An acid is a molecule or ion capable of donating a hydron (proton or hydrogen ion H+), or, alternatively, capable of forming a covalent bond with an electron pair (a Lewis acid). Acidity is measured by pH.
In distilling Acids play a key role in several processes. Mashing enzymes work best in a mildly acidic environment. In fermentation yeast also likes the same but can cause pH crashes leading to stalled ferments. Bacteria create acids that can lead to the same pH crashes. As spirits age in a barrel the tannic acids in the wood decrease the pH levels and it helps increase ester development.
For a distiller it is vital to understand how to measure pH, how to adjust pH, and the natural causes of changes to pH.