Production

Grain preparation

To initiate whisky production, grains are processed to convert their starches into fermentable sugars, which can then be subjected to fermentation and distillation. This process involves malting, milling and mashing the grains.^[28]

Malting

Malt production begins with the soaking of grains in water tanks for a period of three days. The soaked grains are then transferred to large containers where germination occurs, initiating the metabolic process that converts starches into sugars. This process is subsequently halted by drying the grains with hot air, a procedure also known as kilning, at the end of which the product is malt. In the production of Scotch whisky, the air used for kilning is heated by burning peat bricks in a furnace, imparting the characteristic smoky flavour to the malt.^[29]

Milling

Following the malting process, the grains are transferred to a dressing machine, which separates the sprouts from the seeds. The grains are then conveyed to a mill for grinding.^[30]

Mashing

The extraction of sugars to be converted into alcohol is achieved through a process known as mashing. During mashing, the diastase enzyme is activated, which facilitates the conversion of starches and dextrins into sugars. The ground malt is introduced into a mashtun, a large vat containing hot water, and is agitated for approximately thirty minutes or more. The water is then drained from the mashtun, which is subsequently refilled with hot water. This procedure is typically repeated between one and three times. Once complete, the resulting liquid, known as wort, which contains the extracted sugars, is separated from the mash, cooled, and transferred to the washbacks, or fermenting vats.^[31]

Fermentation

During fermentation, a specific strain of yeast, Saccharomyces cerevisiae (commonly known as "brewer's yeast"), is introduced to the sugary wort, which provides the necessary nutrients for its asexual reproduction. The yeast metabolizes the sugars, producing alcohol, carbon dioxide, and congeners, which can influence the flavor profile of the whisky, either enhancing or suppressing desirable characteristics. The fermenting vats are maintained within a controlled temperature range, typically between 10 and 37.8 °C (50 and 100 °F) which is optimal for yeast activity. Distillers ensure consistency by using the same yeast strain to achieve uniformity in the final whiskey product. Fermentation continues for two days or longer until the alcohol content of the liquid reaches between 5% and 10%. Once fermentation is complete, and the yeast ceases to be active, the resulting liquid is referred to as wash or distiller's beer.^[32] Even though the wash is alcoholic it is still organic, which makes it susceptible to contamination by microorganisms that cause rot. Consequently, it is promptly transferred to the still for boiling to mitigate this risk.^[33]

Distillation

The decision to transfer either the entire wash or only the most liquid portion into the still is at the discretion of the distiller, and each choice affects the flavor profile of the final product in distinct ways. Following the transfer of the wash, the still is heated to a temperature sufficient to evaporate the alcohol while remaining low enough to prevent the evaporation of water. Alcohol vapour ascends through the still and is directed to the condenser, which consists of copper tubes or plates, where it condenses back into a liquid form known as the distilled spirit. This distilled spirit is meticulously monitored during its extraction. The initial and final portions of the distillate are deemed undesirable and are therefore redirected back into the still. Only the middle portion, considered the most desirable, is collected in the spirits receiver. At this stage, the distilled spirit is clear and has an ethanol content ranging from 70% to 80% ABV. It is typically diluted before being transferred to casks for maturation, though some distilleries sell it as is at 'cask strength'.^[34]

A still for making whisky is usually made of copper, since it removes sulfur-based compounds from the alcohol that would make it unpleasant to drink. Modern stills are made of stainless steel with copper innards (piping, for example, will be lined with copper along with copper plate inlays along still walls). The simplest standard distillation apparatus is commonly known as a pot still, consisting of a single heated chamber and a vessel to collect purified alcohol.

Column stills are frequently used in the production of grain whisky and are the most commonly used type of still in the production of bourbon and other American whiskeys. Column stills behave like a series of single pot stills, formed in a long vertical tube. Whereas a single pot still charged with wine might yield a vapour enriched to 40–60% alcohol, a column still can achieve a vapour alcohol content of 95.6%; an azeotropic mixture of alcohol and water.

Maturation (Aging)

Whiskies do not mature in the bottle, only in the cask, so the "age" of a whisky is only the time between distillation and bottling. This reflects how much the cask has interacted with the whisky, changing its chemical makeup and taste. Whiskies that have been bottled for many years may have a rarity value, but are not "older" and not necessarily "better" than a more recent whisky that matured in wood for a similar time. After a decade or two, additional aging in a barrel does not necessarily improve a whisky,^[35] and excessive aging will even affect it negatively.^[36] The minimum aging period required for whisky varies by country. In the United States, the minimum aging requirement is typically 2 years, while in Scotland, Ireland, and Canada, it is generally 3 years. ^[36]

While aging in wooden casks, especially American oak and French oak casks, whisky undergoes six processes that contribute to its colour and final flavour: extraction, evaporation, oxidation, concentration, filtration, and colouration.^[37] Extraction in particular results in whisky acquiring a number of compounds, including aldehydes and acids such as vanillin, vanillic acid, and syringaldehyde.^[38] The casks used for aging bourbon whiskey are required to be new (and charred); after being used for this purpose, these casks are typically exported for use in the aging of other whiskies elsewhere. Distillers will sometimes age their whiskey in barrels previously used to age other spirits, such as port, rum or sherry, to impart particular flavours. The size of the barrel also has an effect on the flavour development of the whisky, smaller barrels will contribute more to the whisky due to the higher wood surface to whisky ratio.^[36]

During maturation, up to 45 litres or 12 US gallons of whisky may evaporate from the cask over a 4 year period. This portion is called the angel's share by distillers.^[36]

Packaging

Most whiskies are sold at or near an alcoholic strength of 40% abv, which is the statutory minimum in some countries^[12] – although the strength can vary, and cask-strength whisky may have as much as twice that alcohol percentage. Enthusiasts often prefer cask strength whisky for its concentrated flavors and customizable dilution experience.