compounds, of which some 200 to 300 are easily detected by chemical analysis. The flavouring chemicals include "carbonyl compounds, alcohols, carboxylic acids and their esters, nitrogen- and sulfur-containing compounds, tannins, and other polyphenolic compounds, terpenes, and oxygen-containing, heterocyclic compounds" and esters of fatty acids. The nitrogen compounds include pyridines, picolines and pyrazines. The sulfur compounds include thiophenes and polysulfides which seem to contribute to whiskey's roasted character. Flavours from treating the malt The distinctive smoky flavour found in various types of whisky, especially Scotch, is due to the use of peat smoke to treat the malt. Flavours from distillation 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 remove undesired distillates. 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 of this than vodkas, but significantly less than rums or brandies. Polysulfides and thiophenes enter whiskey through the distillation process and contribute to its roasted flavour. Flavours from oak Whisky that has been aged in oak barrels absorbs substances from the wood. One of these is cis-3-methyl-4-octanolide, known as the "whisky lactone" or "quercus lactone", a compound with a strong coconut aroma. Commercially charred oaks are rich in phenolic compounds. One study identified 40 different phenolic compounds. The coumarin scopoletin is present in whisky, with the highest level reported in Bourbon whiskey. In an experiment, whiskey aged 3 years in orbit on the International Space Station tasted and measured significantly different from similar test subjects in gravity on Earth. Particularly, wood extractives were more present in the space samples. Flavours and colouring from additives Depending on the local regulations, additional flavourings and colouring compounds may be added to the whisky. Canadian whisky may contain caramel