What is the process of sour mashing?

The process of sour mashing involves mixing new mash with backset residue, which is the leftover mash from a previous distillation. This backset contains lactic acid bacteria, which helps to lower the pH of the new mash and introduces desirable flavors and consistency to the fermentation process. The acidity created through sour mashing can improve fermentation efficiency and help prevent the growth of unwanted microorganisms that can spoil the mash. This technique is commonly used in the production of certain types of whiskey, particularly in bourbon production, to enhance quality and character.

The other choices do not relate to sour mashing. Using only malted barley in fermentation pertains to the grain bill composition and doesn't involve the process of integrating backset. Aging whiskey for several years refers to the maturation stage after distillation, and distilling whiskey twice describes the distillation process itself rather than the mashing process, which is crucial in the early stages of production.