The flavouring of whisky is partially determined by the presence of congeners and fusel oils. Fusel oils are higher alcohols than ethanol, are mildly toxic, and have a strong, disagreeable smell and taste. An excess of fusel oils in whisky is considered a defect. A variety of methods are employed in the distillation process to remove unwanted fusel oils. Traditionally, American distillers focused on secondary filtration using charcoal, gravel, sand, or linen to subtract undesired distillates. Canadian distillers have traditionally employed column stills which can be controlled to produce an almost pure (and less flavourful) ethanol known as neutral grain spirit or grain neutral spirit (GNS). Flavour is restored by blending the neutral grain spirits with flavouring whiskies.

Acetals are rapidly formed in distillates and a great many are found in distilled beverages, the most prominent being acetaldehyde diethyl acetal (1,1-diethoxyethane). Among whiskies the highest levels are associated with malt whisky. This acetal is a principal flavour compound in sherry, and contributes fruitiness to the aroma.

The diketone diacetyl (2,3-Butanedione) has a buttery aroma and is present in almost all distilled beverages. Whiskies and cognacs typically contain more than vodkas, but significantly less than rums or brandies.

Flavours from oak

Whisky lactone (3-methyl-4-octanolide) is found in all types of oak. This lactone has a strong coconut aroma. Whisky lactone is also known as quercus lactone.Commercially charred oaks are rich in phenolic compounds. One study discriminated 40 different phenolic compounds. The coumarin scopoletin is present in whisky, with the highest level reported in Bourbon whiskey.

by wikipedia