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What is bourbon? Rules, mash bills and styles explained
A complete guide to bourbon whiskey: legal rules under 27 CFR Part 5, mash bill grains, straight bourbon, bottled in bond, and how it differs from Scotch.
Bourbon is America’s native spirit, defined by strict statutory standards, grain recipes, and virgin oak maturation. Unlike Scotch single malt, which relies on 100% malted barley and aged refill casks, bourbon is driven by the natural sweetness of corn and the potent extractives of freshly charred American white oak.
Whether exploring American whiskey for the first time or comparing it to single malt Scotch, understanding bourbon requires examining its legal standards, mash bill grain ratios, and warehouse ageing dynamics.
The legal rules of bourbon
In 1964, the United States Congress declared bourbon a “distinctive product of the United States.” Today, its production is strictly governed under Title 27 of the Code of Federal Regulations (27 CFR Part 5):
- Made in the United States: Bourbon must be distilled in the United States or its territories. It does not have to be made in Kentucky, though Kentucky produces the vast majority.
- Corn-dominant mash bill: The grain mixture (the mash bill) must contain at least 51% corn. The remaining 49% consists of secondary grains, typically rye or wheat, along with malted barley.
- Distillation limits: The spirit must be distilled to no higher than 80% alcohol by volume (160 proof). Distilling below this threshold ensures that distinctive grain flavours and aromas carry over from the fermented mash into the white dog distillate.
- Barrel entry proof: The spirit must enter the barrel at no higher than 62.5% ABV (125 proof), preserving a balanced water-to-alcohol ratio for optimal wood extraction.
- New charred oak containers: Bourbon must mature in new charred oak containers (almost universally American white oak barrels). Used casks may not be used for primary bourbon maturation.
- Bottling strength: Bourbon must be bottled at a minimum strength of 40% ABV (80 proof).
- No additives: Straight bourbon allows zero added colouring, caramel (E150a), or flavouring agents.
Decoding bourbon label terms
American whiskey labels carry specific statutory designations that tell you precisely how the liquid was made:
Straight Bourbon
To carry the designation Straight Bourbon, the spirit must be aged in new charred oak containers for a minimum of two years. If a straight bourbon is matured for less than four years, it must state its exact age on the label (referencing the age of the youngest whiskey in the bottle). If it is four years or older, an age statement is optional.
Bottled in Bond
Originating from the historic Bottled-in-Bond Act of 1897, this legal quality stamp was created to protect consumers from adulterated spirits. To be labelled Bottled in Bond (or Bonded):
- The whiskey must be the product of one distillation season (January to June, or July to December) by a single distiller at a single distillery.
- It must be aged in a federally bonded warehouse under US government supervision for at least four years.
- It must be bottled at exactly 50% ABV (100 proof).
Single Barrel
Indicates that every bottle in that batch was filled from an individual oak barrel, rather than married across dozens or hundreds of casks. Because each barrel interacts differently with temperature, humidity, and airflow, single barrel bourbons display subtle variations from one barrel to the next.
Small Batch
Unlike “Straight” or “Bottled in Bond”, Small Batch is not legally defined by federal statute. Distillers use it to indicate a whiskey married from a curated selection of barrels (ranging from a dozen to several hundred casks) chosen for a specific profile.
The three primary mash bill styles
The grains in a bourbon mash bill dictate its flavour architecture:
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| Mash bill style | Secondary grain | Flavour profile | Benchmark examples |
|---|---|---|---|
| Traditional Bourbon | 10%–15% Rye | Sweet caramel, vanilla, subtle baking spice | Buffalo Trace, Jim Beam, Wild Turkey |
| High-Rye Bourbon | 18%–35% Rye | Peppery bite, mint, clove, dry oak spice | Bulleit, Woodford Reserve, Four Roses |
| Wheated Bourbon | 10%–20% Wheat | Soft honey, vanilla fudge, sweet bread, red fruit | Maker’s Mark, Larceny, Weller |
In all styles, malted barley makes up roughly 5% to 12% of the recipe. Barley is included primarily because its natural enzymes (alpha- and beta-amylase) convert grain starches into fermentable sugars during mashing.
How climate drives maturation
The maturation of bourbon differs fundamentally from Scotch whisky due to oak and climate:
- Virgin oak: New charred barrels impart high concentrations of wood sugars, vanillin, lactones, and tannins very rapidly compared to refill casks used in Scotland.
- Thermal cycling: In states like Kentucky and Tennessee, summer warehouse temperatures can exceed 45°C, causing the spirit to expand deep into the porous oak staves. In freezing winters, the liquid contracts back out, drawing deep colour and rich caramelised sugars into the whiskey.
- The angels’ share: In dry, hot American rickhouses, water often evaporates faster than alcohol, causing the ABV of the maturing whiskey to rise over time, whereas Scottish humidity typically lowers spirit strength during cask ageing.
Exploring bourbon
To compare current UK retailer offers across the category, visit our bourbon style hub, or read our beginner recommendations in best bourbon for beginners.
Sources and review
Questions
Does bourbon have to be made in Kentucky?
What makes a whiskey legally classified as bourbon?
Can bourbon contain added colour or flavouring?
What is the difference between bourbon and Tennessee whiskey?
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