How Bourbon Whiskey Is Made
An Illustrated Description of the Production of Bourbon
When you ask the master distiller of a Bourbon distillery about the strongest influences in Bourbon production, you get the following answer: the grain, the yeast strains, the new white oak barrels and their storage have the biggest influence on the taste of Bourbon.
What's the reasoning behind this statement, which tells nothing about the particulars of the production process? The answer is as simple as it is convincing. The American distilleries usually have the same production equipment and the same climate. Therefore the distinguishing features of individual Bourbon producers must be sought on another level.
A word on Tennessee Whiskey: Tennessee Whiskey separated from Kentucky Straight Bourbon and the Bourbon Act for marketing reasons. However, the production process of these two Whiskey types is the same, except that Tennessee Whiskey is charcoal-filtered before it is filled into barrels.
On the following pages the production process is described in its chronological order:
Grain Selection and Mixture (Mash Bill)
Each distillery has its own recipe for the grain mixture. The law requires a Kentucky Straight Bourbon Whiskey to have a corn content of at least 51%. However, the corn content is usually higher (between 60% and 80%).
The different grain types are ground separately and stored temporarily. In former times hammer mills were used until it was found out that they heat up the grain too much, which affects the taste. Today the grain is usually crushed in order to open the husk. The following grinding process leads to very fine flour.
Mashbills of Different Distilleries
|Heaven Hill||75||13||12||Different Brands have different Mashbills|
|Jim Beam||77||13||10||Different Brands have different Mashbills|
|Four Roses||75||20||5||Different Brands have different Mashbills|
|Buffalo Trace||90||10||Different Brands have different Mashbills|
|Koval||-||-||-||-||Secret/Single Grain Whiskey|
Fresh spring water is needed for Whiskey production so the starch in the grain can be cooked and the developing sugar extracted. Distilleries were therefore built near springs that yielded enough water.
Kentucky and Tennessee are located on a large limestone layer that filters the water very well. In valleys where the limestone layer is broken, you can find springs that carry superbly clear water ideal for whisky production.
Some of today's distilleries are located in industrial areas (e.g. Buffalo Trace). They don't use spring water but local supply water, which is demineralised or deionised.
Grains consist mostly of starch, but also of small quantities of proteins, fats and trace elements. In chemical terms starch is a polysaccharide. Many sugar molecules are connected in long chains. Each grain type can germinate and can turn starch into sugar and eventually cellulose in this natural malting process. However, not each grain type is well-suited for this technical process. Only barley has excellent abilities to turn starch into sugar during germination, with the help of an enzyme.
For corn, rye and unmalted barley a different solution was needed. These corn types are usually cooked for about half an hour. Early Times cooks with slight overpressure and at higher temperatures in order to reduce the cooking time to approximately 25 minutes.
|Order||Type of wheet||Temperature||Cooking Time|
220°F / 114°C
|Long with pressure|
|2||Rye||170°F / 77°C|
150°F / 66°C
Each distillery in Kentucky and Tennessee has their own yeast strains, which have survived prohibition from 1919 to 1933 in cooled rooms. The yeast strains are so much kept as a secret that the companies have filed patents for their isolated yeasts, which is possible in the United States, unlike in Germany, for example.
Historically, selecting the yeast was a simple process. Containers with a nutrient solution were placed under an apple or pear tree. After some time, natural yeasts on the fruits also accepted the nutrient solution. Small samples of the nutrient solution were then put on carriers, and the individually appearing yeasts were bred in ovens at 35° to 40°C (95° to 104°F). From a single yeast cell you can breed the whole yeast for the alcoholic fermentation.
From a test tube about a pencil tip of yeast is extracted and then mixed with a nutrient solution (malt extract) in a bulb. The pH of the solution must be between 5.4 and 5.8 for yeast reproduction, depending on the yeast.
All objects used for yeast reproduction must first be sterilised in an autoclave so no vinegar bacteria or foreign yeasts contaminate the desired yeast strain.
After approximately half a litre of pure yeast has been produced in this bulb, the yeast is put in a larger container, the so-called 'dona tub'.
A large amount of yeast is produced in the dona, so that later a large container can be filled, which serves as a tank for all the fermenters.
At this stage also parts of the stillage are added. The stillage is formed at a later stage. It is part of the much-praised sour mash process, which will be explained in detail later.
The size of the fermenters varies between 'large' and 'giant'. Since column stills work continuously and can process large amounts of beer, a steady supply must be guaranteed so they don't run dry.
That's why American distilleries have a 'beer well' into which the fermented content (beer) is emptied. The beer well is usually made of stainless steel and is placed amidst the numerous fermenters. The size of the beer well corresponds to the size of the fermenters. The beer well is usually one third larger than the largest fermenter, so the column stills can continue production even if the emptying of a fermenter is delayed.
During alcoholic fermentation yeasts convert the sugar into alcohol and carbon dioxide while also generating heat. Fermentation usually takes three days. An alcohol content of approximately 8% to 9.5% is reached. Few distilleries (e.g. Labrot & Graham) ferment significantly longer, but they also don't reach higher alcohol contents than 10% to 11%. The result is called beer or distiller's beer.
The larger a fermenter, the hotter it gets, since the surface-to-volume ratio decreases with increasing size and the fermenter can't discharge the heat anymore. If the temperature in the fermenter rises above 35° to 40°C (95° to 104°F), the yeasts start dying off until fermentation stops. That's why many fermenters are water-cooled.
Distilleries such as Early Times or Four Roses already evaluate the result of the fermentation. The smell of the beer tells a lot about the future Whiskey. The beer is desired to be aromatic (e.g. having a strong apple aroma). A decrease in the aroma of the beer indicates that the yeast is contaminated, and a new yeast strain is used for the next batch.
After fermentation, the beer is distilled into raw Whisky, the 'white dog'.
All American distilleries (except Labrot & Graham) use column stills for distillation. They were invented by Robert Stein (Haig Co.) in Scotland in 1826 and their pillar-like shape makes a continuous distillation process possible. The basic operation principle is simple: You set up an upright pipe with a height of 5m to 20m and a diameter of 70cm to 150cm. You insert floors with holes into the pipe so there is a connection from the bottom to the top. The edges of the holes are slightly bent upwards so no liquids can flow down through them. Then you insert small tubes so the liquid that accumulates on the floors can flow down to the next floor.
The column is filled with beer in a middle position and heated from the lower end. Thus two opposite flows are created. The liquid beer runs down through the tubes, while the gaseous particles (alcohol vapours) flow upwards through the holes.
The temperature of the column is regulated in such a way that the alcohol is gaseous at the top (78 - 85°C / 172° - 185°F) and the beer is cooking at the bottom (95 - 100°C / 202° - 212°F).
This process can run forever as long as there's enough supply of new beer. While the alcohol is extracted at the top, the water with the fibres and remnants of the grain accumulates at the bottom. This product is called 'stillage' and is processed into animal feed and 'sour mash', which is reintroduced into the fermentation process.
In small column stills an alcohol content of approximately 120 American Proof (60% abv) can be reached at the top. If the columns are taller the alcohol content can be raised up to 80% and more.
After the alcohol has been extracted from the still, the steam is led through the doubler, a copper pot, where a catalytic conversion takes place which improves the taste of the Whiskey. Many column stills need such a doubler since the column floors aren't made of copper. In copper pot stills, as can be found in Scotland, a continuous catalytic conversion takes place. With column stills that lack copper parts, the distillate must be brought into contact with copper externally. That's what the doublers are for.
The vapour is then led into the condenser where it liquefies again and has now become raw Whiskey, which the Americans call 'white dog'.
From the condenser the Whiskey is led through a spirit safe into vats, 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 approximately 20% abv. In this state the aromatic substances can be judged best.
Per bushel of grain (35.24 litres) about 5 US gallons of 100 Proof Spirit are produced (9.5 litres of pure alcohol). Translated to the weight of the grain, approximately 400-450 litres of pure alcohol can be produced from one ton of grain.
Today all American distilleries use the sour mash process. Sour mash means that a part of the distillation residues (stillage) is added to the mash again. After the mash has been prepared with fresh spring water it is chemically neutral, which means the chemical environment is neither acidic nor alkaline. Chemically, this property is measured with pH.
A solution with a pH of 7 is neutral. An acid has a pH smaller than 7. Strong acids have a pH of 3 to 4. If the pH of a solution is higher than 7, it is alkaline. Strong alkaline solutions have a pH of 10 to 11.
After the addition of neutral water, the mash has a pH of approximately 7 (neutral) in which the yeasts can't work properly. The addition of a part of the very sour stillage (pH 5.0 - 5.4) leads to an acidification of the entire mash. After the stillage has been added, the mash has a pH of about 5.4 to 5.8, which is ideally acidic for the yeasts to work properly.
Some Bourbon producers try to make their product look special by using the term 'sour mash'. Other sources speak of a disinfecting effect of the sour mash process on the mash. However, the only purpose of sour mash is to create an acidic environment for the yeasts. The rest of the sour mash story is just marketing.
At the bottom of the column stills the so-called setback accumulates. It consists of water, proteins, fats and the fibres of the mash, while lighter materials such as alcohol and esters are extracted over the head of the column still. This aqueous solution from the bottom is pumped out of the container below the column still from time to time.
First the fibres of the stillage are collected by filters. The remaining liquid is dried in large drums using hot steam. The solid parts that accumulate at the drum walls are then mechanically extracted and given to farmers as animal feed. The feed contains a lot of proteins, fats and trace elements.
Filling of the Barrels
In Kentucky and Missouri companies specialize in the production of barrels from American white oak. The barrels may only be used once for Straight Bourbon Whiskey. They hold approximately 53 American gallons (about 200 litres). Americans often use the term 'barrel' rather than 'cask'. There are various definitions for a barrel. The most famous barrel is the petroleum barrel, which holds 158.97 litres. The normal American barrel, however, holds 31.5 gallons = 119.23 litres. 1 gallon corresponds to 3.785 litres.
The staves of the barrels are first joined but not yet completely closed. Then the staves are made flexible using hot steam and can thus be bent into their oval shape.
The next step is special and lends Kentucky Straight Bourbon its unique taste. The barrels, still open on one side, are held over a small fire. This process is called toasting and makes the wood sugar in the staves on the inside caramelise up to a certain layer. This red layer is later clearly visible when a barrel is disassembled. Toasting takes about 12 minutes.
After toasting the barrel is submitted to an even stronger fire treatment. It is burned on the inside with a large flame for 6 to 12 seconds, which produces a charcoal layer. The thickness of the charcoal layer can vary from grade 1 to grade 4.
Each distillery has their own preferences for storing the barrels.
In the past warehouses with a height of 4 to 5 floors were built. On each floor between 3 and 6 layers of barrels are stored.
The warehouses have a skeleton of bars and girders that make it possible to roll the barrels horizontally. Between this skeleton there are elevators that make the vertical movement of the barrels possible. An ordinary warehouse has a capacity of approximately 20,000 barrels.
In these warehouses a very special climate is created. Below the roof there are very high temperatures in the summer, while at the bottom it stays as cool as if the room was air-conditioned. For temperature equalisation with the outside air a warehouse has many windows.
The Whiskey matures differently on each floor. In the past the barrels were therefore rotated. Rotating means the barrels are moved to different pre-determined positions within the warehouse during maturation, so each barrel can profit from the good positions in the middle of the warehouse. However, a certain part of the warehouse (usually 1/3 of the total capacity) must stay empty for rotation. Maker's Mark was one of the few distilleries that still rotated their barrels in 1999.
The other distilleries have chosen a different way. They no longer rotate but mix barrels from different positions in the warehouse before bottling. With this method you avoid the labour-intensive rotation of the barrels, you use up the whole capacity of the warehouse and you gain an area in the centre of the warehouse where you find extraordinary barrels for small batch Bourbons and single barrel bottlings.
In 1996 several warehouses of the Heaven Hill distillery burned down. The burning Whiskey pushed its way through the distillery down to the river. Thus also the distillery burned down, and the Whiskey burned on the river for the rest of the night.
In order to minimise the risk of fire, today each warehouse has a sprinkler system, and the warehouses are built at a safety distance. As passive safety, the wooden skeleton is built in such a way that a warehouse collapses during a fire and doesn't damage other warehouses.
The Bourbon distilleries have such a high output that they can all afford their own bottling line. Even small distilleries such as Maker's Mark or Labrot & Graham have their own lines.
First the selected barrels are brought to the bottling line by truck and emptied over filters. The charcoal pieces that broke away from the barrel walls during maturation are held back by the sieve.
The Bourbon is pumped into a storage vat and from there fed to the bottling line.
American Whiskey is bottled in a variety of bottle sizes, shapes and materials. Typical sizes are 0.2 litres (flat bottle), 0.7 litres (Europe), 0.75 litres (US market, Japan), 1.0 litres (duty-free) and 1.89 litres (half gallon). The bottles for mass products are more and more made out of plastic (PET).