What substance is the white powdery coating on grapes?

Many people believe that it is pesticide. It is true that in some regions maximum quantity of pesticides are sprayed on grapes. However, white powdery coating on grapes is not pesticide. It is yeast, a type of fungi. Originating as microscopic single cell organisms from grape vine, these cells reproduce in large numbers by splitting and budding, ultimately covering each grape. They survive by consuming glucose in grape and release carbon dioxide and alcohol as by-product, so to say. Both these products cause fermentation in the substance which contains glucose. This is the reason for using yeast in baking bread and cake. Tiny bubbles of effervescence caused by carbon dioxide that yeast releases after consuming glucose contained in wheat expand the dough making it lighter and full of small holes. As there is abundant glucose in grapes, yeast can convert it fully into alcohol within six months.

Additional reading:
Grape (Wikipedia)
Yeast (Wikipedia)

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