Although grain whisky is often overlooked and rarely bottled, it plays a significant role in Scotch whisky production, especially in blended whisky. Its production process is just as complex as other types of whisky, as outlined below.
1. Grains
Many types of grains can be used to produce grain whisky. In recent years, Scotland has preferred wheat (due to cost factors), while corn was more commonly used in the past. Countries like Ireland, Japan, and Canada also use corn. Whisky made from corn tends to be "richer" than those made from wheat. Malted barley is also commonly used in grain whisky formulations.
2. Milling
The grains are ground into powder using a hammer mill.
3. Cooking
The base grains are pressure-cooked to gelatinize (soften) the starch. After the cooking process is completed, malted barley is added.
4. Conversion
Enzymes in the malted barley begin converting the starch into fermentable sugars. Some countries allow the addition of enzymes to accelerate this process, but this is not permitted in Scotland.
5. Fermentation
The wort is pumped into fermentation tanks where yeast is added. Once the conversion is complete, the fermentation process begins, producing what is called "wash." The fermentation time ranges from 48 to 100 hours. The resulting wash has an alcohol content between 8% ABV (for wheat) and 15% ABV (for corn).
6. Distillation Option 1a: Coffey Still (A)
Aeneas Coffey designed this type of still in 1831, and the design is still in use today. It consists of two tall, interconnected columns: an analyzer column and a rectifier column. Both columns are divided into compartments by perforated plates. The wash is pumped into a copper coil that flows from top to bottom through the rectifier column. The coil extends from the bottom to the top of the analyzer column, where the wash is sprayed onto the top tray and trickles down through the compartments. At the same time, hot steam is pumped into the bottom of the analyzer column, rising through the perforated plates. The steam heats the wash and carries the alcohol with it.
7. Distillation Option 1b: Coffey Still (B)
The steam is directed into the bottom of the rectifier column, where it begins to rise once again. Since different alcohols separate (fractionate) at different temperatures, the spirits begin to stabilize in this environment. Heavier alcohols condense on the plates and are collected and sent back to the analyzer column. Only the lightest alcohols can rise to a specific tray at the top of the column, where they condense into a new spirit, typically with an alcohol content of around 90-94% ABV. Despite the high alcohol content, the grain spirit produced by the Coffey still retains more oiliness compared to other distillation methods.
8. Distillation Option 2: Three-Column Distillation
The wash enters the top of the first column (the stripping column), where the volatile "heads" are removed. The alcohol vapor passes through the middle column, which is filled with water. Since fusel oils in the alcohol are insoluble in water, they rise to the top and can be extracted. The mixture of alcohol and water is then drawn from the bottom and diverted to the rectifying column, where it is collected as new spirit.
9. Distillation Option 3: Multi-Column Distillation
The more columns used, the greater the ability to separate individual flavors.
10. Maturation
Grain spirits are typically aged in first-fill bourbon casks. These types of barrels impart vanilla and coconut characteristics to the whisky.