Well, just about everything.
There are many delicious types of whiskey, or whisky, but bourbon and Scotch are two of the most beloved varieties. While both are known around the world and are strongly associated with their places of origin, that’s where the similarities stop. Here are the key differences between the two spirits.
Bourbon is whiskey; Scotch is whisky.
The “whiskey” spelling is used for American and Irish spirits, including bourbon. Conversely, “whisky” is used by the rest of the world, including Europe, Australia, Japan and, of course, Scotland. Regardless of spelling, all whiskey or whisky must be distilled to a minimum of 40 percent and a maximum of 94.8 percent alcohol by volume (ABV).
They have different flavors.
While both can have smokey, charred notes, Scotch and bourbon develop these traits through different means. Bourbon draws its oaky, vanilla-like flavors from the charred surface of the barrels in which it ages, whereas Scotch's smoke comes largely from peat burned in the barley malting process. Keep in mind though that not all scotches have a peaty flavor. In fact, only two of the five Scotch producing regions are known for that signature peatiness.
They're made with different ingredients.
The U.S. government has very strict regulations for bourbon, one of which is that bourbon whiskey must be made from a grain mixture of at least 51 percent corn. The rest is usually a mix of malted barley, rye and wheat. Scotch whisky, on the other hand, must be made from malted barley, which is Scotch’s primary ingredient, along with water and yeast. Scotch producers are permitted to include other whole cereal grains for coloring.
They come from different places.
Contrary to popular belief, bourbon can be made outside of Kentucky. However, to legally qualify as bourbon, it must be made in the United States. Simlarly, in order for a whisky to be called Scotch, it must be made in Scotland. Japanese whisky, for example, is similar to Scotch in many ways. But it's not Scotch, because it's made in Japan.
They have different ABV requirements.
Bourbon must be distilled to no more than 80 percent alcohol (160 proof) and be no more than 62.5 percent when put into casks for aging in new charred oak barrels. Scotch, however, must have an ABV of less than 94.8 percent and is aged in used oak barrels, including those that previously stored Sherry, beer and, yes, bourbon.
They go through different aging processes.
Bourbon has no minimum aging period, but to call your product Straight Bourbon, a specific distinction of quality, it must be aged for no less than two years and have no added coloring, flavor or spirits. Conversely, Scotch must be aged for no less than three years. Within Scotch, though, there are additional distinctions as well. For instance, a single malt Scotch is made with malted barley in pot stills at a single distillery and blended Scotch whisky is made by combining several single malts with other whiskies in column stills.
Scotch also tends to be aged longer than bourbon, with many of the most popular whiskies hitting shelves after anywhere from 12-25 years inside barrels. Part of the reason for this is a difference in climates. Bourbon is predominantly produced in Kentucky, where the climate is quite warm during the summer and therefore bourbon evaporates at a faster rate. This means that the longer bourbon is in the barrel, the lower the yield and the more expensive it gets.