Pellicle

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

A pellicle is a film that can form on the surface of a fermentation. It is created by certain microbes in the presence of oxygen. 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 microbes and some commercial cultures containing non-Saccharomyces yeasts. Therefore it is not possible to identify the microbes(s) that may be present based on the appearance or other characteristics — unless microbiological testing is performed.

Since commercial strains of Saccharomyces (brewers yeast) commonly used for beer and wine do not form a pellicle (with one or two exceptions), it is an indisputable sign of contamination. (See Contamination for more information about the effects of wild microbes and how to deal with them.) On the other hand, pellicle formation is normal on fermentations that contain wild microbes or non-Saccharomyces cultures such as Brettanomyces. For example, a pellicle frequently forms in beers or wines with Brettanomyces and in wines with unpasteurized fruit or honey. Pellicle formation is a hallmark of kombucha and vinegar production. The appearance of a pellicle in these fermentations pitched with pellicle-forming microbes doesn't necessarily 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 observation that oxygen exposure stimulates pellicle formation.

Myths

Lactobacillus always forms a pellicle.
This myth is widespread. Lactobacillus forming a pellicle is considered a rare occurrence, possibly only by uncommon species or strains. None of the commonly used Lacto-only cultures are known to commonly form pellicles. The appearance of a pellicle should be assumed to be a contamination if lab-cultured Lactobacillus was pitched without any other microbes known to form a pellicle.

This myth possibly arose from the process of sour mashing in brewing (or similar method of souring wort with grain), where wild lactic acid bacteria are the focus of attention. However a large variety of other microbes are also present when a wild culture is pitched, and those other microbes are most likely forming the pellicle.

Brewers yeast can form a pellicle.
Some home brewers attempting to make "clean" beers or wines regard pellicle formation as a normal occurrence. This myth stems from brewers with recurrent contamination issues. In reality, unless you pitched a yeast known to form a pellicle, it 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."

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

See also

• See Pellicle at Milk the Funk for more detailed information about pellicles.