What is a precise definition of closed fermentation?

I was reading Papazian's Joy of Homebrewing and in several sections he references "closed fermentation" and "open fermentation." One brief and rather crpytic section is called "Open versus Closed Fermentation."

What, exactly and in one sentence, is "closed fermentation." Likewise, in one sentence, what is "open fermentation."

I've read the book sections a few times and feel like he does not precisely define these terms precisely enough (at least for a novice like me), though the book overall excellent, I think. This is the only thing I don't get.

Is open fermentation simply a method which is done with no lid on the fermenter, or, is it any fermentation process - regardless of a lid being present or not- that allows the krausen to fall back into the beer? thanks for any help

Open fermentation essentially means that the fermenter is open or uncovered, permitting the exchange of ambient air-- for homebrewers, it mostly means fermentation with the lid off. Closed fermentation means that the exchange of air is prohibited, like fermenting with a sealed lid and airlock.

Hope that helps. Cheers!

check this out

http://www.brewingtv.com/episodes/2010/5/17/brewing-tv-episode-4-open-fermentation.html

Sorry, but I am kind of gonna hijack the thread.

Most homebrewers' concerns about open fermentation is cleanliness. If your area isn't suited for an open top something could fall in. I wonder then if there can be a hybrid method.

I recently did a hefeweizen 5 gallon batch in a 7.9 gal pail. I removed the lid for 5-10 minutes every 4-6 hours (when I thought about it). The large head space and then periodically removing the lid should incerase the exposure to oxygen. My thought was to increase the oxygen exposure and airflow to the krausen much like an open fermenter would. At the same time the cover was on most of the time to maintain a clean fermentation. I just bottled last week and don't have a comparison. Does anyone else have experience with this?

thanks zehbaron for the useful response

Got to love the subtle and unique off-flavors of airborne wild yeast and bacteria.