Single malt whisky

Malting floor at Highland Park Distillery.

Malted barley.

Barley is "malted" by soaking the grain in water for two to three days and allowing it to germinate. This process releases enzymes, which convert unfermentable starch (which is insoluble in water and not available for fermentation by yeast) to fermentable sugars. Traditionally in Scotland, germinating seeds were laid on a malting floor and regularly turned. After three to five days, when the optimum amount of starch has been converted to fermentable sugars, germination is halted by heating the seeds with hot air.

The distinctive "pagoda" chimney of a kiln at a distillery in Scotland.

Many distilleries once used a kiln to heat the germinated barley. The "pagoda roof" (many now false) that ventilated the malt kiln can still be seen at many distilleries both in Scotland and in other countries. However, most of the distilleries now use commercial "maltsters" to prepare their malt.

In most cases, some level of smoke from a peat-heated fire is introduced during heating to add phenols, a smoky aroma and flavour to the whisky. Some of the more intensely smoky malts have phenol levels between 25 and 50 parts per million (ppm). Islay malts have a reputation for being the peatiest. More subtle malts can have phenol levels of around 2–3 ppm. Entirely non-smoked (non-peated, unpeated) malts are made by the Glengoyne Distillery, which only uses hot air for drying.

The malt is milled into coarse flour (grist) which is made of husks (70%), grits (20%), and flour (10%), to which three courses of hot water are added to extract the sugars.

The extraction is done in a large kettle (usually made of stainless steel) called a mash tun. At first, the hot water dissolves the sugars (maltose) and enzymes (diastase) in the grist. Then the enzymes act on the starch left over from the malting stage, continuing the conversion to sugar, and producing a sugary liquid called wort. Typically, each batch of grist is mashed three times or so to extract all the fermentable sugars. The first water is injected at about 60 °C, the second portion at about 72 °C and the third and final portion at about 88 °C. The wort from the first two water courses is drained into "washback" vessels for further processing, whereas the third course is retained as the first charge in the next batch.

Yeast is used to ferment malted barley in washbacks.

Yeast is added to the wort in a large vessel (often tens of thousands of litres) called a washback. Washbacks are commonly made of Oregon pine or stainless steel. The yeast feeds on the sugars, and as a by-product, produces both carbon dioxide and alcohol. This process is called fermentation and can take up to three days to complete. When complete, the liquid has an alcohol content of 5 to 7% by volume and is now known as wash. Up until this point the process has been quite similar to the production of beer.

The wash, 5%–7% ethanol, is distilled in copper pot stills, boosting the alcohol content to 60%–80%.

The wash is then distilled. The wash is heated, boiling off the alcohol, which has a lower boiling point than water; the vapour is collected and cooled to condense it back into a liquid form.

Scottish regulations require single malts to be distilled in pot stills. In other jurisdictions, column stills may be used.

The initial distilled spirit produced by a pot still, known as low wine has an alcohol content of about 20 to 30%. The low wines are then pumped into a second still, known as the spirit still, and distilled a second (and sometimes a third) time. The final spirit, called "new make spirit", generally has an alcohol content of 60 to 70%.

Most new-make malt whisky is diluted with water to about 60% alcohol by volume (ABV) or so before it is placed in casks to mature (62.5% is specified as a maximum in U.S. law for making straight whisky). The aged spirit is then diluted with water to reduce it to bottling strength (typically 40-50% ABV). Since large amounts of water are used during the process of whisky production, water supplies are a key factor for the location of any distillery.

Casks maturing at Glenglassaugh Distillery

The "new-make spirit" (above 60% a.b.v.), or unaged whisky, is then placed in oak casks to mature and to reduce its alcohol content to expedition levels (40-45% a.b.v.). By law, all Scotch whisky must be aged for a minimum of three years in oak casks, though many single malts are matured for much longer. The whisky continues to develop and change as it spends time in the wood. Typical maturation periods are in the range of 10-15 years; longer periods of twenty years or more occur, but are rare and costly. During the time it spends in the wood, a significant percentage of each cask's content will evaporate. The lost part is known as the angel's share.

The selection of casks can affect the character of the final whisky. Outside of the United States, the most common practice is to reuse casks that previously contained American whiskey, as US law requires several types of distilled spirits to be aged in new oak casks. To ensure the continuity of supply of used oak casks, some Scottish distilling groups own oak forests in the US and rent the new barrels to bourbon producers for their first use. Bourbon casks impart a characteristic vanilla flavour to the whisky.

Sherry casks are also commonly used. This practice arose because sherry used to be shipped to Britain from Spain in the cask rather than having been bottled, and the casks were expensive to return empty and were unwanted by the sherry cellars. In addition to imparting the flavours of their former contents, sherry casks lend maturing spirit a heavier body and a deep amber and sometimes reddish colour.

Stainless steel shipping containers, however, have reduced the supply of wooden sherry casks, to the extent that the Macallan Distillery builds casks and leases them to the sherry cellars in Spain for a time, then has them shipped back to Scotland.

Other casks used include those that formerly held port wine, madeira, rum or cognac.

A collection of various single malt Scotch whiskies of different ages from different distilleries. These bottles were packaged by an independent bottler, a third-party company that purchases bulk whisky for resale.

To be called a single malt whisky in Scotland, a bottle may only contain whisky distilled from malted barley and produced at a single distillery. The regulations of other countries may allow malted rye.

If the bottle is the product of malt whiskies produced at more than one distillery, the whisky is called a blended malt or vatted malt, or pure malt. If a single malt is mixed with grain whisky, the result is a blended whisky. Single malts can be bottled by the distillery that produced them or by an independent bottler.

The age statement on a bottle of single malt whisky is the age of the youngest malt in the mix, as commonly the whiskies of several years are mixed in a vat to create a more consistent house style. On occasion, the product of a single cask of whisky is bottled without being vatted with other casks, and released as a "Single Cask" offering. However, it is not always clear what the term "single cask" refers to. At least some producers release vatting of multiple barrels that have been matured together for one final period in a larger single cask as "single cask" whisky.[5]

While cask strength, or undiluted, whisky (sometimes having an alcohol content upwards of 60%) has recently become popular, the vast majority of whisky is diluted to its "bottling strength" - between 40% and 46% ABV - and bottled for sale.

Whisky can be "chill filtered": chilled to precipitate out fatty acid esters and then filtered to remove them. Most whiskies are bottled this way, unless specified as unchillfiltered or non-chill filtered. Unchillfiltered whisky will often turn cloudy when stored at cool temperatures or when cool water is added to them, and this is perfectly normal. This cloudy appearance is aesthetic only and does not negatively impact the spirit.

Generally speaking, single malt whisky that is bottled at a proof ≥ 46% ABV (92 proof) does not turn cloudy when cooled (other whiskies and spirits have differing thresholds). Most retail whiskies are bottled at or near 40% ABV for economic reasons, which has now become a modern custom, and therefore chill-filtering is common.

Unchillfiltered, cask-strength whisky (usually > 60% ABV or 120 proof) is generally regarded as whisky in its purest form.

Unlike wine, whisky does not continue to mature in the bottle.

E150A caramel colouring may be added to single malt Scotch whisky prior to bottling,[6][7] to give the whisky a more rich and well-aged appearance. No other additives are allowed in Scotch whisky. This contrasts with the rules governing Canadian whisky production, which allows the addition of other flavourings as well as caramel, and with the rules governing American whiskey, which do not allow additives in "straight" whisky. The use of the caramel additive must be disclosed when the whisky is sold in some jurisdictions, although not in Scotland itself.[6][7]