Which gas makes Orange juice bottles swell? I forgot a bottle of 1L Orange Juice (100% orange juice with pulp) in the fridge for about a week, the bottle was previously open so air got inside as well.

After a week the bottle got inflated and when I opened it I could hear the gas coming out. No odours, orange juice still the same colour (not sure if the same taste, I threw it away).

Which gas was produced and how did it happen? The air that got in seems to have played a part on it (these bottles stay in the supermarket closed for a long time without getting inflated).
I can't imagine the gas was anything except $\ce{CO2}$ formed by the action of microorganisms contaminating the orange juice.  They degrade the organic components of the juice to power their metabolisms, and produce carbon dioxide as a byproduct.  

A common contaminant of pasteurized fruit juice is a genus of bacteria called Alicyclobacillus, but this paper says that Alicyclobacillus bacteria do not cause bottle swelling and do cause a medicinal smell.

By searching Google for ["bottle swelling" "orange juice"], I found an English-language abstract of a thesis from China which implies that the culprit could be a yeast.