And all sour mash is bourbon whiskey but not all bourbon is sour mash. Sour mash is the name for a process in the distilling industry that uses material from an older batch of mash to start fermentation in the batch currently being made, analogous to the making of sourdough bread. The term sour mash can also be used as the name of the type of mash used in that process, and a whiskey made using this process can be referred to as a sour mash whiskey. Sour mash does not refer to the flavor of the whiskey, as is sometimes thought.