Malt whisky /whiskey is usually made from barley, which has a particularly affinity with the malting process, and was also the most easily available grain during the development of the modern whisky industry in Scotland.
Malting itself takes place at the very start of the production process. The barley is soaked in water, turned on a malting floor (see picture) and then dried in a kiln, which catalyzes and then abruptly halts germination, releasing the starch in the grain. During kilning, peat fires are sometimes lit beneath the barley, which is an important factor in the flavor of the finished spirit: peat is a partially decomposed layer of vegetation extracted from boggy ground, which, when burned, gives the barley smoky, salty characters that translate into the finished whisky.
The malted barley is then heated in a mash tun with hot water, converting the starch into fermentable sugars and producing a sweet liquid known as wort. Yeast is added to the wort to start fermentation, resulting in the production of a beer-like liquid of around eight percent alcohol. This "beer" is then distilled twice in copper pot stills; the first distillation makes a rough spirit around 25 percent alcohol that is known as "low wines", and the second distillation – carefully monitored – produces the spirit that will be aged in barrels to become single malt whisky. The shape and size of the stills are crucial factors in the character of the whisky. Each distillery has a unique still that produces the desired expression; many are given names, in the same tradition as ships.
Grain whiskies are made from unmalted grain, usually in continuous or column stills. These can run 24 hours a day rather than on a batch basis, and so are more cost efficient.
Single Malt Whisky / Whiskey
This arguably the finest expression of whisky, made at a single distillery from malted barley, water and yeast. The style is produced in Japan, Ireland and most importantly (and famously) in Scotland. Indeed, single malt Scotch whisky is highly collectable and is easily one of the world's most sought-after and revered liquids, with many examples held in equal esteem with the world's top wines.
In addition, an increasing number of American craft producers have single malt whiskeys in their portfolio. The American Single malt Whiskey Commission was founded in 2016, and defined the category for distilleries in the USA; as of July 2018 the commission's membership had reached 100. However at this time the Alcohol and Tobacco Tax and Trade Bureau had yet to officially recognize the term.
Tradition may have as important a role as notions of terroir in differentiating single malts. A pure water source is usually unique to a distillery and is a source of pride, coming from local springs or rivers. The noted Japanese whisky Yamazaki is named for a suburb of Kyoto that is particularly famous for the quality of its water. But this may not have any role in differentiating the taste of finished product, and not all peated whiskey uses locally sourced peat. Moreover, many distilleries, including many single malts, do not run their own malting floors for reasons of cost and efficiency.
While there may be some terroir factors associated with the distillery location and the environment within the buildings, it is the shape and size of the stills and number of distillations which is usually regarded as having the most significant effect on a young whiskey. Thereafter many other characteristics can be laid over this core character through aging in oak barrels.
Single malt Scotch whiskies age in oak casks for an extended amount of time; mostly between eight and 20 years, but sometimes as long as 50-80 years. Oak maturation helps to soften the harsher edges of the spirit, while imparting its own flavors and allowing these to meld with flavors already present in the whisky. Various finishes are used to create character; casks that once held Bourbon are commonly employed, but Sherry casks are also popular, as are those that once held wine. The environment within the aging warehouses can play a big role in style, but barrels are not totally consistent products and two casks of the same origin filled with the same new-make spirit can produce very different results after a decade or more.
Time consuming, labor intensive production processes mean that single malt whiskeys usually occupy the top end of the whiskey market. Of course, time and care usually translates into a higher-quality product and, as such, single malts are usually savored on their own, with just a drop of water to bring out the spirit's flavors. Bartenders and mixologists are more likely to use more lightly flavored blended whiskies in cocktails.
Blended / Vatted / Pure Malt Whisky
Blended or vatted malt whisky is a combination of two or more single malt whiskeys. In contrast, standard blended whiskey will include grain whisky. Not suprisingly given the components, a skilful blender can make a product that equals most single malts in terms of quality.
Pure Malt is a tricky term; it has now been phased out of use by the Scotch Whisky Association as usage was inconsistent between single and blended malts. The phrase still appears on Japanese whisky labels, but seems to be applied to both types of malt whiskey.