The Chemistry of seasoning and toasting

Seasoning and toasting is a fascinating area of barrel-aging that not everyone is entirely familiar with! Here's a little write-up so you can soak in a bit more information.

Oak destined for barrel-making is usually seasoned-artificially or naturally-to reduce excessive humidity, undesirable aldehydes, and harsh ellagitannins.  Toasting will create a variety of flavors in the wood, depending on temperature and time.

In artificial seasoning, oak planks are heated in an oven (kiln) at approximately 50 °C (122 °F) for up to one month. This is a quick and economical way of seasoning oak but it doesn’t remove as much of the undesirable components. The most expensive barrels are coopered from two- or three-year air-dried wood, which increases concentrations of aromatic compounds. 
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The most dramatic structural and chemical changes in the wood occur with toasting where the inside barrel surface is set over a fire or subjected to another heat source.  Toasting both mellows the tannins in the wood, as well as changes the flavors the barrel might impart from raw wood to more spicy, vanilla notes—toasting actually helps release vanillin from the lignin in the wood. There are varying degrees of toasting—from a light toast to a heavy toast and, as you might imagine, that changes the way they can influence the aged liquid.

Here's the inside of a Squarrel with some toasted staves!

LIGHT TOAST: Provides a slight color change to the wood providing scents of vanilla, caramel and clove.
MEDIUM TOAST: Provides a brown toned wood, and imparts scents of cedar, roasted nuts, vanilla and coffee.
HEAVY TOAST: Provides a dark toned wood, and imparts aromas of coffee bean, charcoal, ginger, nutmeg and toasted bread.

Lignin, cellulose and hemicellulose (wood sugar) are essential components of oak—their chemical bonds are broken during toasting, leading each compound to break down to simpler substances. Tannins also go through a similar process, emitting free ellagic and gallic acids. With enough heat, the tannins can be completely destroyed.

Hemicellulose begins to release its basic sugar compounds, which contribute to toasty aromas. Lignin breakdown leads to the creation of vanillin, oak’s signature flavor, as well as syringaldehyde. At higher temperatures, lignin creates volatile phenols, such as guaiacol, lending smoke notes.

Toasting substantially reduces the plank-smelling aldehydes and the heat causes lignins to degrade into their volatile aldehyde and ketone derivatives.  There is an increase in phenolic aldehydes and ketones and substantial production in guaiacol and syringol and their respective derivatives, all responsible for the smoky, spicy aromas. Heavy toasting also introduces little cresol, which impart a tar-like smell, 4-ethylguaiacol and 4-propylguaiacol, responsible for bacon, spice, clove or smoky aromas. It also results in ketones, which can add aromas of freshly baked bread and caramel. 

The inside of a charred stave Squarrel!

Toasting also increases the amount of oak lactones responsible for flowery, spice, coconut aromas though the incremental effects of heavy vs. medium toasting are not significant. Toasting introduces furfurals or furanic aldehydes, including 5-hydroxyfurfural, which imparts caramel, bread and almond aromas, and 5-methylfurfural, which imparts notes of toasted almond.

Charring increases levels of lignin breakdown products extracted by the spirit.  Although the char layer contains few aromatics, heat penetration to subsurface layers promotes thermal degradation reactions and increases aromatic aldehydes and acids to a depth of 6mm. Although deeper in the stave, the char layer does not hinder their extraction because the disruption of the wood structure during charring increases the penetration of the maturing spirit.  
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The grain is the sum of annual growth rings added each year in the life of a tree. Oak belongs to the family of the “ring porous” hardwoods so the early spring wood tends to be more porous to allow the flow of nutrients to the awakening tree.​

A beautiful close-up of an oak tree!

Although more than 600 different oak species are found globally, only 3 species are suitable for making wine barrels. These are Quercus patraea and Quercus robur, which are mainly found in Europe and Quercus alba which is indigenous to North East America. In Europe oak trees are usually 150 years old before they are used for barrels. Contrary to that North American trees are already used after 60 years. Another important difference between the two species is the appearance of more tyloses or radial rays in American oak. It causes higher density than the European oak and consequently it can be sawn, while European oak needs to be split along the grain to prevent barrels leaking.

As result of this difference only 10 to 14 barrels can be made from a cubic meter of European oak. The utilization of American oak for barrels is 40 to 45%, compared with 20 to 30% of French oak. This is one of the reasons why American barrels are cheaper than European barrels.
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Oak is the primary species used to make barrels.  Its possible to create a broad variety of flavor profiles within the oak species depending on the region from which the oak is harvested, the seasoning of the wood and  the toasting/charring of the barrel. There are additional species of wood used in coopering around the world that can extend the range of flavors obtained.  Squarrel is committed to provide you with a breadth of wood species to best enhance your amazing craft product.  

Soaked Squarrel staves!