The first thing to know about oak is that not all oaks are the same. The quercus genus refers to white oak and tradition dictates that certain types of quercus are used for aging. In America, the most commonly used oak is quercus alba, or American white oak. Bourbon, by law, must be aged in virgin American white oak first. This puts strain on Coopers who compete with furniture makers for this very dense, wide-grained wood. Rums, Scotches, Cognac, and other spirits have more flexibility in what can age. Don’t forget that oak is also in demand for aging wine as well.
Scotches and Cognacs traditionally use quercus robur, sessiliflora or petraea, known as a group as English or French oak, for aging. Many of the forests used for oak, both in Britain and France are incredibly ancient. American oak is, overall, less labor intensive at all stages up to coopering making it cheaper. American oak can often be sawed, but French oak typically cannot. This means the number of particulates imparted into the spirit is greater in American oak. Keep in mind that it can feel like drinking liquified tree if spirits are aged too long.
Besides color, the reason oak is used for aging is what the charred oak imparts into the spirit. Depending on the oak, chemicals like vanillin, guaiacol, eugenol, or cinnamaldehyde are often included in the taste of a spirit. If a couple of these sound familiar, it’s because vanilla is primarily made from vanillin, and cinnamon gets its flavor from cinnamaldehyde. And these crystalize in the wood’s structure during toasting. The percentage of water in the spirit is a universal solvent that breaks down the molecules with the weakest structure.
When tasting Bourbon, Brandy, Scotch, or Tequila, you may taste hints of baking spices. Eugenol, for example, tastes like cloves. This comes entirely from oak aging. The natural flavors of the distillation are considered in relation to how they complement the degree of these phenolic compounds imparted during aging.
The agave used in the production of tequila has its own aldehyde to add into the mix. Isovaleraldehyde is completely natural, and often is perceived as a taste of cocoa. Factor in the vanillin and baking spice flavors and you have a key reason why Añejo tequilas are so different from their Blanco siblings. Another chemical compound of fine tequila is β-damascenone, which is also found in Bourbon whisky. This ketone also exhibits some of the same sweetness that makes this Kentucky distillate typically sweeter than it’s Scottish contemporary.
The strength at which a spirit leaves the still and ends up in the bottle are always different. Tequila is usually distilled twice, although some producers have done third runs for various reasons. Each distillation strengthens the alcohol by volume of the spirit and the most agreeable percentage of this (the heart of the run) is separated for aging. This can be anywhere from 100 to 120 proof.
The chemical rules for distilling apply both in the still and in the barrel. Alcohol evaporates at a lower temperature and pressure than water. If it wasn’t for this wonder of physical chemistry, established as the rules of physics came into being in the early seconds of the universe, we would have no spirits at all. Señor Rio tequila would not exist!
What this means is that while the spirit sits in the barrel, the water will try to evaporate at a faster rate than the alcohol. This “angel’s share” occurs when humidity is high enough and faster when the temperature is warm. The town of Tequila, where Señor Rio is produced, has a temperate climate with some regular humidity. In Fall there is a rainy season and most evaporation of spirit will happen during this time.
Many people ask, quite reasonably, why evaporation can take place when a spirit is shut away in a watertight container like a barrel? The reason is that in the structure of the wood there are pores. These very small holes, depending how small they are, can allow for the evaporated water and spirit to escape. It’s never a lot, 1-3% per year, but it can reduce the quantity of the spirit by a significant portion over time. Considering that Reposado is aged for less than a year and Añejo for a maximum of two, this amount is negligible. An Extra Añejo is typically aged longer. Señor Rio’s Extra Añejo is aged for five years so evaporation will be more noticeable.