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Barrel Maturation


Aging a spirit in a barrel is one of the most transformative, and romanticized, methods in all of spirits production.   The basic concept is straightforward: after distillation, a spirit is placed in a barrel, typically made of oak, which over time imparts the spirit with a brownish amber color and "oak-y" flavors like vanilla and toffee.  The longer a spirit ages, which can be anywhere from few a months to over half a century, the more pronounced these effects will be.


But contrary to popular belief, barrel aging is not just about time (and longer isn’t necessarily better).  Several other elements have an impact including the barrel's size, shape, and condition, the species of oak it is made from, and the climate that surrounds it.  For this reason, I prefer the term "barrel maturation" for this process.  Aging is strictly linear, whereas, for maturity, age is only one of many factors. 


Barrel maturation is employed across virtually every spirit category from whiskey to tequila to gin (sometimes).  On this page, we'll look at the big picture, and dive down the rabbit hole of what happens when a spirit goes into a barrel, as well as how, and why.  

Spirits Will Mature Even without Barrels

Before we get into barrels, it's important to note that spirits will change and mature over time even if there's no wood involved.  This is particularly true right after distillation when the slew of organic compounds within a spirit (which you can read more about here) are especially active.   For this reason, many clear unaged spirits are rested in neutral containers, which are typically made from stainless steel, before being bottling.  This allows harsher flavors to soften and disparate ones to harmonize.  It doesn't add color or new flavors, but for many spirits, it is a very important step.   Sometimes they'll rest for just a few weeks, but it can be much longer.  For example, a component of Yagurara Cachaça's Blue blend rests in a neutral container for 10 months before blending. 


What Happens When a Spirit Goes into a Barrel

When a spirit enters a barrel several things happen at once which facilitates a cornucopia of chemical reactions between the spirit, wood, and oxygen that seeps in through the wood’s pores These are the primary elements at play:

  • Flavor Extraction -  This is the most obvious impact of barrel maturation and it is a big one indeed.  When a spirit comes in contact with wood, flavor compounds such as vanillin (vanilla), oak lactones (peach, coconut), guaiacyl (smoke, barbeque) and eugenol (clove-like) seep out of the wood and dissolve into the booze, imparting the “oaky” flavor we associate with barrel-aged spirits.

Wood also imparts tannins, which are compounds that add astringency and a touch of bitterness, both of which is important to a well-rounded aged spirit. Tannins are also found in grape skins and are a key component in wine.  


While flavor extraction will continue throughout a spirit's maturation time, it is frontloaded.  A large portion of extraction occurs within the first year of aging. 

  •  Flavor Creation - A spirit doesn't just obtain flavors from the barrel, they are created inside it as well.


  • EsterificationWhen compounds extracted from the wood and compounds in the spirit react with one another, it results in the formation of new compounds and thus, new flavors.  The primary example of this is the creation of esters, known as esterification, which are derived through the joining of an alcohol and an acid, both a which are numerous in a barrel full of booze.  As discussed on the fermentation page, these are responsible for a variety of fruity and floral aromas. 


  • OxidationSince wood is porous, air can freely make it inside the barrel, and when oxygen interacts with the spirit and wood compounds, it causes a flurry of redox reactions (aka oxidation) that further influences the creation of flavors. For example, the oxidation of an alcohol molecule will often yield carboxylic acid, which is the most common acid involved in the aformentioned esterification.

  • Evaporation and Flavor Concentration - In addition to oxygen getting in, things can evaporate out of a barrel too, be it alcohol, water or some other compound.  This means the amount of liquid in a barrel will decrease over time, usually between 2-5% per year.   This lost product is famously known as the "angel’s share" and it is one of the primary reasons older booze is more expensive.  Because the longer it sits in a barrel, the less of it there will be.

Which parts of a spirit are lost to the angel's share depends largely on humidity.  In drier climates, more water evaporates than alcohol so the spirit's proof goes up, and the inverse is true in more humid climates.  There's more on this below, in the environmental factors section.  Outside of water and ethanol (alcohol), lighter compounds are also susceptible to evaporation, so the amount of heads (see the distillation page) in a spirit, such as acetaldehyde, are usually be reduced in a barrel.   


The loss of these various components means whatever flavor compounds are left behind will be more concentrated and thus, more potent.   This impact will intensify the longer a spirit ages for.

The Impact of Different Barrels

When selecting a barrel for maturation, there's a lot to consider.  The species of oak, the size of the barrel, and whether it's new or used are just a few of the factors that will shape a spirit's flavor.  Below is an in-depth look at barrels and their many variances.  


Why Oak?

Virtually all barrels used for aging spirits, as well as wine, are made from oak.  Other woods have been experimented with over time, but oak was widely adopted because it possessed several benefits.  The primary advantage, initially, was that it made casks that were airtight.  But in a kind of serendipitous perk, producers soon realized that oak also imparted a pleasing range of complex flavors that were balanced and not overwhelming. 

Different Spiecies of Oak

There are hundreds of species that fall under the oak genus, Quercus. But only a few are used to make barrels for spirits maturation.  The most common species are American white oak (quercus alba) and French oak (quercus robus).  This doesn't just mean oak from the United States or France, they can grow anywhere, though they are generally native to the parts of the world they are named after.   All oak barrels will offer broad flavors of vanilla, fruit, and spice, though there are deeper complexities that distinguish one species from another.  


  • American white oak (Quercus alba) - American white oak tends to be sweeter with more flavors of coconut, dill, peach, and toffee. Naturally, it makes up the majority of barrels made in America, most of which are used to age American whiskey, notably bourbon. By law, bourbon can only be aged in brand new barrels which is why much of the rest of the world ages their spirits in ex-bourbon barrels (more on that below).


  • French oak (Quercus robur) - French oak lends more nuanced flavors of dried fruit, spice, and tannins to spirits.  It tends to have a tighter grain which means it is more porous and allows more oxygen to pass through, thus creating more oxidation.  The tighter grain also means the French oak staves must to be split, rather than sawn like the wider grained American oak.  This added labor makes French oak barrels more expensive.  French oak is best known for aging Cognac.


  • Sessile oak (Quercus petraea) -  This is another common type of European oak.   It is used more for wine and less for aging spirits, though it is notably utilized for some longer aged Cognacs.  Sessile has an even tighter grain than French oak so it is even more porous and gives off relatively subtler flavors and tannins. 


  • Japanese Mizunara oak (Quercus crispula) - This variety of Mongolian oak has seen increased use in aging Japanese whiskeys and is beginning to expand further.  Mizunara has high levels of vanilla, honey, and floral spice flavors. The wood is knotty, and more difficult to make into barrels, which makes it more expensive. 

Barrel Size

Barrels vary in shape and size.  In larger barrels, extraction takes longer because there’s less wood in direct contact with the spirit.  Smaller barrels have a higher wood to spirit ratio, so extraction will happen more quickly.  But this doesn't mean a small barrel will produce the same result as a large barrel in less time.  Other reactions like esterification and oxidation occur at the same rate no matter how the barrel size.  So a spirit aged in a small barrel may resemble an older one in some respects, but it won’t be fully mature. 


Some Common Barrel Sizes:

  • Standard American Barrel, 53 gallons - These are the most common barrel size.  They are predominantly made with white oak and many consider them to be the ideal barrel size for whiskey.  


  • Hogshead, 63 gallons - These are 5 American barrels, dissembled and reassembled to be 4 slightly larger barrels.


  • Sherry Butt, 132 gallons - These are taller and more slender and, as you might imagine, used for aging sherry. Ex-sherry casks are popular in the whiskey industry, particularly those made in the British Isles (Scotch and Irish whiskey).  Often they are produced exclusively for whiskey and seasoned with sherry for 3 years before being shipped to the producer.


  • Quarter Cask, 13 gallons - These are a quarter of the size of an American barrel.  They are often used for "finishing" a spirit for a short period at the end of the aging process.

The Composition of Oak

Oak is largely composed of 3 basic substances: cellulose, hemicellulose, and lignin.  The rest of it is a variety of organic compounds and tannins.  Cellulose and hemicellulose are the actual wood cells and account for 65%-80% of the wood’s total structure.  They are composed of long chains of simple sugars.  Cellulose is harder and is mostly insoluble so it does not supply much flavor, if any at all.  Hemicellulose will partially break down into simpler sugars when heat is applied, which can add sweetness and body to a spirit. 


Lignin has the most flavor potential.  It is the binding agent of the wood cells and contains several desired flavor compounds that impart aromas like clove, vanilla, toffee, smoke, and roasted coffee.   When treated with heat, lignin breaks down making those yummy smelling compounds more available for extraction. Prolonged aging will produce further breakdown of lignin, resulting in more complex and extractable deliciousness.

How Barrels are Made

Barrels are crafted from the inner heartwood of the tree, which is the dry center that supports the green outer layers.  It is cut into staves which are then dried before being formed into barrels.  How they are dried - naturally, indoors, outdoors, in a kiln - and for how long, will have different impacts on the character of the wood and the flavors that it will impart.  Generally speaking, longer natural drying periods (as in, no kilns) is said to produce better and more complex flavors.  Also, drying outdoors, which subjects the wood to the elements, is believed to remove some tannins that can cause unwanted astringency.  That being said, many of your (and my) favorite spirits are aged in kiln-dried barrels indoors.  To see a barrel being formed check out this nifty video from Jack Daniel’s.


Toasting vs. Charring

Once barrel assembly is complete, the inside will be subjected to heat - as you’ll see in the video link above.  This opens up the wood’s pores, allowing for greater contact with the spirit, and breaks down the hemicellulose and lignin into sugars and other desirable flavor compounds for easier extraction.  How a barrel is heated, toasted or charred, will greatly influence the types of flavors it will give off.   Toasting is more delicate.  It is done at a lower temperature and takes 5-6 minutes, resulting in more delicate flavors that are less sweet overall. Charring basically means to burn a barrel with an open flame for about 30 seconds, creating a layer of black char.   This further caramelizes the sugars which gives spirits more intense flavor, sweetness, and color. Char also works as a filter to remove unwanted compounds like sulfur, and other heavier tails elements.  Traditionally, French oak barrels are toasted whilst American whiskey barrels are charred.

New vs. Used Barrels

Whether a barrel is new or used (meaning it previously aged a spirit), is one of the most telling indicators of the flavors it will contribute.  New barrels give off stronger flavors of toffee and vanilla, while used barrels have already had those flavors tapped out, so their effect is softer and more subdued. It's like reusing a tea bag, each time a barrel is used, or “filled”, it’s potency will decrease.  A barrel can be generally be used 4 or 5 times before it no longer has any flavor to give.

New barrels versus used barrels isn’t a question of better or worse.  It’s about the profile you’re after. A used barrel may impart less intense flavors, but that will allow other flavors to come through that wouldn't have otherwise.   As I mentioned above, American whiskey is routinely aged new oak barrels, particularly bourbon, which gives bourbon much of its trademark flavor. Because a barrel is only new once, the bourbon industry churns out most of the world's used barrels, which are relatively inexpensive.   Scotch, rum, and tequila all typically are aged in ex-bourbon casks.  Other types of used barrels the see some action are ex-rum, sherry, Port, Madeira and other types of wine casks.  Cognac typically uses a combination of new and used barrels, they rarely use ex-bourbon casks. 


Used barrels may also be “rejuvenated” by scraping off their inner layer then being re-charred or toasted.  This makes more extractives available, though a rejuvenated barrel will still not be quite as potent as a brand new one.  Still, it is an effective method of prolonging the a barrel’s lifetime.

bourbon barrels

Aging Factors Outside the Barrel

These external factors will also influence how the maturation process will play out. 


The surrounding climate is one of the most overlooked elements of barrel maturation, and it has a multi-pronged impact.



When alcohol warms up, it expands.  So in hotter climates, if a barrel is filled to the brim, the spirit will be pushed into the pores of the wood, which extracts more flavor and creates more breakdown of lignin, thus creating even more extractable compounds.   


In climates that have large seasonal fluctuations in temperature, a spirit will be in a cycle of being pushed into the barrel in the summer and then trickling out in the winter.  The bigger the swing in temperature, the more dramatic this cycle will be. 


This is important to remember when considering a spirit's age relative to where it was made.   For example, in the temperate climate in Scotland single malt scotch is often aged for a bare minimum of 8-10 years before being considered ready for bottling, whereas in the much hotter Caribbean, aged rum can be ready in just a few years.  As the saying goes, three years in Scotland is one in the Caribbean.



The level of humidity will determine whether more water or alcohol will evaporate out of a barrel as part of the angel's share, and thus whether the proof of the spirit goes up or down as it ages.  In more humid climates, more alcohol evaporates than water so the proof of the spirit goes down.  This is the case with scotch, Japanese whiskey, cognac, and many rums.  In drier climates, more water evaporates and the proof goes up.  American whiskey made in Kentucky is a primary example.  So again:


  • High Humidity - Proof goes down

  • Low Humidity (dry) - Poof goes up





Warehouse Positioning

Amazingly, the type of warehouse a barrel is stored in and how will have an effect on the resulting spirit. Barrels can be stacked high, lined in rows, kept in the cellar, and a whole host of other options that will change the result.  Even where a spirit is positioned in the warehouse makes a difference. For example, in American whiskey rickhouses where the barrels are often stacked very high, the barrels on top will age differently than the ones on the bottom because it is warmer towards the ceiling.


Managing the environment within an aging facility is a key area of concern for a spirits producer.  Temperature, ventilation, insulation, air circulation, barrel rotation, and other factors are all taken into account and manipulated as needed.   



Age is probably the biggest value consumers place on spirits, and it's easy to understand why.  Time is a tangible, linear, and easy to measure commodity.  But it's not that simple.  Older spirits are not always better and it depends a lot on the spirit category you're dealing with.  For example, a 5-year-old scotch is very young, a 5-year bourbon is fairly average, a 5-year rum is getting kinda old (especially one from the Caribbean), and a 5-year Tequila is probably too old.  It's all relative.


However, for spirits that are traditionally aged for longer periods, like whiskey and brandy, age is indeed a fair indicator of quality, especially on the front end.  A 4 or 5-year-old bourbon will always be better than a 1 or 2 year old one. But once you get past a decade age becomes less reliable.  Not every spirit is better at 15 years than it was at 10.  At a certain point, spirits become over-aged and are dominated wood with muted flavors that lack nuance.  


Regardless of all this, older spirits will always cost more because the producer has invested more time in them (and people will pay more for them).  Just remember that while price correlates with age, age doesn't necessarily correlate with quality. 


Extreme Aging

That all being said, many spirits that are aged for 15-20 years or more are among the best you will ever taste.  This or a variety of reasons.  The superior qualities of ultra-aged spirits are largely due to prolonged oxidation and take a long time to develop (contrary to popular assumption, flavor extraction, while key, is front-loaded. After the first 12 months it will begin to taper off).  For example, the heralded rancio aroma in Cognac, which occurs when fatty acids oxidize into ketones, doesn't begin to appear for at least 10 years.  


Another reason ultra-aged spirits are so special is that they stood out from the crowd.  Every barrel reaches its "peak" at a different point.  Only a small handful of barrels will keep improving after 20, 30 years.  The producer needs to be paying attention to find that diamond in the rough that can go another extra decade.  Concurrently, they also need to know when to pull the plug and bottle something when it's ready before it begins to decline.


So yes, many of the best Cognacs, single malt scotches and bourbons are very old, but it is not solely because someone has the patience to wait that long.  It's because the producer overseeing the maturation has a keen understanding of all the elements discussed above and knows how to nurture that spirit to reach its full potential, whenever that may be.

Cellar-aged barrels, stacked in low rows.


More Aging Stuff!

It's endless...

Solera System

A solera is a style of blending spirits from barrels over a very long period of time.  It is often employed in sherry, port and some rums productions, among others.  Barrels are stacked in rows on top of one another and the barrels on the bottom row are the ones drawn from for bottling.  But typically no more than 30% is taken at any given time, so the barrel it is never fully drained.  After drawing the spirit, the barrels are then be topped off with contents from the row above, and those barrels are filled from the next row up, and so on.  Any newly made product is be added to the top row.   So barrels used for bottling contains elements of every batch that came before it. Some soleras have been going on for over 100 years, and in theory, today's bottles will still contain traces of the very first batch. Sure it's in very small amounts.  But who cares, it's super cool.

Multiple Barrels and Finishing

While many spirit categories have specific regulations on aging, which will be covered on each spirit's individual page, there is often plenty of room for variety.  For example, many spirits don't just see one barrel in their maturation lifetime.  Sometimes a combination of casks are used. Cognac will often start aging in a new barrel and then transfer to a used one after a year or two. 


Finishing is another common technique that involves putting the spirit into a new barrel for a few months up to a couple of years to add a final layer of complexity.  A finishing cask will usually have some distinguishing characteristic, ex-rum, port, madeira and sherry casks, among others, are commonly used.


Charred barrel.jpg

The inside of a charred barrel.

Enviornmental Facotrs
Heating Barrels
Barrel Size
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