Figure 5 The structures of several compounds that impart unique flavors to whiskey during maturation.

In addition to the structural compounds, oak heartwood contains extractives, especially phenolic compounds, fatty acids, and lactones, which become extracted directly into the raw spirit during maturation.

All of these low-molecular-weight compounds are highly soluble in aqueous ethanol and such extraction, into the spirit, of these compounds originating from the wood of the barrel is an important part of maturation.

The species of oak used in barrel construction also influences the final whiskey flavor. The use of wood from Q. alba results in increased levels of vanillin and of lactones, especially the cis and trans isomers of 4-hydroxy-3-methyloctanoic acid lactone (also known as 3-methyl-4-hydroxy caprylic acid g-lactone, h-methyl-g-octalactone, whiskey lactone, or oak lactone;see Fig. 5), in the maturing whiskey, where they impart a woody aroma, whereas the concentration of tannins is increased with the use of wood from European oaks. In this context, a characteristic component ofoak-matured spirits is eugenol which imparts a clovelike flavor (see Fig. 5) (11,13,14,18).

The use of old sherry casks for whiskey maturation increases in its own right the level of tannins and of sugars such as glucose, arabinose, and xylose in the maturing spirit.

During maturation, the very many chemical reactions, involving both the components of the distillate and the wood-derived compounds, may be variously described as oxido-reductions, esterifications, Maillard reactions, polymerizations, and polycondensa-tions. The first group of reactions are the most important, especially since the oak barrels are permeable to oxygen, and may be exemplified by the oxidation of some of the ethanol to acetaldehyde and acetic acid with the subsequent formation of ethyl acetate.

Maturation over a period of several years results also in the development of the amber color, which for the purpose of consumer acceptability may be standardized with added caramel.

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