What is the primary characteristic that differentiates bourbon from other whiskeys?

The primary characteristic that sets bourbon apart from other types of whiskey is its requirement to contain at least 51% corn in its mash bill. This high corn content is crucial because it not only influences the flavor profile—resulting in a sweeter and fuller taste—but also helps to define what legally qualifies a whiskey to be labeled as bourbon. This specific grain composition is unique to bourbon and is a fundamental aspect of its production, with the remaining components of the mash bill typically including malted barley, rye, or wheat.

Other options do not accurately reflect the key defining traits of bourbon. For instance, while a bourbon aged for under four years may not have an age statement, it does not need to age for five years unless it is labeled as "straight bourbon," which is a different criterion. Additionally, bourbon is not limited to production in Bourbon County; it can be made anywhere in the United States, as long as it follows the defined regulations. Finally, while certain whiskeys may have varied alcohol contents, the legal definitions for bourbon are more focused on its mash bill and production methods rather than specific alcohol content levels, which can also differ significantly among whiskey types.