Blending & proofing
In our three prior installments, we covered fermentation, distillation, and aging. But there are several more critical yet little-known steps before a rum is ready for bottling.
Blending
Distilleries usually make several different flavor profiles of rum, known as marques. Different marques may result from different fermentation protocols or distillation equipment, e.g., stills. Blenders at large distilleries often have a dozen or more rum marques at their disposal, which they blend together to craft your favorite expressions.
A blender's top priority is ensuring an expression's flavor profile doesn't change from batch to batch. However, rum undergoing aging isn't entirely consistent. The same rum put in two "identical cask" yield very different rums years later. Even though blenders continuously evaluate their rum as it ages, they may not have enough of each marque to replicate the blend of a previous batch. Thus, they must skillfully adjust each batch's blend to what they have available.
Blenders must also guide distillers in which marques (and how much of each) to distill and put in casks today to make the rums the market demands a decade or more in the future.
Color Filtration
The vast majority of "white" rum sold today is aged. So why does it have no color? The answer is carbon filtration. A properly constructed carbon filter can remove most of a rum's color with a relatively small reduction in desired flavors. Coconut husks are a common source for filter material – after charring, of course. Large distilleries use various specialized carbon filters to tune which flavor compounds are stripped out and which remain. Spanish heritage rums from places like Cuba and Puerto Rico are particularly known for color filtration.
Chill Filtration
While the organic compounds providing rum's flavor are a tiny fraction of the liquid volume, when a rum cools beyond a certain point, some organic compounds begin to precipitate out, giving the rum a hazy look or even visible particles.
This precipitation becomes more pronounced at lower alcoholic strengths. A cask strength rum might require extreme chilling to achieve a haze, while its 40 percent ABV sibling could become cloudy on a chilly winter day.
Since this haze might concern customers, some brands chill filter their rum before bottling. The rum is slowly lowered to a preset temperature (typically around 0°C) and passed through a filter that pulls out microscopic particles. However, this also removes a certain amount of flavor, so many enthusiasts prefer non-chill filtered rums.
Proofing
Only a tiny fraction of rum is bottled at distillation strength or even cask strength. Diluting a rum to bottling strength isn't just adding the right quantity of purified water all at once. Distilled spirits must have their strength reduced very slowly; otherwise, certain organic compounds convert to fatty acids that taste of soap. (The scientific term is saponification.) Thus, blenders add water in very small increments over many weeks or months.
Coloring
Just as customers don't want a hazy rum, a noticeable difference in color between two "identical" bottles would raise concern. To prevent this, some brands define an optimal color level for an expression, then add spirit caramel (if needed) to match that specification. Commercial spirit caramel, known as E150, is quite bitter, but so little is needed to adjust the color that it has little flavor impact. However, some brands make their own caramel which isn't as bitter, and use enough that its flavor is noticeable.
Every brand has its own approach to preparing rum for bottling. While a few bottle straight from the cask, most use at least some of the above steps. For those seeking the very best rums, understanding these final steps of rum-making processes is important to understand.