Privateer Rum’s Maggie Campbell on Barrel Aging Science – Tropical or Otherwise

The “Wonk” in Cocktail Wonk refers to digging deep to understand as much as possible about spirits and cocktails. Many of the stories here involve the chemistry of aging, as well as the different elements in a spirit – water, ethanol, and other compounds such as esters, aldehydes and higher (heavier) alcohols. It’s these compounds that give spirits their aroma flavor. Each spirit has its own ratios of these compounds, thus making that spirit taste different than any other spirit. The set of aromatic compounds in a spirit are impacted by many processes, including fermentation, distillation, and barrel aging.

There was recently a Facebook post about rum and the aging process. Within the comment threads was a discussion of tropical aging, and whether the high angel’s share (evaporation from a cask) slowed down after the first few years. I’ve heard Caribbean distillers mention that their evaporation rate was higher in the first few years, but I couldn’t pinpoint why. Luckily, Privateer Rum’s Maggie Campbell, who I’ve written about previously, wrote a very compelling comment in that thread which helped my understanding.

I felt that rather than have Maggie’s words be lost in the sands of Facebook time, they should live on in a more permanent, visible form. I asked Maggie for permission to reprint her comment here, with slight reformatting for clarity and unrolling some of her abbreviations.

Maggie wrote:

Tropical aging does have more angel share. A certain amount of this also relates to the tradition for higher entry proof in the Caribbean (proportionally more alcohol that readily evaporates) – but of course consistent 85 °F days is [also] a factor. And many more unique characteristics affect tropical aging beyond that.

It is well studied in formal industry study programs that the molecules that will readily volatilize in ambient conditions evaporate off in the first few years of aging and is related to the cellar conditions.

 Essentially what can turn into a vapor in those conditions does, and what is left behind are the molecules that do not as easily vaporize. What is left behind are heavier, longer chain molecules that may need more energy or specific conditions/structural changes of oxidation etc. to continue to evaporate. It does not magically stop of course; evaporation still happens. It does however slow down on the following years noticeably – everywhere, not just tropically.

But again humidity, consistency of temperature, and many other factors play into the unique expression of tropical aging. And of course, entry proof intersects with the rate of these chemical changes (higher proof – slower, lower proof – faster – due to stability) as well as temperature (higher temps – faster, lower temps – slower).

Clearly there is much more to this conversation than what I’ve covered here but this is part of one aspect.

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  1. Matt, it’s known that bourbon can increase proof in the barrel above the barrel entry proof such that a 2014 GTS can be 144+ proof in the bottle at barrel proof despite being entered in the barrel at no more than 125 proof. Scotch can have a lower barrel proof than its entry proof. These factors are apparently largely determined by the climate in which the barrels are aged. I cannot find much discussion on what happens to rum proof in the barrel, especially in regards to tropical vs continental aging. There is little published data available on rum entry proof, Minnick’s “Rum Curious” aside.

    Do you have any info on entry proof vs barrel proof (at the end of maturation) for rum?

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