Sour Mash Bourbon (Whiskey), on a label, is both marketing designation and a type of whiskey. The Sour Mash name refers to the process used to create product continuity from batch to batch of whiskey and is used in all whiskeys not labeled Sweet Mash. From a global perspective, there are less than 10 sweet mash labels among the hundreds of American whiskey labels currently in production.
The Sour Mash Process was introduced to American Whiskey and perfected by Dr. James C. Crow in the early to mid-1800’s. It was developed by applying the scientific method to bourbon distillation to control the result of the distillation. The goal was to achieve the same taste and flavor from batch to batch. Dr. Crow did so by cutting pieces off of the fermented mash containing the yeast from one fermentation and introducing it into the fermentation of the next batch of whiskey. This controls the growth of bacteria and creates chemical continuity from batch to batch ensuring a much smaller margin of error in flavor from batch to batch. This process is the backbone of American Whiskey.
A neat tidbit about Dr. Crow is that he wasn’t in fact a doctor at all*. Upon arriving in Kentucky in the early 1820s he claimed to be educated at Edinburgh in Scotland. Research after his death revealed that he didn’t attend Edinburgh at all. Although his impact was great, his tall tale just adds to the charm of American Whiskey and it’s winding and storied history.
*Credit to noted Bourbon Hall of Fame Historian Mike Veach