Gracie squeezed some grapes and kept the juice in a vessel for 5 days at room temperature. On the fifth day she found that the juice tasted sour and appeared frothy.
What caused this change?

Question

Gracie squeezed some grapes and kept the juice in a vessel for 5 days at room temperature. On the fifth day she found that the juice tasted sour and appeared frothy.
What caused this change?

anonymous · Answer

Fermentation had taken place.  
The process of fermentation turns grape juice into an alcoholic. During fermentation, yeast interact with sugars in the juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide (as a by-product).