Single malt Scotch whisky is defined in law as a whisky distilled at a single distillery from malted barley and water alone, in pot stills, and matured in oak casks in Scotland for a minimum of three years. Within those constraints lies enormous variation, but the production sequence is broadly consistent.
The process begins with malting. Barley is steeped in water until it germinates, activating the enzymes that will later convert starch into fermentable sugars. Germination is halted by drying the barley in a kiln. Where peat is used as fuel — as it traditionally has been on Islay and in parts of the Highlands — the smoke contributes phenolic compounds that will persist through distillation and maturation.
The dried malt is milled into a coarse grist and mixed with hot water in the mash tun. This process, known as mashing, dissolves the sugars into a sweet liquid called wort. The wort is drawn off and transferred to washbacks, large vessels traditionally made of Oregon pine or larch, where yeast is added and fermentation begins.
Every distillery is a slightly different answer to the same question.
Fermentation typically runs between 48 and 90 hours, producing a beer-like liquid known as wash with an alcoholic strength of around 7 to 9 percent. The character of the fermentation — the yeast strain, the temperature, the vessel material — has a marked influence on the final whisky.
Distillation follows, almost always in copper pot stills. The wash is first distilled in the wash still, producing a low-strength distillate known as low wines. These are then distilled again in the spirit still, and only the middle portion of that second run — the heart, or the cut — is collected for maturation. The remainder is redistilled.
The new-make spirit, clear and fiery, is then filled into oak casks at a strength of around 63.5 percent alcohol by volume. Maturation, which begins the moment the spirit meets the wood, is the longest and least predictable stage. Over years and decades the whisky draws colour and flavour from the cask, loses volume to evaporation (the angel's share), and softens in character.
Only after the minimum three years have passed can the spirit be legally called Scotch whisky. In practice, single malts are almost always matured much longer.