What Is Bourbon? A Guide to America’s Original Whiskey

Whether savored neat or mixed into an Old Fashioned or Mint Julep, bourbon may be the spirit most often associated with the United States. Designated “America’s Native Spirit” by a 1964 Congressional resolution, bourbon is nearly as old as the U.S. itself, with origins that date to the American Revolution, and likely further. The primarily corn-based whiskey has since become the stuff of songs, legends, and plenty of cocktails.

From common terminology like single-barrel and sour mash to designations like bottled-in-bond and small batch, here are the answers to some of your most commonly asked questions about bourbon.

What Defines Bourbon?

Bourbon is a type of primarily corn-based whiskey that is made in the United States and aged in new charred-oak barrels. Although it legally must contain 51% corn in the mash (or total makeup of grains), most bourbons are often made with anywhere from 60–86% corn. The other grains in a bourbon can vary, but may include rye, wheat, or barley. There are no regulations as to how long bourbon must age, but to be labeled straight bourbon it must age for at least two years in new charred oak.

What’s the Difference Between Bourbon and Other Types of Whiskey?

Bourbon is a subcategory of whiskey, which is a catch-all term for a spirit that is distilled from a fermented grain mash, often stored in barrels before bottling. Bourbon legally must be distilled with a minimum of 51% corn. Whiskey can be made anywhere in the world, whereas bourbon is only produced in the United States. Compared to other types of whiskey, bourbon typically shows a sweeter profile, with notes of caramel and vanilla. The charred oak in which it ages has compounds like vanillins, lactones, and tannins, which are imparted into the liquid, giving bourbon its characteristic profile and color. 

Bourbon usually has a heavier body and a sweeter, less spicy profile than rye whiskey (which also must be made in new charred oak barrels, but with at least 51% rye content, rather than corn). It is very similar to Tennessee whiskey, which also must be made from a mash bill that contains 51% corn and aged in new charred oak barrels. However, Tennessee whiskey must be made in Tennessee and typically undergoes a charcoal filtering process before maturation that produces a slightly mellower profile. 

Bourbon vs. Tennessee Whiskey: The Main Differences

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How Is Bourbon Made?

While there are some variations in individual producers’ styles and distillation techniques, most bourbon follows a similar process.

1. A master distiller determines the bourbon’s mash bill, or recipe that details the proportion of grains to be used. These grains are milled and then combined with water, before being heated. As the mash cooks, enzymes convert the grain’s starches into fermentable sugars. After cooling, yeast is added to kickstart fermentation.

2. Distillers store the mash in a vat from one to two weeks and allow it to fully ferment, which creates ethanol, or alcohol. Notably, unlike other global styles of whiskey, the entire mash is fermented—grain solids included—rather than the liquids being siphoned off beforehand. Another signature is the sour mash, a common practice in which a small portion of residual grain material from an older batch of bourbon is included in the new bourbon mash’s fermentation, along with additional yeast. 

3. The mash is then transferred to a still. While some whiskey producers will strain off solids and only use the resulting liquid, called distiller’s beer or wash, others distill the mash on-grain.

4. Distillation occurs. Most bourbon is distilled twice, first in a column still (or Kentucky-style beer still), then in a copper pot still. Once it reaches 80–125 proof, distillers place the spirit in new charred oak barrels for aging (at least two years for straight bourbon, at least four years for bottled-in-bond bourbon, and often longer).

5. Distillers may dilute the product with water before and/or after distillation to reach their desired strength.

6. The final bourbon whiskey is bottled.

Bourbon History

In the late 18th century, immigrants from countries including Scotland, Ireland, France, and Germany increasingly relocated to the U.S., bringing with them distilling techniques that they applied to native North American agricultural products like rye and corn. 

Whiskey became particularly popular after the American Revolution, as decreased sugar and molasses imports from Britain throttled rum production. The whiskey industry thrived in Kentucky for a multitude of reasons. Slave labor in the state factored heavily into spirits production, while the state’s network of waterways, like the Kentucky, Mississippi, Ohio, and Tennessee Rivers fueled distribution. Kentucky’s fertile soil was also conducive to growing corn, while a climate that included hot summers and cool winters proved well-suited to the barrel-aging process. 

Although the origins of bourbon as we know it today are largely unknown, Kentucky producer Dr. James C. Crow has been loosely credited with introducing the sour mash technique in the 19th century, in which a small portion of older mash is added to a new mash to help prevent bacterial spoilage. 

Whatever the true origins, the term “bourbon” began appearing in newspaper advertisements by the 1820s. Throughout the next century, legislation like the Bottled in Bond Act of 1897 and orders like the Taft Decision of 1909 helped to codify bourbon regulations and vernacular. Although Prohibition and World War II presented setbacks, the Kentucky bourbon industry rebounded, and by 1963 bourbon was the top-selling spirit in the United States. In 1964, Congress declared bourbon a “distinctive product of the United States,” limiting its production to the U.S. 

Common Bourbon Designations

Bourbon has several sub-classifications, which denote the style and processes used to create it. Here are some of the more common designations you’re likely to find listed on a bottle. 

Bottled-in-bond

Bottled-in-bond originated as a consumer protection law in 1897, when Congress passed the Bottled in Bond Act to sideline producers who were adulterating their product. The designation denotes a spirit made under particularly stringent regulations. These rules stipulate that the product must be crafted by one distiller at one distillery during a single season, bottled at exactly 100 proof (50% ABV), and aged in a federally bonded warehouse for at least four years. The bottled-in-bond label has become more popular in recent years, particularly with higher-proof bourbon coming back into fashion.

Straight Bourbon

Straight bourbon must be aged for at least two years in charred new oak. An age statement is required if the liquid ages for fewer than four years. Straight bourbon may include blends of two or more bourbons, provided they are produced in the same state.

High rye

Most bourbons rely on rye as the secondary grain, but high-rye bourbons have a particularly high proportion of rye in the mash bill. Though there are no legal standards for a bourbon to be classified as high rye, offerings with this label will generally have a slightly spicier profile, though not as much as rye whiskey. 

Single barrel

These bourbons have been bottled from a single cask, which has not been blended (or vatted) with whiskey from other barrels. Though often highly sought after by aficionados, these offerings will have a less consistent profile than non-single-barrel options.

Small batch

This bottling is made by combining the liquids from a small number of selected barrels, ensuring consistency across bottlings. However, the term is largely unregulated, meaning the actual number of barrels from which the whiskey is sourced could range from single digits to thousands.

Wheated, or wheater

Wheated bourbon is made with a mash bill that contains a higher-than-normal proportion of wheat. The mouthfeel is often described as soft, rounded, and creamy. 

Bourbon Glossary

The full range of bourbon, whiskey, and distilling terms can be staggering. Consider the glossary below a starting point to understanding some of the terminology used to describe the spirit, as well as various aspects surrounding its production.

ABV
ABV, or alcohol by volume, is a measure of how much alcohol, or ethanol, is contained in a liquid. Bourbon must contain a minimum of 40% alcohol by volume (80 proof). In the U.S., a bottle’s proof is simply twice the stated ABV number.

Aging
Bourbon is aged in new charred oak barrels after it is distilled. Bourbon generally does not have a legal requirement for length of aging, but those labeled straight bourbon must be aged for at least two years.

Age statement
An age statement tells how long the whiskey has aged in a barrel. An age statement is only required for bourbon if it has aged for less than four years. In the case of blends, a bourbon’s age statement must state the age of the youngest whiskey included in the blend.

Angel’s share
Angel’s share is the distillate that evaporates through the wood from the cask during maturation. As the whiskey matures, the angel’s share decreases. Although the amount allowed is not codified, Kentucky bourbon typically has a higher angel’s share than Scotch whisky.

Barrel-entry proof
This refers to the proof of the bourbon when it enters the barrel, which can be up to 125 proof (62.5% ABV). Bourbon may be distilled at a maximum of 160 proof, after which distillers will “proof it down” with water before it goes into the barrel. 

Batching/Mingling
Batching refers to combining whiskey from different casks to ensure a consistent flavor profile across barrels.

Beer still
A beer still, also known as a column still, includes linked chambers separated by perforated plates. Most bourbon is distilled twice, first in the beer still, then a second time in a copper pot still. 

Cask finishing
Whiskeys are often transferred from one cask to another for a secondary, shorter period of maturation. For example, a bourbon may be matured in new American oak but finished in a cask that formerly held oloroso sherry or rum to impart additional flavors.

Cask strength
Cask strength refers to a spirit that has not been diluted with any additional water after going into the barrel. Cask-strength bourbon will generally have an ABV between 60 and 65%. 

Charring
All bourbon is aged in charred oak. Charring refers to exposing the interior of barrels to open flames. The amount of time that the oak is seared will determine its char, or toast, level. The wood may be exposed from 40–60 seconds to as long as three minutes. As barrels char, the wood sugars become caramelized, producing compounds like vanillins, lactones, and tannins that influence the profile of the whiskey stored within it. 

Cooper/Cooperage
A cooper is a person who makes wooden casks, barrels, vats, and similar vessels from staved timber. Cooperage refers to the trade, or the facility in which the cooper makes their barrels. 

Distiller’s beer
Also known as the wash, the distiller’s beer is the fermented liquid that goes into a still. 

Distillation
While fermentation creates low levels of alcohol from a sugar-rich liquid base, distillation is the process in which this alcohol is removed and consolidated into a stronger spirit. Using single batch pot stills or continuous column stills, distillers heat the wash to the point at which the alcohol converts to vapor, separating itself from the other ingredients in the still. This vapor rises to the top of the still, before being condensed back into a higher-proof liquid form, creating the distilled spirit. 

Distillation proof
This refers to the strength of bourbon when it comes out of the still, which can be up to 160 proof (80% ABV). It is often “proofed down” with water before being transferred to barrels. 

Doubler/Thumper
Also known as a pot still, a doubler or thumper is used for the second distillation of whiskey. Its name comes from the distinct sound that heated steam creates in the still.

Expression
An expression is a different version of whiskey from the same producer. Variations could include the age, distillation process, or barrel type. 

Malting
Malting refers to germinating sprouted grains, such as barley, before stopping the process by drying the grains and adding them to the mash bill. This helps convert starches to sugars and enzymes that contribute to the fermentation process.

Mash
The mash is a slurry made from the unfermented grains and water, used to create a whiskey. 

Mash bill
A mash bill is the recipe for the composition of grains in a whiskey. In the case of bourbon, the mash bill must contain at least 51% corn.

Pot still
Most bourbon is twice-distilled. Typically, the spirit undergoes its second distillation in a single-batch copper pot still, also known as a doubler or thumper. 

Proof
Proof is a statement of the measure of ethanol, or alcohol content, in a beverage. In the U.S., it is twice the numerical value of the alcohol-by-volume, or ABV. Bourbon must be bottled at no less than 80 proof (40% ABV).

Rackhouse/Rickhouse
A rackhouse is a warehouse where barrels of whiskey are stored. 

Sour mash/Setback
Sour mash is a small portion of used, older mash that is added to a new bourbon mash, helping to ensure consistency across batches and lowering the pH of the new mash.

Sweet mash
Although still relatively uncommon, sweet mash is a term denoting a bourbon made in absence of the sour mash step. Distilleries will instead only use fresh ingredients and yeast to create the mash, highlighting the natural grain flavors more but running a higher risk of bacterial contamination issues.

Uncut
Uncut bourbon, also known as barrel-proof, is not filtered with water before bottling. 

Unfiltered
Bourbon is usually chill-filtered when it comes out of the barrel to remove any sediment. Unfiltered whiskey, on the other hand, is not chill-filtered. Advocates say this helps to preserve flavors that are sometimes stripped in the filtering process. 

Wash/Wort
Also known as the distiller’s beer, wash or wort is the fermented liquid that goes into a still. 

Washback
A washback is the container in which the distiller’s beer, a beer-like liquid formed from a mixture of wort and sour mash, is created. It is typically made of sturdy materials like wood or stainless steel. 

White dog
White dog, sometimes called white whiskey, is the raw, clear, and unaged whiskey distillate that comes out of a still and has spent no time aging in a barrel after distillation.