In Scotland, they used to make a fermented whey drink called blaand. Like kvass and kumis, it was a sort of cottage industry beverage. My brother had kumis in Khazakstan that was actually fermented from cow whey, rather than the traditional mare's milk. From what I understand, fermented whey was just left to ferment in a cask; I guess picking up wild yeast. Or perhaps they used some of a previous batch as a starter. I had a room ate who was experimenting with catching wild yeasts and ferment things, but I found the taste always too sour for me. I think that maybe I'll try pitching a little ale yeast into my whey this weekend when I make my cheese, in a fermentation bucket with an air lock and see what happens.
Dave in CT