Grain whisky ordinarily refers to any whisky made, at least in part, from grains other than malted barley, such as whisky made using maize (corn), wheat or rye. Grain whiskies may also contain some malted barley (and are required to if produced in Scotland). Whisky made from only malted barley (or primarily from malted barley) is typically called malt whisky rather than grain whisky (although barley is a grain). Most American whiskeys and Canadian whiskys are grain-based.
Grain whisky definition
In Scotland, "malt whisky" must use a 100% malted barley mash and must be distilled in a pot still, whereas grain whisky is typically distilled in a continuous column still in a manner that results in a higher percentage of alcohol by volume (ABV), but less flavorful spirit. Because of this practice, grain whisky is seldom bottled by itself in Scotland, where it is instead manufactured primarily for blending with malt whisky to create blended whiskies, which account for over 90% of all Scotch whisky sales. The comparative lightness of the clearer, more-neutral-flavored grain whisky is used to smooth out the often harsh characteristics of single malts. Occasionally well-aged grain whiskies are released as single grain whisky if made at one distillery or blended grain whisky if combining spirits from multiple distilleries. It is important to be aware that the phrase "single grain" does not refer to using a single type of grain to make the whisky – indeed, it is against the law to produce whisky in Scotland that is made from a single grain unless the grain is barley – in which case it would be considered a "malt whisky" rather than a "grain whisky". Instead, "single grain" refers to using a single distillery to make a whisky from at least two types of grain – one of which is required to be barley.
Outside Scotland, the use of continuous column stills and the use of a non-barley mash are not so closely associated with the production of "light" whisky (whisky with little flavor due to distillation at a very high ABV). For example, nearly all American whiskey is produced using column stills, and all American whiskey that is labeled as straight whiskey (including straight Bourbon and straight rye) is required to use a distillation level not exceeding 80% ABV. Because of this constraint, much of the American whiskey may actually be less "light" than some Scotch or Irish single malt pot still products. In the United States whiskey produced at greater than 80% abv is formally classified as Light whiskey and cannot be labeled with the name of a grain or called malt, bourbon or straight.