Not always true - there is also aerobic fermentation. You are right in that many ferments require anaerobic conditions however it really just depends on the des

Not always true - there is also aerobic fermentation. You are right in that many ferments require anaerobic conditions however it really just depends on the desired result and bacteria involved. Kombucha, for example, requires oxygen and is mostly an aerobic fermentation. In order to carbonate, however, it goes through a short anaerobic step at the end (which also causes it to switch from producing organic acids to producing alcohol). This is usually done during the bottling process since the bottle is sealed.