American Whiskey

All nine TTB whiskey categories — complete specifications

The following table documents all nine TTB-defined whiskey categories from 27 CFR §5.22(b). All ABV figures are stated in the TTB's original proof notation — converted to % ABV alongside (US proof = 2 × % ABV).

Mash bills — the grain recipes

The TTB does not require distillers to publicly disclose their mash bills — the specific percentage of each grain used. The following are representative mash bill styles documented in industry reference literature, not legally mandated formulas.

Colour key: ■ Corn  ■ Rye  ■ Wheat  ■ Malted barley — approximate proportions only, not regulatory standards

Bottled-in-Bond — a 19th century consumer protection law still in force

The Bottled-in-Bond Act of 1897 — enacted by the US Congress in response to widespread adulteration of whiskey with additives including tobacco, iodine, and industrial alcohol — created a government guarantee system for whiskey quality. Whiskey meeting the Bottled-in-Bond standard could carry that designation as assurance that it was genuine distilled spirit without adulteration.

Kentucky — geography, geology, and ageing science

Kentucky produces approximately 95% of the world's Bourbon supply — not by legal requirement but by accumulated advantage. Four geographic and climatic factors documented in academic literature explain this concentration:

Tennessee Whiskey — the Lincoln County Process explained

The Lincoln County Process — mandatory for Tennessee Whiskey under the 2013 Tennessee Distillery Act — involves filtering or steeping the new make spirit through or in sugar-maple charcoal before or during ageing. The charcoal is made by burning sugar-maple wood and is packed into large vats (typically 3 metres deep at major producers).

The process removes some heavier congeners from the new make spirit and adds trace phenolic compounds from the charcoal. The result is a slightly smoother character relative to unfiltered Bourbon made with an identical mash bill, though the sensory differences documented in controlled tasting studies are relatively subtle. The process is named for Lincoln County, Tennessee — the region where the practice historically developed — though the modern Jack Daniel's distillery is located in Moore County, which was separated from Lincoln County in 1871.

The chemistry of new charred oak maturation

The new charred oak requirement — unique to most American whiskey categories — creates a maturation chemistry distinctly different from the re-used cask maturation used in Scotch, Irish, and most other whiskey traditions. Three chemical layers in the barrel contribute different compounds:

The char layer (inner surface)

The char layer — produced by toasting the interior of the barrel with a direct flame — is a porous layer of converted cellulose and degraded wood components. It functions as a physical filter (adsorbing some sulphur compounds, fusel alcohols, and aldehydes from the new make spirit) and a chemical source (contributing activated carbons and lactones). The char level — rated 1 to 4+ by thickness — directly affects the depth of filtration and the rate of wood chemical extraction. A #3 char (the most common in Bourbon) produces approximately 3–4mm of carbonised wood.

The caramelised wood sugars layer (toasted zone beneath char)

Beneath the char layer, the heat of charring creates a caramelised zone — wood sugars converted to complex brown-coloured compounds including caramelans and humins. This zone is the primary source of Bourbon's characteristic vanilla, caramel, and toasted grain notes. The key chemical markers: vanillin (vanilla), syringaldehyde (smoky vanilla), and guaiacol (smoke) are all extracted from this zone as ethanol-water solution penetrates the stave during warm seasons.

The intact wood layer (outer stave)

The outer portion of the stave, unaffected by char, contributes tannins (ellagitannins from oak) and lactones (oak lactones, particularly cis- and trans-3-methyl-4-octanolide — the "coconut" note in new American oak). American white oak (Quercus alba) contains significantly lower ellagitannin concentrations than European oak (Quercus petraea/robur), but much higher concentrations of oak lactones — explaining the distinctively coconut-vanilla-heavy character of American oak maturation versus the more tannic, drier character of European oak.

Sour mash — the process, not a category

Sour mash is a production technique widely used in American whiskey production but not a legally defined category under TTB regulations. In sour mash fermentation, a portion of the spent grain and liquid from a previous fermentation (the "backset" or "setback") is added to the new fermentation batch — typically 20–25% of the new mash volume. The backset is acidic (pH approximately 3.5–4.5), which lowers the pH of the new fermentation to approximately 5.0–5.5.

This pH control serves documented functions: inhibiting bacterial contamination (most spoilage bacteria cannot survive at pH below 5), providing a consistent chemical environment for yeast activity, and contributing to batch-to-batch flavour consistency over time. Virtually all major Kentucky Bourbon and Tennessee Whiskey producers use sour mash. "Sweet mash" — fermentation without backset addition — is used by a small number of craft producers to produce lighter, cleaner-flavoured spirits, as documented in TTB industry guidance.