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 often an indisputable sign of contamination. However, in fermentations that contain wild microbes or non-Saccharomyces cultures such as Brettanomyces, the formation of a pellicle is a normal occurrence. For example, a pellicle frequently forms in beers or wines with Brettanomyces, in wines with unpasteurized fruit or honey, and it's a defining characteristic of kombucha and vinegar production. Furthermore, the appearance of a pellicle doesn't necessarily mean anything good or bad is happening in the fermentation, it merely indicates the presence of the microbes.

"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, then congratulations, you have a pellicle!

See also

• See Contamination for more information about the effects of wild microbes and how to deal with them.
• See Pellicle at Milk the Funk for more detailed information about pellicles.

Myths

Lactobacillus 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 issues with recurrent contamination. The fact is, 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 large yeast labs to verify that their strains so not produce a pellicle:

• Statement from Wyeast: "Wyeast Laboratories, Inc. does not have any commercial Saccharomyces strains that form pellicles."
• Statement from Fermentis: "None of our strains (Fermentis or Red Star) have that ability [to form a pellicle] in any circumstances."
• Statement from White Labs: "Pellicle formation isn't common with our Saccharomyces strains. We have seen it with WLP644 and WLP700 but that's likely it."