Bourbon - Sour Mash
Today all American distilleries use the sour mash process. Sour mash means that a part of the distillation residues (stillage) is added to the mash again. After the mash has been prepared with fresh spring water it is chemically neutral, which means the chemical environment is neither acidic nor alkaline. Chemically, this property is measured with pH.
A solution with a pH of 7 is neutral. An acid has a pH smaller than 7. Strong acids have a pH of 3 to 4. If the pH of a solution is higher than 7, it is alkaline. Strong alkaline solutions have a pH of 10 to 11.
After the addition of neutral water, the mash has a pH of approximately 7 (neutral) in which the yeasts can't work properly. The addition of a part of the very sour stillage (pH 5.0 - 5.4) leads to an acidification of the entire mash. After the stillage has been added, the mash has a pH of about 5.4 to 5.8, which is ideally acidic for the yeasts to work properly.
Some bourbon producers try to make their product look special by using the term 'sour mash'. Other sources speak of a disinfecting effect of the sour mash process on the mash. However, the only purpose of sour mash is to create an acidic environment for the yeasts. The rest of the sour mash story is just marketing.