Acetic acid is an organic acid produced by heterofermentative bacteria (like F. sanfranciscensis) alongside lactic acid, ethanol, and CO₂. It is responsible for the sharp, vinegary "tang" associated with San Francisco-style sourdough. Beyond flavour, it is a powerful antifungal agent.

Lactic acid is an organic acid produced by Lactic Acid Bacteria (LAB) during the fermentation of carbohydrates. It is responsible for the mild, creamy, yogurt-like acidity in sourdough bread. It lowers the pH of the dough, activating enzymes and extending shelf-life by inhibiting mould growth.