How American Whiskey is made

An illustrated description of bourbon making

Whisky is whiskey? Not quite. American bourbon is based on the same production principle as Scotch whisky or Irish whiskey, but small but subtle differences have developed over the years. Local conditions in particular have given bourbon its distinctive character. If you want to know exactly how bourbon whiskey is made, you've come to the right place. On the following pages you will find everything about the production process. However, the result of each distillery tastes different. If you ask a master distiller of a bourbon whiskey distillery which influences in the production have the strongest effect on a good bourbon, you will get the following answer: The grain, the yeast strains, the fresh white oak barrels and their storage have the greatest influence on the taste of the bourbon whiskey. What is the reason for this statement, which now contains nothing about the details of the production process, but rather ingredients? The answer to this question is as simple as it is convincing. American distilleries generally have the same production equipment and the same climate. So the differentiator between bourbon producers falls on a different level.

A word about Tennessee whiskey. For marketing reasons, Tennessee Whiskey has separated itself as a Southern whiskey from Kentucky Straight Bourbon and the Bourbon Act (Act). However, except for additional filtration through charcoal prior to barrel filling, the production process of these two types of whiskey is identical. Below is a description of the production process in its chronological order:

  1. Grain selection and blending (Mash Bill)
  2. Water
  3. Grain cooker(Cooker)
  4. Yeast preparation
  5. Alcoholic fermentation
  6. Distilling process
  7. Sour Mash
  8. Animal feed
  9. Filling into barrels
  10. Storage
  11. Bottling

Grain selection and mixture (Mash Bill)

Each distillery has its own recipe for the corn blend. The legal requirement for a Kentucky Straight Bourbon Whiskey is a corn content of at least 51%. As a rule, however, the corn content is significantly higher and lies between 60 and 80%.

The other components of the grain blend are rye (rye) and malted barley. The latter two grains make up about 10 to 15% each. Few distilleries add wheat to their grain blend (e.g.: Bernheim or Maker's Mark). Wheat makes a whiskey softer and smoother on the tongue. If wheat is added, the proportion is usually around 10%.

The different types of grain are ground separately and stored temporarily. The mills used to be hammer mills until it was discovered that the grain heats up too much and the taste suffers. Today, the grain is usually crushed to open the grain hull. The subsequent milling process results in a very fine flour.


Whiskey production requires fresh spring water to boil the starch in the grain and to dissolve the resulting sugar. When the distilleries were founded and settled, particular importance was therefore attached to a spring with sufficient water output.

Kentucky and Tennessee lie on a large layer of limestone, which filters the water excellently. In valleys where the limestone layer is broken through, springs usually emerge that carry wonderfully clear water that is ideally suited for whiskey production.

Cereal cooker (Cooker)

The cereal grain mainly contains starch. In small quantities, the cereal grain also contains protein, fat and trace elements. From a chemical point of view, starch is a poly-sugar. Many sugar molecules are linked together in chains. Every type of grain germinates and can convert its starch into sugar and finally cellulose in this natural malting process. However, this is not always easy to do technically for all types of grain. Only barley has a very good ability to convert starch into sugar during germination with the help of an enzyme.

For maize, rye and unmalted barley, they have therefore come up with something different. These grains are usually cooked for about half an hour. With Early Times, you cook under slight overpressure, at a higher temperature, to reduce the cooking time to about 25 minutes.

1.Maize114°C/220°FLongest cooking time (with overpressure)
2.Rye77°C/170°Fmedium cooking time
3.Barley66°C/150°Fshortest cooking time (sensitive)


The grain mixture is then cooled before the yeast is added to the fermenter.

Yeast processing

Every distillery in Kentucky and Tennessee has its own yeast strains that survived Prohibition from 1919 to 1933 in refrigerated rooms. The yeasts are so closely guarded as a secret that the companies have applied for patents for their isolated yeasts. Unlike in Germany, this is possible in the United States of America.

The selection of yeast in history was done in a simple way. One placed vessels with nutrient solution under an apple or pear tree and waited until natural yeasts on the fruit also accepted the nutrient solution. Then one placed small samples of the nutrient solution on carriers and incubated the individual presenting yeasts in temperature-controlled ovens (35 to 40°C). From a single yeast cell, one can then propagate the entire yeast for alcoholic fermentation.

The amount of yeast corresponding to the tip of a pencil is taken from a test tube. This yeast quantity is mixed with nutrient solution (malt extract) in a glass flask. Important for the propagation of the yeast is the correct pH value of the solution, which must be between 5.4 and 5.8 depending on the yeast.

All objects in which the yeasts are propagated must first be sterilised in an autoclave so that vinegar bacteria or foreign yeasts do not displace the desired yeast strain.

Early Times - Yeast Tanks
Early Times - Yeast Tanks
Early Times - Yeast tank from the inside (before propagation)
Early Times - Yeast tank from the inside (before propagation)
Early Times - Yeast tank from the inside (strong propagation)
Early Times - Yeast tank from the inside (strong propagation)

After about half a litre of pure yeast has been produced in this glass flask, this yeast is added to a larger vessel, the so-called 'Dona Tub'.

Distilleries usually produce different bourbons with different yeasts. There is therefore a separate yeast tank for each whiskey produced. The propagation of the yeasts via the Dona therefore only takes place from time to time when the yeast stock has been contaminated by acetic bacteria or foreign yeasts.

Alcoholic fermentation

After the grain mixture from the cooker has cooled down to about 25-30°C, it can be added to a fermenter together with a larger amount of yeast, in which a beer with about 9% alcohol by volume is produced by fermentation. At this point, parts of the stillage that is later produced are also added. The stillage is part of the much vaunted Sour Mash process, which will be explained in detail later.

Early Times - Large-scale fermenter
Early Times - Large-scale fermenter
Cypress wood fermenter
Cypress wood fermenter

The size of the fermenters varies from 'large' to 'huge' depending on the distillery. Since the continuously operating distillation columns can process large quantities of beer, they must be constantly replenished so that the distillation columns do not run empty.

The Beer Well

For this reason, American distilleries have a so-called 'Beer Well' into which the fermented fermenter contents (beer) are emptied. The Beer Well is usually made of stainless steel and is located in the middle of the numerous fermenters. The size of the Beer Well is determined by the size of the fermenters. As a rule, the Beer Well is one third larger than the largest fermenter so that the distillation columns can continue to produce even if the contents of a fermenter are pumped over late.

During alcoholic fermentation, yeasts process the sugar into alcohol and CO2. At the same time, heat is released. Fermentation usually takes three days. An alcohol content of about 8 to 9.5% vol. is achieved. A few distilleries (e.g. Labrot & Graham) ferment considerably longer, but do not achieve higher values than 10 to 11% vol. either. The resulting product is called Beer or Distiller's Beer.

The larger the fermentation vessel, the more it heats up. This is due to the fact that the ratio of surface area to volume decreases as the size of the fermenter increases and the fermenter is therefore no longer able to dissipate the heat. If the temperature in the fermentation tank exceeds 35 to 40°C, the yeasts gradually die until fermentation stops. Many fermentation tanks therefore have water cooling.

The result of the fermentation is already evaluated at distilleries like Early Times or Four Roses. The smell of the beer says a lot about the later whiskey. The brewer attaches importance to an aromatic beer, which can smell strongly of apple, for example. If the aroma of the beer begins to diminish, this indicates that the yeast is contaminated and a new yeast strain is used for the next batch. After alcoholic fermentation, the beer is distilled into raw whiskey, the White Dog.

Firing process

With one exception, the distillation process at all American whiskey distilleries (exception: Labrot & Graham) takes place on a column still (distillation column). The column-shaped distillation column allows continuous dist illation and was invented by Robert Stein (Haig Company) in Scotland in 1826. The basic principle of operation is simple: a vertical tube is set up, 5 to 20 metres high and 70 to 150 cm in diameter. Perforated bottoms are inserted into this pipe so that a connection is made from the bottom to the very top. The edges of the holes are bent up a little so that any liquids cannot run through directly at the bottom. In addition, small tubes are inserted so that the liquid collecting on the bottoms can drain off to the next bottom.

If you now fill the column with beer in the middle position and heat the column from below, two opposite flows will result. The liquid beer flows downwards through the tubes, whereas the gaseous components (alcohol vapours) flow upwards through the holes.

The temperature of the column is adjusted so that alcohol is gaseous at the very top (78 - 85°C) and the beer is boiling at the bottom (95 - 100°C). This process continues indefinitely as long as enough beer is added to the column. While the alcohol is being removed from the top, the water with the fibres and remains of the grains falls to the bottom of the column. This product is called 'stillage' and is processed into animal feed and added back to the fermentation process as sour mash.

Small distillation columns yield an alcohol content at the top of about 120 American proof (60% alcohol by volume). If the columns are higher, the alcohol content can be increased even further. Up to 80% vol and more.

After the alcohol is removed from the distillation column, the vapour is passed through the doubler, a copper container. In it, a catalytic conversion takes place that improves the taste of the whiskey. This doubler is necessary in many distillation columns because the equipment, or the column bottoms, are not made of copper. In copper pot stills, as in Scotland, a constant catalytic conversion takes place during distillation. In column distillation plants without copper components, extra contact of the distillate with copper must be ensured. These doublers were developed for this purpose.

The raw whiskey, which the Americans call 'white dog', is produced in the downstream condenser.

The alcohol vapour that is liquefied again in the condenser is fed via an alcohol vault into collection containers from which either the barrels are filled or trucks are loaded for transport.

The White Dog is regularly tasted directly after production, for which it is diluted to about 20% alcohol by volume. In this state, the aromatic substances can be best assessed.

Four Roses - Tasting of the White Dog
Four Roses - Tasting of the White Dog

Per bushel of grain (35.24 litres), about 5 US gallons of 100 proof spirit are produced (9.5 litres of pure alcohol). If you convert this with the weight of the grain, you get about 400-450 litres of pure alcohol from one tonne of grain.

Sour Mash

All American distilleries today work according to the Sour Mash process. Sour mash means that part of the distillation residue (stillage) is added back to the mash. After the mash has been prepared with fresh spring water, it is chemically neutral. This means that the chemical milieu is neither acidic nor basic.

Chemically, this property is evaluated with the pH value. Neutral is a solution with a pH value of 7. In the case of an acid, the pH value is less than 7. Strong acids have a pH value of 3 to 4. If the pH value is greater than 7, one speaks of an alkali (base). Strong alkalis have a pH value of 10 to 11.

After the addition of neutral water, the mash has a pH value of about 7 (neutral), in which the yeasts cannot work optimally. The addition of part of the very acidic stillage (pH 5.0 - 5.4) leads to an acidification of the entire mash. The pH value after the addition of the stillage is about 5.4 to 5.8. Sour enough for the yeasts to do their work optimally. Some bourbon producers try to give their product a special addition with the term 'Sour Mash'. Other sources speak of a disinfecting effect of the sour mash process on the mash. The sole purpose of sour mash is to set a pH value that is favourable for the yeasts. The rest of the sour mash story belongs to the realm of marketing.

Pet food

The so-called setback accumulates at the bottom of the distillation column. It consists of water, protein, fat and the fibres of the mash, while the more volatile substances such as alcohol and esters are discharged over the top of the distillation column. This aqueous solution at the bottom of the distillation column is pumped out from time to time from the container below the distillation column.

First, the fibre portion of the stillage is collected by sieves. The remaining liquid portion is dried in large drums with the help of superheated steam. The solid components that adhere to the wall of the drum are then mechanically stripped off and sold to farmers as cattle feed. The cattle feed is very rich in protein and fat and also contains a large amount of trace elements.

First, the fibre portion of the stillage is collected by sieves. The remaining liquid portion is dried in large drums with the help of superheated steam. The solid components that adhere to the wall of the drum are then mechanically stripped off and sold to farmers as cattle feed. The cattle feed is very rich in protein and fat and also contains a large amount of trace elements.

Filling into barrels

In Kentucky and Missouri, companies have specialised in the production of barrels made from American white oak. The barrels may only be used once for Straight Bourbon Whiskey. They have a capacity of about 53 American gallons (approx. 200 litres). The term barrel is very often used in the American context. There are various definitions of a barrel. The most famous barrel is the petroleum barrel with a volume of 158.97 litres. However, the normal American barrel contains 31 ½ gallons = 119.23 litres. 1 gallon is equal to 3.785 litres.

The staves of the barrels are first joined together and not yet completely closed. Then the staves are made pliable with superheated steam and can be bent into their oval shape.

But now a special feature takes place that gives the Kentucky Straight Bourbon its special taste. The barrels, still open on one side, are held over a light fire. This process is called toasting and causes the wood sugar in the staves to caramelise to a boundary layer on the inside. This reddish layer is also clearly visible later when a barrel is taken apart. The toasting process takes about 12 min.

After the toasting process, the barrel is subjected to a much stronger fire treatment. It is burnt out from the inside with a large flame for 6 to 12 seconds. This gives the barrel a charcoal layer on the inside. When making barrels, the thickness of the charcoal layer is specified (grades 1 - 4).

After filling, the barrel is sealed with a 'bung' and transported by truck to the warehouses.


Each distillery has found its own arguments and preferences for the different ways of storing the barrels. In the past, warehouses were built that were 4 to 5 storeys high. Between 3 and 6 layers of barrels are stored on each floor.

Wild Turkey - Warehouse Skeleton
Wild Turkey - Warehouse Skeleton

For this purpose, the warehouses have a skeleton of beams and girders that allow the barrels to roll horizontally. Between this skeleton there are lifts that allow the vertical movement of the barrels. An ordinary warehouse has a capacity of around 20,000 barrels.

A very special climate is created in these warehouses. Under the roof, temperatures are very high in the summer, whereas on the ground it remains as cool as in an air-conditioned room. To allow the temperature to equalise with the ambient air, a warehouse has many windows that are opened when needed.

The whiskey matures very differently on the floors. In the past, therefore, the barrels were usually rotated. Rotating the casks involves moving them around the warehouse in predetermined, different positions during storage so that each cask gets the benefit of the good positions in the middle of the warehouse. However, for rotating, a certain part of the warehouses must remain empty. Usually this is 1/3 of the capacity. Maker's Mark was one of the few distilleries that still rotated their casks in 1999.

Other distilleries have taken a different path. They deliberately no longer rotate, but mix barrels from different positions in the warehouse before bottling. This approach saves the labour-intensive rotation of the barrels, makes full use of the warehouses and gains a region in the centre of the warehouse where excellently matured barrels for small batch bourbons and single barrel bottling can be found.

In 1996, several warehouses at the Heaven Hill distillery burned down. The burning whiskey made its way through the distillery down to the river. As a result, the distillery also became a victim of the flames and the whiskey continued to burn on the river all night.

Today, attempts are made to avoid the risk of fire by installing a sprinkler system in every warehouse and by building the warehouses at a safe distance from each other. As passive safety, the wooden skeleton is designed in such a way that the warehouse collapses in the event of a fire and does not damage other warehouses.


Bourbon distilleries have such a large output that all distilleries can afford their own bottling plant. Even small distilleries like Maker's Mark and Labrot & Graham have their own facilities.

First, the selected barrels are brought to the filling plant by truck and emptied through sieves. The charcoal pieces that have come loose from the barrel wall during storage are retained by the sieve.

Jim Beam - Barrel emptying
Jim Beam - Barrel emptying

The bourbon is pumped into a storage tank and from there a bottling line is fed.

American whiskey is bottled in a wide variety of bottle sizes, shapes and materials. Typical sizes are 0.2 litre (flat bottle), 0.7 litre (Europe), 0.75 litre (US market, Japan), 1.0 litre (travel value), 1.89 litre (1/2 gallon). The bottles are increasingly made of plastic (PET) for mass products.