Toasting refers to the heating of wood to produce flavor compounds not found in raw wood. Different from Charring, toasting affects the wood deeper. Charring only affects the surface. Toasting breaks down the molecular structure of oak, pyrolyzing the lignins, hemicellulose and cellulose. Properly seasoned and slow toasted, the byproducts are delightful nuances that react synergistically with spirits resulting in perceived nostalgic flavors and aromas. Toasting causes a decrease of the astringency in the wood, matched with an increase of aromatic substances, ones that already exist in the wood and new ones formed through exposure to heat.
The degree of toast in a barrel has a profound impact on the flavors of a spirit. Soft or light toast gives a wide array of aromas. Most pronounced are coconut, cream, and white chocolate. Used alone, however, this light toast can sometimes result in an unpleasant, resinous quality. Medium toast is the most commonly used since it provides good balance and complexity. While the coconut notes (lactones) decrease, other volatile substances increase, such as vanilla, caramel, coffee and bread. On the palate it is characterized by sweet and spicy notes combined with vanilla caramel and milk chocolate as well as toast, smoke, spice, and leather. High toast or medium plus, results in more pronounced spicy and smoky notes, graffito, dry leaves, gunpowder, and black chocolate. Very high toast imparts intense, powerfully smoky notes. While this goes well in certain spirits, it risks giving the final product an overly roasted character.