Originally Posted by Yooper
Well, I've never used bananas in beer. But I make banana wine (recipe is posted). It's like any other fruit- the sugars are simple sugars (fructose) and easily fermented by ale or wine yeast. But it wouldn't have the diastastic power (enzymes) to convert other substances. It'd be like adding pineapple to a mash- simple sugars, not amylase.
Banana wine sounds awesome!
But it seems that as a fruit ripens, amalyase is actually present (either in the peel or in the fruit itself). I don't know how much I trust this source, but it references two enzymes, amalyase and pectinase.
Now, any beer created from this would most likely have no body because, as you said, the sugar present would be fructose. But it would be fermentable...in theory. The difficult thing, as far as I can tell, is getting the starches to dissolve in water and getting the enzymes out of the fruit. I'm not sure it would actually add anything to the beer, but it may be a little...unique, if very water-like.