Active rye sourdough starter in a jar.
The Trick To Reviving That Sourdough Starter That's Been In Your Fridge For Months
By Haldan Kirsch
If your sourdough has been sitting in the fridge and you were working with a pretty mature sourdough, then it might be able to be revived. Crave the Good claims that it was able to restore a sourdough starter that had been forgotten for four months, and though this might not hold true for every starter, some could come back to life with a little TLC.
However, if your sourdough starter appears moldy, it's best to throw it out, but it can be hard to tell when something fermented has gone bad. If it seems thick and chunky and smells musty or moldy, it's got to go, but if your sourdough starter is thin, watery, and still has an acidic, vinegary smell, then it's likely safe to use.
To revive a starter, let it come to room temperature before mixing equal parts sourdough, water, and flour, and let it sit out to ferment. Another three or four days of regular feedings will help it reach maturity again, and it should be safe to store back in your refrigerator — just try to remember it this time!