A straight whiskey must have a minimum of 51% and a maximum of 80% of one grain in its mash bill. It must be distilled to a maximum of 80% abv, and it must be aged for a minimum of two years in new charred oak barrels; whiskies that are less than four years old must state the age in the label.
A blended whiskey must be at least 20% straight whiskey. The rest will be bulked up with neutral, grain spirit.
Sour Mash Whiskey
Sour mash is a method of ensuring consistency from one fermentation batch to the next, and avoiding microbial spoilage of the fermenting beer. A portion of acidic residue from one distillation will be added to the mash about to be fermented for a later batch. Most American whiskies are produced using various versions of a patent still. However, because they are distilled to a much lower strength than gins, vodkas or even grain whiskies, they emerge from the stills with many characterful flavour compounds, but rather harsh alcohol. The hot ageing environment and the use of active new casks means that drinkable spirit can emerge after only a couple of years, though most whiskies are aged for much longer than this.
Bourbon is a straight whiskey that must be at least 51% maize. Although it takes its name from Bourbon County in Kentucky, where most is produced, it can be made anywhere in the United States. The whiskies are stored in very hot warehouses, which results in an accelerated ageing process and a rapid extraction of tannins, sugars, colours and coconut and vanilla flavours from the wood. Although a few premium brands use pot stills, most use continuous stills. Unlike scotch whiskies, which may use caramel to adjust the colour, all of the colour in Bourbon must come from the ageing process.
During the ageing process barrels that have particular characteristics may be bottled separately as Small Batch Bourbons. This can also take place with other categories of whiskies such as Kentucky Straight Rye.
Rye whiskey in the United States is a straight whiskey that must be made with a mash bill of not less than 51 per cent rye. This gives whiskies with a particularly fiery, spicy character.
A Tennessee whiskey must satisfy all the regulastions of a straight whiskey, except that in addition it must be distilled in Tennessee and the spirit must be filtered through maple wood charcoal before ageing. The latter process, also known as Lincoln County Process, was probably introduced by Russian or Polish settlers who had experience in vodka production. Although charcoal used for filtering vodka is of very high standard of purity, and strips colour and flavour from the spirit as well as making it less harsh, the charcoal used in the Lincoln County Process is much less pure and gives a very smoky character to the whiskey.