Aging: what goes on inside the barrel?

In previous posts, I talked extensively on what I learned about oak trees, coopers, and the noble art of barrel making. But why is a barrel so important for aging? What actually goes on inside the barrel that makes your rum or whiskey taste so much better, after a few years? What chemical reactions take place and how can you put them to good use, as a Master Distiller, aiming for the best possible spirits?

Basically, four chemical reactions happen, when a brandy, rum or whiskey ages in a barrel:

  1. Extraction;
  2. Esterification;
  3. Oxidation and Evaporation;
  4. Filtration.


When the new made spirit touches the wood , it extracts wood particles like tanins, vanillins, lignins and wood sugars. Envision making tea, where water touches the herbs and extracts particles and colour. That’s extraction, just as what’s hapening (at a slower rate) inside your barrel.

Now, the speed and quality of extraction are very much influenced by toasting and/or charring. Toasting and charring open up the wood, increasing both the surface area where spirit and wood mingle, as well as the overall particle transfer.

A heavier toast will have opened up the wood more and created a longer and more intense caramelization process. That’s why heavier toasted barrels give off a more intense taste at a quicker rate than barrels that have seen only light toasting.


Esterification means so much as taste formation. It is the chemical process where alcohols and organic compounds, in a slightly sour and wet environment, react into new molecules that can be smelled and tasted. This family of tasty molecules is called “esters”. More esters equals more taste.

What barrel aging does, or – actually – what the process of extraction does, is introduce huge amounts of various organic compounds (right, the tanins, vanilins, lignins and wood sugars) into an already sour, wet, and alcohol rich environment. More organic particles create more chances for ester molecules to develop.

Some of these esters will have a fruity character. Others have a more rooty, nutty stance. Situated in between are the esters with tastes most associated with the basic ingredient of the spirits you are producing: molasses for rum, grapes for brandy, and grain for whiskey. Important. I will get back to that later on!

Oxidation and Evaporation

A barrel breathes. The spirit inside it slowly evaporates, thus introducing more air and oxygen into the barrel.

In a warmer climate the overall amount of evaporation is bigger than in a colder climate. In a drier climate, relatively more water evaporates, so the barrel content will gain in ABV, even when the total content is getting smaller. In a wetter climate, relatively less water evaporates, so the whiskey inside will loose a few points of alcohol. Why is the evaporation of alcohol and the introduction of oxygen important? Here we go:

Just as with distillation, evaporation favours smaller, lighter molecules. Just as in the first part of your distillation run, the headsy alcohols are over-represented in the evaporation. Evaporation helps you get rid of excessive heads!

Oxidation helps control the heads faction as well. Remember those esters with a fruity taste? They are formed where headsy alcohols, like acetones, methanol, and acetic compounds react with organic (carbon) particles. Oxidation helps esterification. In a degressive order:

  1. By helping transform headsy alcohols into fruity flavours;
  2. By helping transorm lignins and tanins into vanilla flavours;
  3. By helping transform tailsy alcohol into rooty and nutty flavours.


Toasting opens up the wood. Charring, in addition, also creates a charcoal layer on top of the wood. Since charcoal is a very good filtering agent, it helps filter out the bigger molecules present in the spirit. The bigger molecules? Right, the tailsy alcohols like propanol, buthanol, and furfural. These tailsy alcohols are bigger than ethanol molecules and are “catched” by the charcoal layer.


Barrels are essential for growing up a great whiskey, rum or brandy. Barrels introduce unique tastes of its own as well as organic particles that enable esterification (taste formation on a molecular level).

Because barrels breathe, headsy alcohols evaporate, while more fruity, rooty, nutty, and vanilin notes develop over time. If you choose to age in a charred barrel, the charcoal layer can even help you with tails control.

Mastering Wood as a prerequisite to becoming a Master Distiller

How can a Master Distiller apply the knowledge shared above, and put it to good use? In various ways. Here are a few examples. For a more open discussion, please see the iStill Form ( from today onwards. Okay, the examples of how a Master Distiller can benefit from the various chemical reactions that take place inside the barrel:

  • A newly made spirit with quite dirty cuts for heads and tails will develop into a more complex product over time, while total amounts of headsy and tailsy alcohols will actually be slowly reduced during the barrel aging time;
  • A rum, whiskey or brandy distilate, that barrel ages in a hot climate, will loose relatively more headsy components to evaporation, thus creating a less fruity nose and taste;
  • If you have all the time in the world (say 12, 16 or 20 years), and you live in a cold, wet climate, you may decide hardly to cut at all. Just use second hand barrels (to prevent over-extraction) and give them a fresh layer of char for tails control. Time will create a very complex product, while mild evaporation and prolongued oxidation control the final amounts of heads.

Do you want to know more?

I hope I made my initial point clear: barrel ageing is essential to making a great brandy, rum or whiskey. I also hope I succeeded in explaining what factors need to be taken into account and how your now improved knowledge on barrel ageing can help you be a better Master Distiller. But there’s more to be learned and shared.

How about the influence of the ABV on taste development, the effects of fill strength, for example? And how does barrel maturation influence the amount of acids, esters, aldehydes, sulfur compounds, and sugars in your drink? The influence of using bigger or smaller barrels?

And how about that spirits perfect barrel we are designing? What properties will it have? How much will it cost? And do you think it is something we should bring to the market place?

So many questions that still deserve answers. Please, keep on following the iStill Blog and we will dive in deeper soon!

Complexity and character come at a price. European Oak is cleaved, limiting nett yield to 20 – 25% instead of 50% …


One thought on “Aging: what goes on inside the barrel?

  1. Pingback: Whiskey Reviews: Daviess County Kentucky Straight Whiskey and Daviess County Cabernet Cask Finish - The Whiskey Wash

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