The art of aging
In our prior two installments we learned how sugarcane is turned into raw rum. While rum is frequently sold unaged - think Martinique rhum agricole blanc, or Jamaican “overproof” rum - most rums are further transformed via aging in wood casks prior to sale.
While rum was stored in wooden casks since its earliest days, the original intent wasn’t necessarily to refine its flavor. Rather, casks were a convenient way to store rum for transport as metal and plastic tanks were many years in the future. However, it was discovered that rum (and other spirits) that spent significant time in wooden casks was more pleasurable to drink.
Naturally, we might ask – why is this?
Rum undergoes many simultaneous chemical transformations while in a cask. Certain organic flavor compounds are concentrated, while unpleasant flavors slowly escape through the wooden walls. New flavors are introduced, as the wooden cask walls hold pleasing flavor compounds like vanilla, coffee, and coconut which seep into the rum. This is known as extractive aging because the flavor compounds are extracted from the wood. Other flavors such as fruity notes develop over many years due to the constant intermingling of the liquid and the oxygen slowly entering the cask through its walls. This is known as oxidative aging.
The ever-so-slightly porous nature of wood also allows both water and alcohol—the main components of rum—to evaporate. In hot climates like the Caribbean, a cask loses around 10 percent of its liquid volume from evaporation in the first year or two; a cask filled with 200 liters might only hold 180 liters (90 percent of 200 liters) after one year.
In subsequent years, evaporation slows to about seven percent annually. Nonetheless, a once full cask may hold less than half its original contents after ten years. In cooler climates like northern Europe, evaporation is much lower—about 2 percent annually. Naturally, rum aged in Jamaica ages differently than the same rum aged in Scotland, despite equal time in the barrel. This gives rise to the terms tropical aging for Caribbean rum, and continental aging for rums aged in Europe. These evaporative losses are whimsically known as the angel’s share.
Not just any wood works when aging rum. Many woods are “leaky” or add unpleasant flavors. It’s oak, with its many varieties, that age the overwhelming majority of the world’s rum. White oak (aka Quercus alba, or American oak) is relatively inexpensive and easy to work with. The American whiskey industry uses staggering amounts of American oak casks. Because U.S. regulations only allow whiskeys like bourbon to age in new casks, once-used casks are sold by the millions to age rum and other spirits.
The other type of oak frequently used to age rum is European oak, or its subtype, French oak. It requires more effort to work with, but European oak casks have longer useful lifespans. It’s the wood of choice for European spirits, and many such casks end up in the Caribbean. As you might expect, the two types of oak impart different flavors to rum.
Casks are nearly always toasted or charred during construction. The heating process creates desirable flavor compounds in the wood that are extracted during aging. Charring a cask also adds a thin layer of carbon which acts as a filter to remove certain undesirable flavors from the rum during aging. In some countries, primarily of Spanish heritage, additional carbon filtration is an intrinsic part of the aging process.
With some many variables at play, it’s easy to see that aging rum well is extraordinary complex to master. However, when perfected, as Cuba’s maestro roneros seek to do, the act of aging and blending rums is considered an art form.