Refers to the quality of being acid and/or sourness or tartness. It is one of the main properties used to characterize a loaf of bread and influences the taste and keeping quality of the loaf. Breads leavened with commercial yeast and with a shorter fermentation usually have lower acidity, which causes it to go stale sooner and to have less flavour complexity. On the other hand, naturally-leavened breads with longer fermentation are more acid, which produces the recognizable tart flavor of sourdough and allows it to last longer.

The acidity in most homemade breads is provided exclusively by the yeast as a product of it's metabolism during the consumption of sugars in the dough.