American craft breweries make some truly tasty sour fruit beers, but most are fermented with cultured yeast strains, not airborne wild yeast and bacteria.
In Belgium, though, the tradition of spontaneous fermentation holds strong, and true Belgian lambic ferments only with wild yeast. Some of it is destined to become dry, cidery gueuze (a blend of young and old lambics) while some will have fruit added to it. For traditional fruited lambics, brewers and blenders add whole fruit (cherry pits and all) to a blend of aged and young lambics, causing fermentation to start again as the yeast consume the fruit sugars. The final beers are traditionally left unsweetened.
If you haven't tasted a traditional Kriek or Framboise before, you're in for a surprise. They're funky and acidic, with hints of shoe leather and wet dog. These days, you'll frequently see fruit beers with added sweeteners and fruit juices—lots of folks love them, but we encourage you to try the real thing sometime. Traditionally sour fruit lambics may not be beers for beginners, but they're a palate-expanding experience we highly recommend.
Disclosure: The Boon Kriek and Drie Fonteinen Oude Kriek were review samples.