How do I know if my sourdough starter has gone bad and needs to be thrown out?
A healthy starter is very hard to spoil, because its acidity keeps pathogens out; a dark liquid on top (hooch) or a sour smell is normal, not bad. Mold on the side of the jar usually forms in dry spots the live starter cannot reach. You do not need to toss it: scoop out a small clean portion (1 to 2g is plenty), feed it fresh, and after a few feedings it is back to normal.
Answered by Hendrik, The Bread Code
From the book
Mature sourdough starters should not go moldy unless the conditions of the starter change. I have seen mold appearing when the starter is stored in the fridge and the surface dried out. It also sometimes forms on the edges of your starter’s container, typically in areas where no active starter microorganisms can reach. Simply try to extract an area of your starter that has no mold. Feed it again with flour and water. After a few feedings, your starter should be back to normal. Take only a tiny bit of starter: 1 g to 2 g are enough. They already contain millions of microorganisms.
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