Sour mash is a process in the distilling

Distillation

Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

 industry that uses material from an older batch of mash

Mashing

In brewing and distilling, mashing is the process of combining a mix of milled grain , known as the "grain bill", and water, known as "liquor", and heating this mixture...

 to start fermentation

Fermentation (food)

Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...

 in the batch currently being made, analogous to the making of sourdough

Sourdough

Sourdough is a dough containing a Lactobacillus culture, usually in symbiotic combination with yeasts. It is one of two principal means of biological leavening in bread baking, along with the use of cultivated forms of yeast . It is of particular importance in baking rye-based breads, where yeast...

 bread. The term sour mash can also be used as the name of the type of mash used in that process, and a whiskey made using this process can be referred to as a sour mash whiskey. Sour mash does not refer to the flavor of the whiskey, as is sometimes thought.

The sour mash process was developed by either Dr. James C. Crow

James C. Crow

Born in Inverness, Doctor James C. "Jim" Crow was the Scottish creator of the sour mash process for creating Bourbon Whiskey.Dr. Crow graduated in medicine from Edinburgh University in 1822. He moved from Philadelphia to Kentucky in 1823 and began working for a distiller, bringing his scientific...

 or Dr. Jason S. Amburgey while they were working at the Old Oscar Pepper Distillery (now the Woodford Reserve Distillery

Woodford Reserve

Woodford Reserve is a brand of premium small batch Kentucky Straight Bourbon Whiskey produced by the Brown-Forman Corporation. It is distilled at the company's Woodford Reserve Distillery, and marketed under the former company name for the distillery, Labrot & Graham. The distillery is located in...

) in Woodford County

Woodford County, Kentucky

Woodford County is a county located in the heart of the Bluegrass region of the U.S. state of Kentucky. As of 2000, the population was 23,208. Its county seat is Versailles. The county is named for General William Woodford, who was with General George Washington at Valley Forge...

, Kentucky

Kentucky

The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...

.

In the sour mash process, the mash – a mixture of grain

GRAIN

GRAIN is a small international non-profit organisation that works to support small farmers and social movements in their struggles for community-controlled and biodiversity-based food systems. Our support takes the form of independent research and analysis, networking at local, regional and...

, malt

Malt

Malt is germinated cereal grains that have been dried in a process known as "malting". The grains are made to germinate by soaking in water, and are then halted from germinating further by drying with hot air...

 and water – is conditioned with some amount of spent mash (previously fermented mash that still contains live yeast). Spent mash is also known as spent beer, distillers' spent grain, stillage, and slop or feed mash, so named because it is used as animal feed. The acid introduced by using the sour mash controls the growth of bacteria that could taint the whiskey and creates a proper pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 balance for the yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 to work. An established and active strain

Strain (biology)

In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...

 of live yeast is introduced into the mash that is to be fermented. By using an established and known fermented "sour", this fermentation process controls the introduction and growth of foreign bacteria and yeasts that could damage the whiskey, and improves the consistency and quality of the liquor, so that every bottle tastes as close to the same as possible.

The sour mash process is used for the production of nearly all Bourbon

Bourbon whiskey

Bourbon is a type of American whiskey – a barrel-aged distilled spirit made primarily from corn. The name of the spirit derives from its historical association with an area known as Old Bourbon, around what is now Bourbon County, Kentucky . It has been produced since the 18th century...

 and Tennessee whiskey

Tennessee whiskey

Tennessee whiskey is Straight Bourbon Whiskey produced in the state of Tennessee. This definition is legally established under the North American Free Trade Agreement and at least one other international trade agreement that require that Tennessee whiskey be "a straight Bourbon Whiskey authorized...

.