Cask power
How cask maturation can transform sugarcane spirits
Ian explores the ins and outs of rum maturation
Drinking rum evokes various reactions – to the flavour, the experience, the occasion. This may result in a smile that says a lot, or a verbal reaction that says it all. There’s no need to understand the intricacies of the ageing process to appreciate a rum. But for some of us, details are a vital part of the enjoyment and appreciation of a rum.
The range of aged rums continues to grow broader and older, but whatever the age, the quality remains the same. It’s the flavours that change. Longer or shorter ageing isn’t better or worse, it’s different, and whether more senior or more youthful flavours are preferable is entirely personal.
The influence that ageing has on rum was only discovered accidentally. Rum was originally consumed unaged, and exported with this intention from the Caribbean to Europe from the late 18th century. Rum travelled in barrels, a standard choice at the time for storing various items including foodstuffs. It was also a very fortunate choice for rum, which interacts readily with a cask. Arriving in Europe weeks or even months later, the rum was found to taste better than it did on embarkation, although why wasn’t understood.
Knowledge of the ageing process has deepened enormously, but this is such a complex process it remains only partially understood. At least three principal influences are clear: the cask used for ageing, evaporation and oxidation.
Oxidation refers to air entering the cask, the vital component being oxygen which dissolves in the spirit. This instigates a range of reactions that mellow and refine a rum, which includes promoting fruit notes.
“Oxidation and oxygenation also modify existing aromas, with pineapple, for example, transforming into dried fruit notes which gives the rum greater complexity,” says Loic Lionnet, assistant cellar master at Grays Rum in Mauritius.
How influential evaporation can be depends on the rate, which rises and falls in line with the temperature. At Grays Rum, the temperature is 15ºC in winter, and reaches 34ºC in summer, which accounts for evaporation rates of 7-8 per cent of the contents of the cask per annum. This compares to 6 per cent at Appleton Estate in Jamaica, where temperatures range from 21ºC and peak at 40ºC from March to November.
In the Philippines, where Tanduay is based, the temperature is around 23ºC during rainy seasons and 30ºC in summer. Master distiller Myrna Delos Santos says, “Being in a tropical country, we have an annual evaporation rate of 5-7 per cent, compared to around 2 per cent per year in Scotland. So, a year of ageing in the Philippines could be equal to three years of ageing in Scotland.”
Scotland’s climate is similar to England, but creating a microclimate can change all that and increase evaporation rates to around 6-10 per cent. By harnessing geothermal heat, emanating from the deepest hole in the UK at more than 5,280m, the Cornish Geothermal Distillery Company will fuel a series of ultrahigh-tech, carbon-neutral biomes, housed within a ‘mega-dome’. Climate control will provide a tropical microclimate for maturing rum in Cornwall that won’t depend on weather patterns.
“The temperature will reach 30ºC in the biomes, but we are also using this to enable tropical plant life to thrive, which will balance the humidity to create a unique landscape for the barrels,” says Ryan Chetiyawardana, a founding partner of The Cask Club/Cornish Geothermal Distillery Company.
The ageing environment works in conjunction with the influence of the cask in which a rum is aged. This usually means a previously used, ‘seasoned’ cask, with different cask types contributing an individual range of aromas and flavours.
A favourite choice throughout the rum world are ex-Bourbon barrels. There’s a good reason for this, as these barrels provide vanilla, honey, coconut, hazelnut and fruit flavours, together with a light, dry sweetness. For comparison,
casks previously used to age sherry add dried fruit, fruitcake, raisins, walnuts and a rich, indulgent sweetness.
Casks also vary in size, with Bourbon barrels having a capacity of around 200 litres, while sherry casks are either a hogshead (250 litres) or a butt (500 litres). Moreover, casks can also be downsized by a cooper – the smaller the cask, the more intensely it influences the rum.
Leon Swallow, senior distiller at England’s Cotswolds Distillery, says, “We used cream sherry casks to age our rum; these added raisins, nuts, cinnamon, nutmeg and some vanilla and sweetness to the spirit character, which had fruity, pineapple, banana and caramel notes, with a hint of molasses. We used quarter casks (125 litres) as we wanted them to deliver a big hit of flavour, and took regular samples to see how the rum was progressing. These casks worked really well.”
There is also the option of applying a ‘finish’. A rum initially aged in ex-Bourbon barrels, for example, can be ‘finished’ by spending a further six to 24 months in another type of cask to gain additional flavours.
Finishing was pioneered in Scotland by malt whisky – this was a dynamic innovation in the mid-1990s, though also the continuation of a traditional practice. During the late 19th and early 20th centuries, Scotch whisky was shipped from Scotland to British colonials around the Empire in casks rather than bottles. Meanwhile, various styles of fortified wine, including Madeira, port and sherry, were shipped to Scotland in casks and bottled locally, leaving plenty of empty casks ready to be re-filled with whisky and exported.
As several months could elapse between casks being filled and arriving at a long-haul destination, these whiskies acquired a ‘finish’ of additional flavours, which received an enthusiastic welcome.
The starting point for a finish from Grays Rum is a blend of rum aged for seven years in ex-Cognac casks, which contribute smoothness and a dry sweetness, and virgin French oak casks that add spice and fruit. This blend then undergoes a finishing period of one year in Moscatel casks, which add a honey-like sweetness with white fruit notes and very sweet white grapes.
It’s a perfect example of finishing also being a great beginning, as more rum distilleries are now experimenting with finishing and creating an exciting new era.