In Defence of Artificial Aging pt.2 - It's Not the Size of Your Stave, It's What You Do With It.

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Previously on Bottlestore Galactica we looked at the different types of wood from around the world that go into making barrels, and the complex ways their character makeup can effect your finished spirit. So you've chosen your wood type, now you're ready to make your barrel; but how big should you make your barrel, and how does that change taste? What's the difference between toasting and charring? What about temperature? Humidity? Atmosphere? And what about that very bugbear we are trying to banish, time?


In Defence of Artificial Aging, pt.2
It's Not the Size of Your Stave, It's What You Do With It.

The Grain

One factor that was discussed in the previous entry was that of wood with a loose vs tight grain, with the postulation that barrels made up of tightly-grained wood were better suited for wines, whereas looser grained wood was better served aging spirits. But how do you know if your barrel is tight or loose grained?

Well on the off chance you are not a fourth generation French cooper, an easy method to ascertain a barrel's grain is simply by looking at the stave at the end of your barrel and counting.

A French cooperage places the following designation on grain grading:

nb: GPI= Grains per inch

>17 GPI: Extra Fine Grain
12-16 GPI: Fine Grain
8-11 GPI: Medium Grain
8Coarse Grain


Now grain size depends on how fast a tree grows during summer - so trees growing in warmer climates will have a broader, looser grain - see Quercus Alba, the American White oak. 

Barrel Size

The size of the barrel a spirit is aged in has a huge affect on how much of the barrel's character is absorbed into the wood; that is, how many of the wood solubles are dissolved into the spirit and at what concentration. Smaller barrels have a higher ratio of oak in contact with the contents of the barrel and therefore will release a higher concentration of flavors and colors into the the total content of the spirit than would occur in a larger-size barrel.

In this way a smaller barrel can give the desired effects of long-term maturation faster than a barrel with greater volume, assuming the spirit is maintained in identical conditions in both cases. A smaller barrel can therefore 'age' a spirit faster than a large barrel: keep this fact in mind for later.

Another way that a producer can speed up the 'aging' process is to increase the surface area of oak exposed to the wood without increasing the overall volume. One such way to do this is to use corrugated staves, a practice that is used at Maison Vicard in Cognac.

In order to calculate the interior surface area of a barrel you can follow this formula below: 

SA = (2 x Pi x r x (A-(L+I)) + (2 x Pi x (r x r)) 

Where SA: Surface area,

             Pi: 3.14

              r: Radius of the barrel top

              A: Actual Length

              L: Lip of Barrel (Stave Width)
              I: Interior Lip 

Regularly to calculate the surface area of a cylinder you would use the following:

SA = (2 x PI x r x H) + (2 x PI x (r x r))

But this doesn't take into account the irregular shape of a barrel, nor does it take into account the thickness of the staves, and it would only give you the outside surface area of the barrel.

When measuring your barrel, be sure to measure the Actual Length (A) and not the Height (H) of the barrel; the barrel's irregular shape means that the height measurement will give you an innacurate measurement, whereas Actual Length will account for the shifting exterior circumference of the barrel as an average (assuming the stave width, L, is a uniform constant).

Once you have your interior surface area in square units (cm or inches, we don't discriminate), you can do a quick calculation of your liquid volume divided by the surface area to see how large a percentage of your liquid comes into contact with the oak.

Percentage of liquid in contact with oak = ((volume/interior surface area)/volume) x 100

LC(%)=((V/SA)/V) x 100

Once you have this figure you can start to get an idea of how increased contact with oak can speed up the maturation process, and affect the concentration of wood flavors. I haven't done it myself yet but it would be and interesting experiment to take a series of different sized barrels and age identical lots of spirit under controlled conditions. This way you will be able to demonstrate how much of an effect spirit-to-wood contact has on the final spirit.

Below is a table of the most common barrel styles used in the global liquor industry, and their characteristics:

Barrel Name	Capacity	Wood Type (Traditional)	Characteristics & Notes	Used For (Traditional)
Gorda	700 Litres	American Oak	Large size and low wood-to-oak ratio means this size is seldom used for maturing whiskey, intended instead for marrying blended whiskeys.	Blended/Vatted Malts
Maderia Drum	650 Litres	French Oak	Short, fat barrel. Very Thick staves.	Madeira Finishing: whiskey
Port Pipe	650 Litres	European Oak	Tall, thin barrel.	Port Finishing: whiskey
Butt	500 Litres	European Oak	Popular in the Spanish sherry industry – tall, thin design.	Sherry Finishing: whiskey
Puncheon	500 litres	Spanish Oak	Thick staves, short fat barrel design.	Rum, Sherry Finishing: whiskey
Barrique	300 Litres	European Oak	The traditional use of barrique casks by the wine industry has led to the spirits industry using this barrel type to give a wine finish.	Wine Finishing: whiskey
Hogshead	225 Litres	Traditional: European Oak Contemporary: American White Oak	Barrel size derives from 15th century brewers-unit. While traditionally made of European oak, the shift in demand and oak prices sees most contemporary hogshead made from American oak.	Scotch, Irish Whiskey
American Standard Barrel (ASB)	200 Litres	American White Oak	Smaller barrel size and wide-grain Quercus Alba meant faster spirit maturation vs. traditional hogshead	Bourbon, Irish & Scotch whiskey (ex-bourbon)
Quarter Cask	50 Litres	No standard	Also known as a firkin.	Small batch whiskeys, dodging the tax man.
Blood Tub	40 Litres	No standard	Long, oval shape designed to be carried on horseback. Awesome name.	Brewing beer, small batch whiskeys.

Information from whiskeyforeveryone

In the last few years we've also seen the rise of the personal barrel, popular in bars and with home distillers. For the most part they are American oak (although certain French vodkas are starting to hand out 3 liter barrels of Quercus Petraea) and come in variations of 1, 2, 3 and 5 liters and have no specific title. 


Charring and Toasting

Once the wood is chosen and the barrel has been constructed, coopers can further adjust flavor profiles that can be extracted from the wood by charring and toasting the barrels, using a range of methods from flash-toasted to naked flame to 'cook' the wood.

We've already discussed how the American coopers industry would dry and cure their oak by kiln as opposed to the traditional French air-drying industry. The process by which the French shape their actual barrels is by bending their staves by heating them with the use of a wood-fired brazier, before heating to either light (5-10min), medium (10-15min), or heavy (15-20min) toasting. American barrels are shaped by using a steamer to soften the oak staves, then using a gas torch to char the interior of the barrels, grading as light (15s under torch), medium (30s), or heavy char (45s), with even heavier char (1min<) available to craft distillers.

There is no definitive proof of who begun charring oak barrels and when they did, although plenty of bourbon producers have claimed that they begun the trend that now defines the category.

In fact the earliest record of charring barrels comes from a book stored in the United Distillers archive in Louisville, Kentucky. Unfortunately, there is no cover to the book or author listed, just an uncredited, handwritten note that references the year 1854. It appears to be in the common style of farmer's almanacs of the times, which would provide a wide variety of information on everything from when to plant crops to how to cure meat, usually including information on home-made tinctures and tonics.

It includes this passage:

“Q: Why are water and wine casks charred on the inside? 
A: Because charring the inside of a cask reduces it to a kind of charcoal; and charcoal (by absorbing animal and vegetable impurities) keeps the liquor [liquid] sweet and good.”

So water and wine casks were already undergoing charring at the time this was written, and was common enough that his information was thought vital enough to be included in what seems to be a general almanac. No proof exists, then, that any one producer was exclusively using charred oak prior to it becoming the norm and then a legal requirement, but as to why they were charring oak and how and where it originated these facts are lost to the mist of time.

So while there is no definitive answer to the question of who started charring barrels, the processes of heat treatment via charring and toasting have different effects and produce different flavors. For example toasting helps converts complex insoluble sugars (hemicellulose) in simpler sugars, which is turn break down into caramelization products like fufural and maltol amongst a host of others, which will in term develop into the structures that will give color, as well as notes of cooked and burnt sugar and of course 'toasty notes' which primarily come from the fufural. 

Different temperatures of toasting will break down the hemicellulose to different levels, which will in turn produce different simple solubles and therefore different flavor profiles as a result of maturation from the finished barrel. 

Graph from World Cooperage

Charring on the other hand not only creates a whole different range of flavor compounds (vanillin, eugenol, syringaldehyde), but also has creates a larger contact surface area as the wood cracks and converts to charcoal. A larger surface area means a much faster interplay between wood and the now developed sugars and solubles, providing an easy passage for spirit to enter the wood and absorb the sugars and solubles that create the flavor of a matured spirit. 

The charcoal also acts as a filter, absorbing sulfurs and esters from the spirit, smoothing out your spirit from the rough white spirit to the aged final product.


Relative Atmosphere: Temperature & Humidity

Once the barrel is treated is filled with spirit to be aged and stored in a warehouse for the length of it's maturation. The vast majority of these storage houses will not be climate controlled; indeed it is the natural climate and relative atmosphere that characterizes regional liquors, imbuing them with the spirit of their land of origin.

For example, take two spirits, a Scotch whiskey and a Caribbean rum, both aged at seven years, both matured in an ex-bourbon barrel of comparable size. Both of these spirits will go into the barrel from the distillery crystal clear, and only develop their color from time spent in oak. In the vast majority of cases the rum will be much darker than the Scotch. 

This is because of the two factors of relative atmosphere as it affects maturation of spirits: temperature and humidity. 

For quick reference, the average annual temperature in Havana, Cuba varies from 16c(60.8f) to 32c(89.6f), and humidity from 50% to 97%. Islay Airport, conversely, records an average annual temperature range of 2c(35.6f)to 17c(62.6f) and a humidity range of 59% to 99%.

So while they have a similar humidity, Scotland is far colder annually than Cuba (look out for our future blog post, Grass is Green and the Sky is Blue). 

Obvious conclusions aside, what does the temperature have to do with the maturation process? Higher temperatures affect the oxidization reactions of the wood solubles, increasing the levels of fufural, tannins and aromatic compounds. 

However, high temperatures also accelerate the oxidization of ethanol, which will convert into ethyl acetate, the strong, sweet nail-polish remover note that is often present in rums produced in high-temperature regions, which can be either stripped via charcoal filtering, extended aging, or masked with added sugars.

Low temperatures avoid this issue of rapid oxidization, but as a result the aging process takes a lot longer to achieve comparable results. While it may be advantageous to age spirits at high temperature to cut down on aging time and increase turn around between distillery and market, you risk adding harsh esters and acetones to your spirit which then have to be masked with additives.

At high humidity ethanol evaporates faster in your barrel, whereas at a low humidity it is the water that evaporates at a quicker rate. 

Beyond just altering the final proof of your products, the portion that evaporates will change the character of what remains in the barrel. Both water and ethanol are solvents but for different compounds; water contains the caramel coloring and the wood sugars, whereas ethanol contains the lignins, vanillin and tannins.
 
So at an extreme high or low of humidity would it be possible to evaporate the entirety of the ethanol or water content of the spirit? Well no, as we covered earlier when we were looking at freeze distillation thermodynamic law dictates that there shall never be an absolute separation of ethanol and water in a solution. As you would never be able to truly freeze 100% ethanol off of water, also it is impossible to separate the two components via evaporation.

Therefore, high-humidity maturation will result in a sweet, dark spirit, but without the nuance of character that comes from the complex soluble compounds in the oak. It will take a lot longer to draw these flavors from the oak, but if aging in a high-temperature region, rapid oxidization will starts to produce unpleasant esters and acids that can ruin a barrel of spirits. 

This is not to say all esters created during maturation. are unpleasant and unpalatable, as some aromatic esters are highly prized in spirit production.


In Kentucky there is the greatest fluctuation in both temperature and humidity; from -4c(24.8f) to 31c (87.8f) and 41% to 94% annually. This, coupled with a high diurnal temperature variation (the difference between daytime and nighttime temperatures) means that bourbons aged in non-climate controlled warehouses have a huge amount of temperature fluctuation inflicted upon them. 

This means both the oak and the spirit inside will expand and contract on a daily basis, the oak becoming more porous as the day heats up while the spirit pushes itself deeper into the wood, dissolving solubles in the deep grain of the charred barrel. Capillary motion, similar to how a pair of lungs works, pulls the liquid into the staves, diffusing the whiskey throughout the cell structure of the wood.  As night falls and temperature drops, the oak will contract, forcing the absorbed spirit back into the mix, bringing with it extracted flavors from deep in the porous, loose grained American oak. The elastic expansion and contraction drives the interplay between spirit and wood, the humid afternoons drawing out the angel's share as it dissipates into the air, further driving rich, toasty notes into the bourbon.

I've just realized that description sounds vaguely erotic. 

Nonetheless, the temperature & humidity in Kentucky creates an ideal atmosphere for quick maturation while maintaining an environment that discourages oxidization and the development of unpleasant flavors.

Another potential atmospheric factor is the mineral content of the air - more specifically, the salt content of atmosphere in locations where spirit is aged adjacent to the ocean, particularly whiskeys from the Islay region of Scotland. 

'Salty' is a tasting note popular with tasters and distillers alike for describing their product, but there is little information on the science behind the evident salty taste. Indeed 'salt' as a descriptive is a relatively recent development; "Salty" as a descriptive wasn't used until 1978.

The commonly accepted theory is that the briny sea air of Islay leaches sodium chloride and other saline metals into the wood, which is then absorbed into the final spirit, giving the final product a salty note, with most distilleries going so far as to suggest that, without confirming it as actual fact. That is probably for the best, as some Islay distilleries age their whiskeys on the sea front with the salt waves washing against their warehouses, where some age their whiskey in a warehouse far inland where you can maybe see the ocean if you stand on the roof on a clear day and squint real hard, yet they both claim it is the relative atmosphere that cause salty notes in their whiskey

Others have opined that it is the water used by distillers that imparts these briny notes, that is mostly disregarded as distillation removes the vast majority of sodium solids from the spirit, as the brine waste is denser than water or ethanol and can be separated in a still with ease. Others claim it is the peat used to smoke the malt, claiming you can taste the difference between peat harvested on Islay and on the mainland (most famously between Bruichladdich's Octamore 1 & 2), but this is all subjective evidence. 

As to whether or not there is any scientific evidence behind the perceived 'salty'-ness of Islay whiskeys due to the relative atmosphere, it is not publicly available as far as I can find. I'm sure Suntory, Beam Global or Diageo has researched this but they are keeping their findings close to their chest.

As always, if you know anything to the contrary on this, please feel free to send me a message and I will be happy to publish a correction. But that still begs the question, what changed since 1978 to make everyone start recognizing salty notes in Islay whiskey? Was there a conscious choice amongst whiskey makers to change something to introduce this flavor? Or was is simply a trend followed by whiskey enthusiasts & profilers, disseminated by Michael Jackson (not that one) and accepted as fact as the popularity of Islay whiskey has increased over the last few decade?

Without proof I'm still inclined to tend towards the latter, and while there have been various studies published on phenol content, gas chromatography and esters present in Islay whiskey, there is nothing so far that has covered saline or similar heavy metal content in whiskeys that are described as having such a flavor.

Alcohol Proof, Thieving Angels and Time

We've already covered how different fields of humidity cause a dominant portion of either water or ethanol to evaporate in a cask, which strip certain flavors and introduce others. This loss of volume can either raise or lower the final proof of an aged spirit, but it is a hard constant to graph, as the many variables of changing atmosphere, constantly changing ratios of ethanol to water and other solids, trade between oak and spirit, potential of oak density change due to temperature, and lack of universal oak grain as a constant, all of which is taking place in a barrel over the course of years, make for a incredibly complicated set of variable to try and track.

What we know for sure is that a higher alcohol percentage will have a faster extraction rate, it will also draw a higher amount of tannins from the wood, which can cause bitterness and astringency in high levels. Higher abv also means in most cases more water will have to be added to dilute the final product for general consumption (believe it or not, not everyone loves Smith & Cross and overproof whiskey), which dilutes the effects and flavor of the barrel aging in the first place, defeating the purpose of trying to speed up aging with a higher abv product. 

Therefore, most distillers accept that 55-65% alcohol is optimum for maturation, dissolving a higher ratio of wood sugars and caramel coloring from the oak then the tannins it will pull. Furthermore, studies have confirmed that this ABV is the best for maintaining the porous nature of the oak, facilitating a better passage for water to evaporate, meaning in most cases the final spirit will have a higher ABV then the spirit that went into the barrel pre-maturation, with a higher concentration of lignins, vanillins and oak lactones, all good things for creating a complex and nuanced spirit.

It also is worth noting that that the combination of high annual temperature and humidity is a reason many rum producers in tropical climates prefer ex-bourbon 'refill' barrels to mature their spirits in. As a general rule of thumb each time a barrel is refilled it will take twice as long as the previous aging period to extract flavor compounds from the wood, as the spirit has to diffuse deeper into the cells of the oak. New make barrels with high level of lignin & tannins will extract too heavily in high-proof spirits, so the abundance of ex-bourbon barrels allows for a more controlled aging in tropical climates, so the spirit can pull color and sugar from the wood without becoming too bitter and astringent.

The loss of volume by evaporation is called the angel's share, born of the old belief that angels were taking a certain percentage of distilled spirits as some sort of holy tax. As adorable as this is as a story, and as many bars and cocktail menus around the world have profited from this superstition, the fact is that for most cases, you can expect around 7-10% loss from the volume of a standard oak barrel annually. This is even higher in tropical climates, around 10-15%.

From the position of a major supplier, this mean you are potentially 'losing' hundreds of liters of delicious whiskey to freeloading angels every year, which can equate to hundreds of thousands of dollars. This realizations have led to some attempts at arresting this loss, ranging from the slightly ingenious (the case of a certain rum house installing a cooled corrugated iron roof above its barrels to collect evaporation and funnel it back into other barrels) to the bizarre (Diageo's experiments in cling-filming their barrels to try and decrease evaporation).

While this may amuse some and raise hackles in others, it does call to light the pedestal upon which we have placed barrel aging, and the common misnomer that older is automatically better. While better aging can indicate a spirit with more complex character, often the price of a bottle or a nip is directly related to the amount of time a spirit has spent in a barrel.

To clarify I'm not saying all super-aged whiskeys are inherently bad, I've had some stunning 27, 30, 35 year old whiskeys. But I've also tasted bottle with a retail price north of $500 that taste like chewing wood with a chaser of old sugar syrup and sherry.

Excessive aging can cause high levels of astringent and drying tannins, unpleasant esters and the development of cask spoilage in the form of brettanomyces, a yeast strain that can develop during oxidation (not necessarily a bad thing, if you like the funky horse-must and saddlery aroma it creates). These factors are often compounded by the fact that after a whiskey spends a certain amount of time in wood, it stops tasting like whiskey and starts to just taste like smooth, woody chocolate. 



So if we can decrease maturation times through the use of barrel sizes, wood surface area to spirit ratio, char and toast, temperature and atmosphere, surely we can recreate a facsimile of an aged spirit by exploiting these factors? 

What other woods can we use to age spirits, and what flavors and characteristics can we extract from them? How are other, global brands speeding up the maturation process to get an aged product into the hands of consumers who increasingly believe that age=quality? How can you achieve their results in your home or bar? And is artificial aging unethical, or a necessary evil? Or if you can match the quality of an aged product in a fraction of the time, is it evil at all?

Find out next time, as usual, right here on Bottlestore Galactica