this does not mean that each successive fill also delivers the same flavour profile, in a progressively milder format. It.s actually a case of obtaining varying, rather than subtler flavours from different fills. Then again, it.s also a case of imparting an .appropriate. influence to the particular house style of the spirit. A lighter, unpeated malt, for example, may principally be aged in second-fill sherry casks, to ensure that the oak influence doesn.t predominate.

    .The first-fill of Spanish oak sherry casks gives a dark mahogany whisky, with rich, woody spices such as cloves and ginger, toffee sweetness and dried fruits,. says David Robertson. .When refilled there is a less robust oak influence and more new-make spirity character coming through, together with more apples, esters and floral notes, while the tannins manifest themselves as light spices (such as cinnamon) rather than oak..

    Similarly, a first-fill bourbon barrel offers a different influence to products further down the line. .A first-fill bourbon barrel has more natural sweetness and body, the second-fill allows more products of oxidation to shine through and gives more of a floral top note, though greater oxidation also reflects the damper ageing conditions of a traditional warehouse,. says Dr Bill Lumsden, Glenmorangie.s Head of Distilleries and Maturation.

    Spirit within a first-fill bourbon barrel also has optimum contact with the charred layer, though the char.s active influence does diminish during maturation, not to mention physically breaking down within the cask. (Consequently, when the spirit is emptied after the first-fill, some charcoal also leaves the barrel, reducing the level of char available during the second-fill, which is also less active anyway). This gives a lower level of vanilla and .burnt heather. notes which stem from the char, while simultaneously increasing the level of oak influence. Another principal difference between successive fills is the level of wood extractive liquid remaining within the staves of the cask. Not simply a case of residue bourbon or sherry, this liquid also incorporates additional wood-derived compounds, which could total around 75 cl in a 200-litre bourbon barrel, or 10 litres in a 500-litre sherry butt.

    The largest amount of wood extractive liquid is present within a first-fill, and released during the first few years, with some wood extractive liquid remaining in the second, but only a minimal amount is released from a third-fill. Traditionally this residue was considered an important element, though the nature of the oak, European or American, is now established as the primary influence during maturation, and the wood extractive liquid a supplementary influence.     This page last modified Tue, 20 Jan 2015 20:51:05 -0800