Most bread rises primarily due to the CO₂ produced during alcoholic fermentation as yeast feeds on sugars. Sourdough bread is unique in that it also has an initial stage, and requires a "starter" to act as a leavening agent. Michaela keeps a sourdough starter, refreshing it often to keep the starter alive and active. She adds a portion of the starter to dough when making sourdough bread. What is the BEST description of the process that occurs in the starter to give sourdough bread its famous tangy flavor?