Maturation Basics

How does aging or maturation of spirits work?  

There are four basic mechanisms that take place during the aging of distilled spirits.  They happen in order, and then repeatedly from the minute the spirit, wood, and air interact, until the aged spirit is bottled.  The four mechanisms are partitioning, digestion, oxidation, and esterification.  

What is Partitioning in the maturation process?  

Partitioning is the integration of distilled spirits and the wood. This includes the infusion of the distilled spirits with soluble (or solubilized) components of the wood.  The infusion of soluble components like sugar, vanillin, tannin, and other compounds provides an important part of the final flavor profile.  Partitioning also includes the sequestration of small molecule congeners from the distilled spirits into the wood and wood-char structures.  

Congeners are compounds produced during fermentation and distillation that make their way into the distilled spirit.  Some congeners produce foul flavors and it is important that these are removed during the barrel aging process.  One means of removing them is via partitioning.  Partitioning is the first mechanism to take place, and it continues to take place throughout the aging and maturation process.  

What is Digestion in the maturation process?  

Digestion is the next step in the maturation process.  As the distilled spirit (mostly water and ethanol) penetrates the matrix of the wooden tiles, the wood (hemicellulose and lignin) dissolves slowly over time.  As these compounds dissolve they create species that contribute to the flavor and mouthfeel of distilled spirits. Partitioning and digestion are notable in that they are the two processes that take place during infusion.  Infusion alone does not constitute maturation but is an important part of the process.  Infusion is an important spice, but just like any home cook can attest, it is possible to oversalt your dish.

What is oxidation in the maturation process?  

The third maturation mechanism is oxidation.  In a barrel, atmospheric oxygen reacts with a variety of molecules that are present in the distilled spirit after it has been slowly infused with the wood and with the alcohol (ethanol) itself.  The liquid will deplete oxygen from the headspace of a vessel as maturation occurs.  A traditional barrel breathes: air comes in, and water and ethanol go out.  But this process is extremely slow.  To enhance oxidation MOBA actively introduces air into the spirit through the central aeration stone.  Critically, the oxidation of alcohol leads to organic acids, which drive the final mechanism, esterification.

What is esterification in the maturation process?  

The fourth mechanism is esterification.  Esters are a class of molecules that create aromas and flavors that are found in all properly aged distilled spirits.  Esterification happens as organic acids (such as acetic acid/vinegar) react with alcohols (such as ethanol/drinking alcohol).  While esters are only present in tiny amounts in the final product, their presence is notable in the flavor profile.  This mechanism is also slow in a traditional wooden barrel but is accelerated by heating the requisite components in MOBA.