The primary characteristic of "sour mash" whiskey is that it uses a portion of mash from a previous distillation. This process involves taking a small amount of mash that has already fermented and mixing it with fresh mash for the new batch. This method introduces lactic acid bacteria, which helps to control the pH levels and can enhance the fermentation process, producing a more consistent and flavorful product. It also contributes to the distinct taste profile associated with sour mash whiskeys, making this practice an essential aspect of their production.
The other options do not accurately define what makes sour mash whiskey unique. While sour mash can be distilled at varying proofs, it is not defined by its distillation proof. Aging in charred barrels is a common practice for many types of whiskey, not just sour mash. Lastly, while some sour mash whiskeys may have a higher percentage of rye, this is not a defining feature of all sour mash whiskeys, as the mash bill can vary.