By 1850 America’s population had grown to twenty-four million and that year Americans consumed 51,833,473 proof gallons of distilled spirits, not including moonshine and bootleg. A proof gallon is one gallon of 100 proof whiskey and that was the measure then and is still the measure used to calculate federal taxes, with a higher proof taxed proportionately more and a lower proof less. America’s thirst pushed distilling away from local stills and toward commercial production and standardization. 

Before standardization distillers found their results on more or less an empirical basis. Gerald Carson writes in The Social History of Bourbon that a distiller had his own formula, techniques and discoveries about which he was less than garrulous. A recipe printed in the Lexington Gazette in 1823 describes just how empirical the old methods were by instructing the distiller to let the cooked mash stand until cool enough that the distiller can bear his hand four inches within the surface of the mash with no more pain than a slight stinging sensation at the ends of his fingers.

An early innovator who had a major and documented hand in shaping American whiskey during these times of rapid change was Dr. James Crow, a Scot who migrated to Kentucky in the 1820s. Crow, a physician with an interest in chemistry, introduced scientific measure into the industry with such basic instruments as the saccharometer and thermometer. He worked at Old Oscar Pepper Distillery which eventually became Labrot and Graham Distillery. Today Woodford Reserve Distillery stands on the same space. He insured the quality of his own brand, Old Crow, by aging it in charred oak barrels and he advanced standardization from batch to batch by introducing the sour mash method of fermentation.

This process calls for a portion of the previous fermentation (usually about 25%) to be added to the new batch of unfermented mash. The old fermentation is ‘sour mash’ because the sugar has been consumed by the yeast. This produces a higher acidity in the mash that protects against undesirable bacteria and promotes continuity of character from one batch to the next. The sour mash is called backset, setback, thin slop, or yeast back, depending on the distillery. All American whiskey today is made using this process, whether specified on the label or not.