Can an opened plastic container of refrigerated grape juice ferment in my frig?

Question

I left a semi-full, yet sealed plastic container of 100% grape juice in my refrigerator for about 30 days. this morning when I drank a glass, it felt carbonated...

Answer 1

I really dont think it will actually ferment without yeast and heat.

Answer 2

Yes, People use to ferment juice all  the time.  It is a little unusual if the grape juice was purchased at a store since the manufactures of bottled grape juice do not want this to happen in transportation so the juice goes though steps to stabilize the juice so it will not ferment.  But once opened the juice can have the proper type of yeast introduced through the air to start the process.  The top on the container would not prevent fermentation from taking place since the yeast would already be in the juice.  

If you want to do it on purpose make the juice without heating(grape and apple work well) the proper yeast is already in the air we breath.