Hi Vale, a vinegar smell isn't a bad sign, but it alone doesn't indicate starter maturity. In order to consider your starter "mature" it needs to be rising predictably—that means two or three days of rising consistently after having been fed at room temperature. In addition, you want your starter not to smell unpleasant—so no off odors like stinky feet or acetone. A vinegary smell is more likely when the starter is past its peak of rising and in need of another feeding, nothing to worry about, but not exactly the goal either. If you're trying to determine if your starter is ready to add to your bread recipe, you want it to be at its peak of rising, or just beginning to fall. I hope this helps! Let us know how it goes. 

Barb

• Reply