What substance is added to grapes bring about fermantation?

No substances are added to bring about fermentation in grapes. Fermentation is a natural process that occurs when yeast converts the sugars in grapes into alcohol and carbon dioxide. The yeast that is responsible for fermentation is naturally present on the skins of grapes. When the grapes are crushed, the yeast comes into contact with the sugar and begins to ferment, producing alcohol and carbon dioxide.