Barrel aged rum: the magic of maturation
Ageing rum has to be one of the more thrilling experiences of this distiller's journey yet. Every so often I take a wee sample from an oak barrel to see how the spirit is maturing -- it's a delight every time.
What is barrel ageing?
In a nutshell, ageing rum involves putting distilled spirit into a wooden barrel, most commonly oak, either French or American. (The rum industry typically uses ex-bourbon American oak barrels, and if you're interested in why, read this.)
Inside the barrel a chemical tango is underway between the spirit and the wood, a push and pull dance. The most visible transformation is in the colour. All spirits come off the still as clear as water, whether the distilled spirit is sugar-based (rum), grain-based (whisky) or grape-based (brandy). It is the wood which imparts a golden amber colour to the spirit, over time giving the characteristic colour of a 'brown' spirit. But the most remarkable change is in the organic flavour compounds the wood imparts including vanillin (vanilla), oak lactones (coconut), eugenol (clove) and guaiacyl (smokiness). All together they provide the 'oaky' flavour we associate with aged spirits.
Wood doesn't just donate flavours, the dance actually creates new flavours too. The organic chemical compounds extracted from the wood interact with the spirit to form new compounds, new flavours. This is called esterification, a process which originally starts with fermentation where alcohol compounds bond with acids to create flavours, from raspberry to coffee. Rum is typically a very high-ester spirit, full of fruity, floral notes like banana and peach.
Finally, since barrels are porous they breathe and this allows oxidation that also creates brand new flavour compounds; while the evaporation concentrates flavours.
Ageing vs maturation
Picture a 10,000-litre wooden vat filled with spirit. Now picture a 100-litre barrel filled with spirit. Which will mature more quickly? The 100-litre barrel offers so much more surface area of wood to spirit for the chemical interactions to occur. You can age both vessels for the same arbitrary amount of time, but the spirit in the smaller container is going to come out significantly more mature.
It's important to note that spirits will also mature over time without wood. After distillation there are a tonne of organic compounds that are active and which benefit from resting in neutral containers, like stainless steel. This allows the spirit to soften and it marries disparate organic flavour compounds for a smoother, more harmonious spirit. Some distillers let their spirt rest in stainless steel for up to a year! Our unaged expression, Blanco, is left to rest in stainless steel for 4-6 weeks; combined with a very slow distillation is what makes Blanco so smooth.
To char or not to char
Toasting and charring the inside of an oak barrel will have a great impact on the flavour profiles of the spirit. A toast subjects the wood to heat at a lower temperature for longer and provides more delicate, subtle flavours. A charr burns the barrel with an open flame for not longer than a minute -- the charcoal cleans the spirit of impurities and provides sweeter caramel notes and more colour.
What more goes into ageing?
So much more! Ambient temperature, seasonal variations in temperature and humidity, single casks or blends, solera systems, new or rejuvenated used barrels, and ultimately, how long the spirit spends in wood.
Under the 1901 Excise Act, in Australia 'rum' must be stored in wood for a minimum of two years before it can be legally defined 'rum'. Around the world definitions vary, most with no minimum time. The Australian craft rum scene works around this by releasing 'cane spirits' or 'aged cane spirits' while they put down barrels for the future.