Bourbon Whiskey

Bourbon (bou’r’bon) – 1. “The Whiskey That Made Kentucky Famous” and 2. a whiskey distilled primarily from corn and aged in charred oak barrels.

Distilling Process

Bourbon whiskey[1] distilling has remained fundamentally unchanged for the past two hundred years.  Distilling can be broken down into four steps.  Industry terms and modern legal requirements are highlighted in bold and italics, respectively.

Step One – Raw Materials:

Each distiller has a grain or mash bill made of different proportions of distiller’s grains - corn, rye, wheat and malted barley.  The exact formula is often a family secret (passed down through generations) and is proprietary.  Bourbon whiskey must contain at least fifty-one (51%) percent but not more than eighty (80%) percent of corn.  In most cases corn will represent around seventy (70%) to eighty (80%) percent of the grain bill.  For rye whiskey, the corn is replaced by rye.

The bulk grains are fed into a roller mill, where they are ground into a coarse meal.  By law no coloring or flavoring agents can be added to the natural ingredients.

Step Two – Mashing & Fermentation:

The meal is mixed with limestone water and cooked at around one hundred twenty degrees in a large vessel called the mash tub.  Limestone water is pure, naturally filtered through the region’s limestone basin - high in calcium (hardness) and free of iron.  This is ideal distilling water.  As the mixture is cooled, the distiller adds malted barley.  This cooked liquid is known as sweet mash.  This process converts the starches contained in the grains into fermentable or soluble sugars.

The distiller adds yeast to the sweet mash.  Yeast converts the soluble sugars into alcohol during fermentation over the next three to five days.  This process takes place in fermentation tubs.  For the sour mash process, a portion of the previous mash is added to the next day’s batch.  This is called setback.  This preserves the flavors of the whiskey by blending a batch with the one before and then the one after.  This maintains a consistent pH level.  After fermentation is completed, the finished liquid - distiller’s beer - is then consolidated into a large tank called the beer well.

Step Three – Distillation:

The distiller’s beer is then heated to near boiling and passed through the whiskey still.  The alcohol, which is more volatile (boils at a lower temperature than water), is carried off with the steam and condensed in the condenser or still baffles.  This liquid is low proof whiskey and known as low wine.  The low wine whiskey is redistilled in the doubler into high wine.  The high wine whiskey is then adjusted to the desired proof by adding distilled water and is stored in the finished whiskey tanksProof[2] is the scale for measurement of alcohol content and denoted at exactly twice the percentage of alcohol.  One hundred proof would equal fifty percent alcohol.  For bourbon, it must be distilled at less than one hundred sixty proof and racked into barrels with an entry proof of less than one hundred twenty five.

The new whiskey (a clear liquid) is then racked into new charred white oak barrels and sealed with a wooden dowel, called a bungA barrel contains fifty-three gallons of new whiskey.  The charring converts a portion of the wood to charcoal and provides the new whiskey with its caramel color.  As whiskey ages, the harshness is slowly dissipated and the mellow flavors develop.

Step Four – Maturation or Aging:

The filled barrels are then stored in a bonded warehouse, under government supervision.  Over the first year, roughly six percent will be lost due to evaporation and leakage.  Thereafter, approximately three percent will be lost annually.  This lost whiskey is known as the angels’ shareStraight and bonded whiskies are aged a minimum of two years and four years, respectively.  Most bourbon is aged from four to eight years.  Used or seasoned barrels are not reused for bourbon but may be used for other whiskies.

After the aging period is finished, the whiskey is then withdrawn from the warehouse and the barrels are then dumped in the gauging room.  The whiskey is then filtered and any adjustments are made to the proof by adding distilled water.  The finished whiskey is bottled and the excise taxes paid.  A proof gallon is one gallon of whiskey at one hundred proof.

To qualify as Kentucky Bourbon, the whiskey must be distilled and stored in the Commonwealth for at least one year.  To qualify as Bottled in Bond, the whiskey must be distilled at the same plant where it is bottled, aged for four years and bottled at one hundred proof.  Bourbon is not bottled at less than eighty proof.  Single batch bourbon is bottled from a single barrel and small batch bourbon is bottled from a select few barrels.


[1]  Traditionally, whiskey is spelled with the “e” in the United States and whisky without the “e” in England, Ireland and Canada.

[2] Early distillers measured the strength of whiskey by mixing gunpowder with whiskey and then setting it on fire.  If the flame burned blue, it was “proven” or roughly fifty percent alcohol.  If it failed to ignite, it was too weak, and if it burned with a yellow flame, it was too strong.


William M. Ambrose, Bottled In Bond under U. S. Government Supervision, Limestone Press, Lexington, 2008.