Single
malt whiskey is a type of Whisky, distilled by a single distillery in
a pot still, using malted barley as the only grain ingredient.
"Malt" indicates that the Whisky is distilled from a single "malted" grain. Not all grains can be malted (rye is another grain which can be malted), but in the case of single malt Scotch, barley is always the grain used.
"Single" indicates that all the malts in the bottle comes from a single distillery. Multi-distillery malts are usually called "blended malt", "vatted malt" or (deliberately confusing, perhaps) "pure malt".
Production
All single malt Scotch goes through a similar batch production process, as outlined below. At bottling time various batches are mixed together or vatted to achieve consistent flavours from one bottling run to the next. Even still, some variation does occur.
Water
Water is used in all phases of the production of Whisky. It is added to the barley to promote germination, it is mixed with ground barley grist to create a mash and it is used to dilute most Whisky before maturation and once again before bottling.
Most distilleries use different water sources in the various steps.
Most new-make malt Whisky is diluted to about 63.5% before it is placed in casks to mature. These days, many distilleries are using distilled water for diluting Whisky before it is casked as well as for diluting the Whisky to bottling strength (40-46% Alcohol by Volume (ABV)) after maturation. Others, like Jura or Bruichladdich use water from local burns or springs to dilute new-make before it is casked. Much new-make Whisky is shipped in tanker trucks to central warehouses where local tap water is used to dilute it before casking, and again at bottling time.
Since huge amounts of water are used during the process of Whisky production, water supplies are a key factor for the location of any distillery.
Malting
Barley, yeast
and water are the only ingredients required in the production of single
malt Scotch.
The barley used to make the Whisky is "malted" by soaking the grain in water for 2-3 days and then allowing it to germinate to convert starch (which is insoluble in water and not available for fermentation by yeast) to fermentable sugars.
Traditionally each
distillery had its own malting floor where the germinating seeds were
regularly turned. Most of the distilleries use commercial "maltsters"
who prepare each distillery's malt to exact specifications, but the
"pagoda roof" (many now false) which ventilated the malting
floor can be seen at nearly every distillery.
The distinctive "pagoda" chimney of a kiln at a distillery
in Scotland.
Mashing
The malt is milled into a coarse flour (grist), and added to hot water 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 an sugary liquid called wort. Typically, each batch of grist is mashed three times or so to extract all the fermentable sugars.
Fermentation
Yeast is added to the wort in a large vessel called a washback. Washbacks are often made of Oregon Pine or stainless steel.
The yeast feeds on the sugars and as a by-product produces both carbon dioxide and alcohol and 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.
Distillation
The wash is then pumped into a copper pot still,
known as the wash still, to be distilled. The wash is heated, boiling
off the alcohol, which has a lower boiling point than water; the vapor
is collected in a condenser which has been submerged in cool water. The
lower temperatures cause the vapor to condense back into a liquid form.
This spirit, known as low wine has an alcohol content of about 20 to 40 %. The low wines are then pumped into a second pot still, known as the spirit still, and distilled a second time. The final spirit called "new make spirit" generally has an alcohol content of 60 to 70%.
Much of the body, or mouth feel, of the final Whisky is believed to come from the size and shape of the stills used in its production.