Such a simple question invites a simple answer, but a triviality can't capture the truth.
A dictionary will tell us that “whisky” and “whiskey” are variant spellings of the anglicized pronunciation of the Gaelic word uisge or uisce. This from Gaelic “uisge beatha” derived from Latin “aqua vitae”. Aqua vitae in ancient alchemy is a term for a concentrated ethanol solution, but that doesn't properly differentiate modern whiskey from Scandinavian akvavit, the Ukrainian akavita, Polish okovyta, or French eau de vie; all terms derived from aqua vitae. What are the distinguishing features of the Gaelic derived whiskey from other aqua vitae ?
The major ingredient of whiskies are grain (or malted grains), water and yeast. This excludes fruit based brandy and eau de vie, but doesn't differentiate whisky from akvavit or various grain based vodkas (neutral grain spirits). The unique characteristic of whisky comes from the process, more than the basic ingredients. The means of mashing or brewing is distinct. The method of distillation is different and this allows the yeast and grain components and the mashing method to play a role in aroma and flavor. The extended storage in charred barrels is both an ingredient and a process. I intend to explore these differences in future notes.
Nations as diverse as Australia, India, Japan, Taiwan have recently developed significant whisky production.The spelling “whiskey” is used by Irish and US distillers with Scotland, Canada and nearly every other nation using the “whisky” spelling. I will use the spelling that seems appropriate throughout.
Many different grains, the seeds of grasses, are used for the production of whisk[e]y. Here is a little sketch of the major distillers grains. The process of malting will be discussed later.
Barley (Hordeum vulgare):
Barley originated in the Middle-East at the dawn of human agriculture and was used for beer in early Mesopotamia and Sumeria. But barley traveled West and north across Europe and to the Americas and beer culture followed rapidly.
Single Malt Scotch Whisky and many contemporary Irish Whiskey is made entirely from malted 2-row barley. Scotch distilleries each had their own 'malting floor' for production until the recent era where malting is sometimes centralized. A large majority of beers (aside from wheat beers and weizens) are made from 2-row barley. 2-row barley has a milder less phenolic flavor compared to 6-row “horse feed” barley, but let's not underestimate 6-row. Many pre-prohibition US beers were made with 6-row barley combined with corn or rice to dilute the phenolic flavors. Many North American whiskies are made from 6-row barley since these phenolic flavors are vastly reduced or eliminated by distillation. Obviously malted barley creates the flavors we call 'malty'. Unmalted barley is sometimes used in whisky production too, and it provides a nice hearty grainy flavor of it's own.
Corn (Zea mays, or maize):
Maize in whiskies for obvious reasons a North American invention. Maize is the central grain of the Americas, and was grown by early agriculturalists of southern Mexico, Central America, and northern South America 7000 years ago. Corn lacks cold hardiness, but is well adapted to managed agriculture in mid-northern latitudes of North America, so it's inclusion by Kentucky distillers is not a mystery. The corn used in distilling is common #2 yellow corn, which is alternatively used for cattle feed, corn flour, high-fructose corn syrup, corn oil, and fuel ethanol. Unlike other grains considered here, corn has a high level of oils, around 6% of dry-weight versus around 2% in barley, wheat and rye. Although yeasts enjoy the nutritive value of the extra vegetable oils, they are unacceptable in beers where they ruin the foamy head.
Corn leads to a simple grassy, and sweet flavored whiskey. US bourbons are required by law to use at least 51% corn in by weight their grist (grain mix) while other factors boost the flavors of bourbon. The US designation, “corn whiskey”, requires is at least 80% corn and can be sold without any barrel aging.
Wheat (sp. Triticum):
Wheat, like barley was developed in the early Middle-East agriculture era and spread widely. It has an unusually high degree of gluten (sticky proteins insoluble in water) and so can be used to make impressively raised breads. Red & white wheat differ in husk phenolic content, soft vs hard wheat differ in protein content. All have been used successfully in whisk[e]y.
We all know wheat from the flavor of bread and especially whole wheat bread, and this nice plain flat wheat flavor appears in wheat beers and wheated whiskies. The famous Bavarian 1516 Reinheitsgebot (German Beer Purity Law) was less about 'purity' and more about definitions, prices and preventing avid 16th century wheat-beer fans from using up the wheat needed for bread.
Among whiskies we have a few examples of Wheat whiskey (>51% wheat) . But we also have some bourbons that, after the required 51% corn and some malted barley, use wheat as opposed to rye as the secondary grain. 'Maker's Mark' is a classic example of a wheated bourbon. More often bourbons include a fair bit of rye.
Rye (Secale cereale):
Rye is an unusual crop in many ways. Its agriculture was developed quite late compared to barley, wheat or corn. It was cultivated widely along the Rhine and to the British Isles only in the Middle Ages. Rye is very cold tolerant, and can withstand drought as well as wet, short growing seasons. It's more tolerant of poor soil conditions than many other grains. Rye was introduced to the Americas by early Germans immigrants who appreciated it's use in bread as well as distilled beverages. But the rye was rapidly adopted by the Celtic heritage immigrant distillers for it's characteristic flavor and growth potential on marginal farmland.
Rye grain provides an unmistakable spicy fruity aroma that can be imparted to whiskey. There are “low rye” and “high rye” bourbons that contain perhaps <20% vs ~35% rye in the grist. US Rye whiskey (>51% rye) was the most popular whiskey in the US until prohibition, due undoubtedly to the impressive rye flavors. By the 1980s there were only 4 common brands of rye whiskey sold in the US. In the past decade there has been a resurgence of rye to it's rightful place among the worlds great beverages. It should be noted that 51% rye in the grist gives a very pronounced rye flavor, and whiskies distilled from a higher percentage of rye were typically made for blending purposes. Notwithstanding, several 95% rye and 100% rye whiskies are on the market today. Canadian whiskies are often called “rye whisky”, however there is no legal requirement to use rye grain in these, and many use only a small amount of rye in the grist.
There is no end to the small grains that have been made into whiskey . Triticale (a rye x wheat cross) has the proper heritage for whiskey production. Oats, like maize are unusually oily, and are used in a few rare whiskies. A few examples of whiskies containing spelt, sorghum, millet and others exist.
What can be simpler than water; H2O, two hydrogen atoms and an oxygen ? But water as it appears in surface water, wells and from municipal supplies is a different matter. These waters contain various dissolved minerals that impact whisky making.
Whisky making uses water at many stages of production. Clean potable water is used for the creation of malt from grain. Similar water is used for the cereal cooking and the mash cycle and is the main part of the 'wash'. Relatively pure water may be needed to supply steam in the case of steam driven distillation or mashing systems. A good deal of water is used for the cooling of distillation products from vapor into liquid phase, where the only requirement is that the cooling water is non-corrosive. Once distillation is complete the product may be diluted with water to a fixed concentration for barrel aging and again for bottling at a specific proof. Beyond all of these uses a vast majority, at least 75%, of water is specifically for cleaning, I'll avoid the issue of common cleaning and cooling water and focus just a few factors that impact whisky.
The first item is the US whiskey lore of hard limestone rich waters used by distillers. In contrast most water used to make Scotch is fairly soft, so hard, alkaline limestone water is not a requirement. Limestone or calcium carbonate slightly dissolve in water leaving calcium and bicarbonate ions in solution. The bicarbonates represent temporary hardness and alkalinity and this buffers the water from pH changes by acid or base additions, or by fermentation. This unfortunate buffering and the potential of scale formation (carbonate precipitation) are all negative for the brewing and distilling process. The main advantage of limestone rich waters is that they are typically low in iron. Iron has a particularly bad impact on the wash flavor, and if it appears in the whisky can cause an unattractive black color.
Another issue is that modest hardness water improves the stability of enzymes in the mash, and are important to yeast metabolism, however this is entirely due to the metal ions (calcium, magnesium, and trace amounts of manganese and zinc). The ideal water for malting and mashing would be nearly neutral in pH and have some calcium and magnesium salts as permanent hardness, with little bicarbonate content. Very hard water can have a negative impact on yeast performance, and a few uncommon ions can be devastating.
Chlorinated water from municipal supplies will create very strong bad tasting chlorophenolic compounds in the wash. Surface water and shallow well water has other issues with decaying plant matter providing phenolic compounds, algae blooms or geosmin producing microbes causing unpleasant flavors. It is disputable if these flavor can pass through distillation, but they do interfere with a sensory analysis of the wash.
The blending water used to control the alcohol strength into the barrel and later the bottle must be neutral pH, low in hardness, and without chlorine or iron. Reverse osmosis or distilled water is often used, although soft potable water is as well.
Lore and marketing myths aside, water impacts the flavor and look of whiskey enough to matter.