All spirits are stored for a time between distillation and bottling.
Maturation, however, is intended to make a positive impact on the aroma, texture, and color of a spirit. Oak is the most commonly used wood when aging spirits, because of its durability, flexibility, being watertight (but not airtight) and the flavors imparted to the maturing spirit. Up to 70% of a cask-matured rum's character comes from the interaction with the oak, so it is important to understand the nature of a barrel.
The most common variety used for aging rum is American white oak, which contributes flavor notes of vanilla, coconut & sweet spices. Producers in the French Rhum world use predominantly French oak, which adds more tannins & a savory spiciness.
Staves for the barrels are cut lengthwise from the heartwood of the oak tree to have the fibers of the wood running vertically on the barrel. This makes the wood watertight but still allows for air and alcohol vapor to move in and out of the wood. The oak is either air-dried or kiln-dried before being shaped into a barrel. The inside of the barrel is charred to make flavors and color more available for the spirit to extract. The charred layer also acts as a filter, trapping unwanted fusel alcohols and sulfur, resulting in a smoother spirit.
The rum industry gets the majority of casks from the bourbon world. Bourbon, by law, must be aged on new oak, which opens up a huge second-hand market of ex-bourbon barrels. A new barrel will impart more flavor and color than a used barrel, which is why a 4-year-old bourbon can appear very dark in color. Luckily for us, there is still plenty of flavors left to extract.
The size of the barrel will also have an impact on the spirit. In a smaller barrel, more liquid is in contact with the wood and will in turn extract more flavor.
Now that we have a better understanding of the oak vessel, let’s get into the mechanics of maturation.