Pellicle

The pellicle on a cider pitched with wild microbes, courtesy of Dr. Adam Bittner

A pellicle is a film that can form on the surface of a fermentation. It is created by some types of yeast and bacteria, when oxygen is present. A pellicle may be white, off-white, or clear and it generally has a powdery, waxy, or slimy appearance, often with bubbles or wrinkles (never fuzzy or hairy). It should not be confused with mold, kraüsen, "yeast rafts", or films from oils.

Pellicles can be formed by a variety of wild yeast and bacteria. Therefore it is not possible to identify which microbes(s) may be present based solely on the appearance of the pellicle or its characteristics — microbiological testing must be performed for identification.

Since commercial strains of brewers yeast used for beer and wine do not form a pellicle (except for sherry/flor yeast and possibly WLP644), it can be an indisputable sign of contamination. (See Contamination for more info.)

On the other hand, pellicle formation is normal for fermentations that contain wild microbes or non-Saccharomyces cultures such as Brettanomyces. For example, a pellicle frequently forms in mixed-fermentation beers or wines/ciders/meads with Brettanomyces or unpasteurized fruit or honey, and pellicle formation is a hallmark of kombucha and vinegar production. The appearance of a pellicle in these products doesn't reflect what may be happening in the fermentation with regard to quality or progress, it merely indicates the presence of the microbes. In wine, adequate pH-adjusted sulfite levels post-fermentation can inhibit these microbes and therefore prevent the formation of a pellicle.

"Kahm yeast" is a colloquial term for a pellicle, and it is a misnomer because it is not always created by yeast. Similarly, the pellicle on kombucha is often mistakenly called the "SCOBY".

Pellicle test

If you aren't sure whether an unknown film is a pellicle, you can do what we call a "Forced Pellicle Test", and it's easy:

1. Add a small sample of the liquid into a sanitized jar or other container.
2. Cover it loosely with foil and leave it undisturbed in a warm location.
3. Check on it in 1-2 days. If a significant film has formed (that is not fuzzy or hairy), then congratulations, you have a pellicle!

If you still aren't sure, leave it undisturbed for several more days and check again. Any fuzzy or hairy growth is mold.

This test is based on the fact that oxygen exposure generally stimulates pellicle formation.

Myths

Myth: Brewers yeast can form a pellicle.
Truth: With the exception of sherry (flor) yeast, it does not. This myth stems from brewers with recurrent contamination issues. In reality, unless you pitch a yeast known to form a pellicle, any pellicle was definitely created by wild microbes that were somehow introduced into the beer or wine.

We have reached out to some of the major yeast labs to verify whether their strains produce pellicles:

• Wyeast: "Wyeast Laboratories, Inc. does not have any commercial Saccharomyces strains that form pellicles."
• Fermentis: "None of our strains (Fermentis or Red Star) have that ability [to form a pellicle] in any circumstances."
• White Labs: "Pellicle formation isn't common with our Saccharomyces strains. We have seen it with WLP644 and WLP700 but that's likely it."
• Omega: "I am not aware of any of our Sacch strains being able to form a pellicle. We have a handful of "wild" Sacch strains and haven't seen it with them either."
• GigaYeast: "We bank and grow hundreds of yeast strains and have never encountered a domesticated brewers yeast strain that exhibited pellicle formation."
• Imperial: "We have not run trials on pellicle formation with Saccharomyces strains specifically as this usually isn't a consideration when using them in the short duration fermentation process."
• Lallemand: "None of the Saccharomyces cerevisiae strains available through Lallemand for brewing or wine production are known to produce pellicles under tested laboratory conditions. No pellicle formation observations have been made in the presence of pure cultures."
• Escarpment: "None of our Saccharomyces strains are capable of forming pellicles/films. Among Saccharomyces this seems to be fairly limited to sherry flor yeasts, which have some modifications in flocculation genes that seems to help them form rafts/films. "

On his blog, Dr. Matt Humbard discusses the ability of Saccharomyces to form a pellicle.

Myth: White Labs WLP644 forms a pellicle.
Truth: It is questionable whether WLP644 Saccharomyces "Bruxellensis" Trois can actually form a pellicle. When this culture was first released, it was noted to form a pellicle. However, there was a shift in quality control practices in 2017 with the Left Hand vs. White Labs lawsuit. Since then, there have been no validated reports of this strain forming a pellicle. Nevertheless, White Labs continues to claim that WLP644 can possibly form a pellicle, as noted above.

Myth: You can rack from under a pellicle to remove the contamination.
Truth: This is completely false. Contaminating microbes are present throughout the liquid. For example microbes like Brett, Lacto, and Pedio are able to be grown from bottles of mixed-fermentation beers 30+ years later, every though those were obviously not bottled with the pellicle from the barrels.

Myth: All wild microbes (e.g. contaminants) form a pellicle.
Truth: Not all wild mixed yeast/bacteria cultures form pellicles. This is easy to demonstrate by those of us repeatedly (and intentionally) capturing wild cultures. The genes that cause pellicle formation aren't always turned on, even if they are present.

See also

• See Pellicle at Milk the Funk for more detailed information about pellicles.
• Pellicle photo collection on HomeBrewTalk.com
• See Contamination for more information about how to proceed when your fermentation has an unexpected pellicle.