I use a technique called the french orleans process, or 'surface fermentation'. Fresh hard cider is inoculated with a living cider vinegar (mother) and exposed to open air where it can ferment. Plenty of small farm producers use this method. The mother can take the form of a physical cellulose mass that has naturally grown (similar to a kombucha scoby), or it could simply be some actively fermenting vinegar. Either way, this method is essentially relying on wild acetobacter bacteria and likely will have a succession of different bacteria throughout fermentation. Each new batch of fresh hard cider will also bring in its own nutrients and bacteria that could influence the fermentation complexities. The results are generally complex and evolving. Similar to wild fermented natural ciders and wines, the flavors and aromas may be mind blowing to some, but a turn-off to others. And there is almost always variability from batch to batch and year to year.
Most large scale operations rely on an acetobacter fermenter of some kind. A single strain of acetobacter bacteria is introduced to the fresh hard cider through rapid oxidation and a vinegar can be made in a matter of hours, or a few days. The results can be very predictable and clean, but perhaps more one dimensional.
