Pellicle

Pellicle from The Yeast Bay Brussels Brett Blend; courtesy of Dan Pixley

Lactobacillus pellicle; courtesy of Matt Humbard

A pellicle is a subtype of biofilm that is formed on the surface of a beer. A biofilm is a community of microorganisms that are connected to one another through the use of material that is present in the environment, such as proteins, long chains of sugars, or live (or dead) cells. It is often formed by Brettanomyces, Pediococcus, or Lactobacillus. It can also be formed by Saccharomyces in rare occasions .

Introduction

Pellicles form when the surface of the beer is exposed to oxygen One theory is that the formation of a pellicle allows the organism to access the small amount of oxygen that is present in the headspace of the fermentation vessel. Another theory, and one that may be less accurate according to Dr. Matt Humbard, is that the pellicle protects the beer from other microorganisms . Popular thought is that the formation of a pellicle is not indicative of the quality of the sour beer that is being produced; it is only an indication that oxygen has entered the fermentation vessel and that the microbes are reacting to that exposure.

Note About the Science and Terminology

(This section in progress)

As with some things in science that are new or not greatly explored, terminology isn't always agreed upon or fully established. Pellicles are a good example of this. It's been stated on this wiki and elsewhere that a pellicle is a subtype of biofilm. However, the accepted definition of a "biofilm" in biology doesn't describe pellicles that we see in fermentation. The traditionally accepted definition of a "biofilm" is an aggregate of microorganisms where the cells adhere to each other on a surface . Pellicles in beer do not attach to a solid surface, they appear on the "air-liquid interface" (the surface of the beer). They are also not colonies within an adhesive . To make matters even more confusing, the accepted definition of a "pellicle" in biology is either the outer boundary of a protozoa cell , or a protein film that forms on the surface of teeth .

Recently, there have been studies that define a "pellicle" as we see in fermentation as a type of biofilm that forms on the air-liquid interface of a liquid (see references) . Although these studies don't directly look at beer pellicles, this indicates that the definition of a pellicle as stated in this wiki may become more widely accepted in the scientific community.

The bottom line is that what is referred to as a "pellicle" in fermentation has not been widely studied, and the terminology is not widely established yet scientifically. Indeed, when researching "biofilms" and "pellicles", results will mostly refer to the more well established topics. Research done on traditional biofilms cannot necessarily be extrapolated to the pellicle phenomenon seen in fermentation.

Images

• See Google search images.

External Resources

• Beer Microbiology – What is a pellicle? A PhD in Beer blog. Dr. Matt Humbard. 01/30/2015.