The first time you brew a wild or sour beer, you may be surprised at what shows up in your fermentor. In fact, go ahead and run a Google image search on “beer pellicle.” I’ll wait.
Eww, right? A pellicle (pronounced “PELL-uh-kull”) is the gooey, slimy, bubbly, fuzzy layer of nastiness that may appear on the surface of beers fermented with Brettanomyces, Lactobacillus, or Pediococcus. It’s a type of biofilm, which is a colony of microbes that huddle together and float like a raft on the surface of certain kinds of beer.
If you intentionally inoculate beer with Brett, Lacto, or Pedio, then pellicle formation is a sure sign that wild and sour bugs have established themselves in generous numbers. Pellicles don’t form on beer fermented with straight up Saccharomyces, though, so the presence of one may be an indication of contamination (also called an infection).