What is the white residue that is often on grapes and why doesn't washing the fruit get rid of it?
The foggy white “bloom’’ sometimes found on grapes is a yeast known as saccharomyces cerevisiae. It grows in soil and then becomes airborne, settling into the skins of the fruit as they mature and continuing to propagate until the grapes are harvested. Though it’s often more visible on red and purple grapes, the yeast also lives on green grapes. This yeast “bloom’’ is waxy and water-insoluble, protecting the flesh of the fruit from harmful fungi and bacteria, but also making it difficult to rinse off.
There is no need to scrub too hard, however: The yeasty residue is completely harmless and tasteless.