If fermenting in a crock or jar, look out for a wispy white substance that may form on the surface of the liquid and around the edges of your fruit. This is kahm yeast, a topical fungal bloom that can flourish before your fruit has fully fermented and acidified its juices. Kahm yeast is harmless, but it can add an off-flavor if it gets disturbed and mixed into the liquid. When you spot some kahm yeast, carefully spoon it off and discard.