Pellicle

From Brewing Forward
Pellicle on cider
A pellicle on cider pitched with wild microbes

A pellicle is a film (or "velum") that can form on the surface of a fermentation. It is created by some types of yeast and bacteria, when oxygen is present.[1] 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.[1] 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.[2]

Since commercial strains of brewers yeast used for beer and wine do not form a pellicle (except for sherry/flor yeast[3][4] and possibly WLP644[5]), the presence of a pellicle is often a sign of contamination.[6] (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.[7] For example, a pellicle frequently forms in mixed-fermentation beers, kombucha, vinegar, and wines, ciders, and meads with Brettanomyces or unpasteurized raw ingredients such as fruit or honey. The appearance of a pellicle in these products doesn't reflect what may be happening during fermentation with regard to quality or progress, it merely indicates the presence of the microbes.[6] 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,[8] 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[edit]

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.[1][6][9]

Myths[edit]

Myth: Brewers yeast can form a pellicle.
Truth: Commercial Saccharomyces yeast NEVER forms a pellicle, with the exception of sherry (flor) yeast. This myth stems from brewers with recurrent contamination issues.[10][11] 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."[12]
  • Fermentis: "None of our strains (Fermentis or Red Star) have that ability [to form a pellicle] in any circumstances."[13]
  • White Labs: "Pellicle formation isn't common with our Saccharomyces strains. We have seen it with WLP644 and WLP700 but that's likely it."[14] When asked to further clarify WLP644: "I don't have any specifics on when a pellicle has formed but I know our R&D team has seen it. It likely has to do with how active the culture is/pitching rates. If you have a really active fermentation you likely won't get too much pellicle growth as it gets broken up."[15]
  • 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."[16]
  • GigaYeast: "We bank and grow hundreds of yeast strains and have never encountered a domesticated brewers yeast strain that exhibited pellicle formation."[17]
  • 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."[18]
  • 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."[19]
  • 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."[20]


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

Myth: White Labs WLP644 forms a pellicle.
Truth: White Labs says that WLP644 Saccharomyces "Bruxellensis" Trois can possibly form a pellicle, but doesn't provide specifics about under what circumstances it may occur. However, the consensus among brewers is that it does not form a pellicle, as there have been no recent validated reports of pellicle formation with this strain.[21][22] Therefore when using a WLP644 culture, it is wise to regard any significant pellicle formation as a likely contamination.
Myth: You can rack from under a pellicle to remove the contamination.
Truth: Contaminating microbes are present throughout the liquid. For example, it's common knowledge that microbes like Brettanomyces, Lactobacillus, and Pediococcus are able to be grown from bottles of commercial mixed-fermentation beers, even though a pellicle was obviously not put into the bottles.
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 (intentionally) capturing wild cultures. The genes that cause pellicle formation aren't always turned on, even if they are present.[23]

See also[edit]

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

References[edit]

  1. ↑ a b c Pellicle. Milk the Funk wiki. Accessed April 2020.
  2. ↑ Widmayer C. Comments from a professional fermentation/yeast researcher. Milk the Funk Facebook group. Feb 2020.
  3. ↑ Eldarov MA, Beletsky AV, Tanashchuk TN, Kishkovskaya SA, Ravin NV, Mardanov AV. Whole-genome analysis of three yeast strains used for production of sherry-like wines revealed genetic traits specific to flor yeasts. Front Microbiol. 2018;9(965).
  4. ↑ Van Mulders SE, Ghequire M, Daenen L, Verbelen PJ, Verstrepen KJ, Delvaux FR. Flocculation gene variability in industrial brewer’s yeast strains. Appl Microbiol Biotechnol. 2010;88:1321–1331.
  5. ↑ 644 Explanation. White Labs. Accessed April 2020.
  6. ↑ a b c Humbard M. Beer microbiology – what is a pellicle? A Ph.D. in Beer – A Study of Beer and Fermentation Science. 2015.
  7. ↑ Pellicle photo collection. HomebrewTalk forum. 2010.
  8. ↑ Colleen. What is kahm yeast & is it safe? Fermentools. 2018.
  9. ↑ Nakagawa Y, Arai Y, Toda Y, et al. Glucose repression of FLO11 gene expression regulates pellicle formation by a wild pellicle-forming yeast strain isolated from contaminated wine. Biotechnology & Biotechnological Equipment. 2017;31(1):120–127.
  10. ↑ Thin white film floating in secondary? HomebrewTalk forum. December 2007.
  11. ↑ Post your infection. HomebrewTalk forum. April 2020.
  12. ↑ Logsdon, Tamara (Wyeast QC Lab Coordinator and Brand Manager). "Pellicle formation." Personal email correspondence received by Adam Bittner. April 22, 2020.
  13. ↑ Pizarro, José (Fermentis Regional Sales Manager for North America – East & Gulf of Mexico). "FW: Other request from Fermentis Website." Personal email correspondence received by Adam Bittner. April 22, 2020.
  14. ↑ Taylor, Kara (Head of Laboratory Operations at White Labs). "Pellicle Formation." Personal email correspondence received by Adam Bittner. April 23, 2020.
  15. ↑ Taylor, Kara (Head of Laboratory Operations at White Labs). "Pellicle Formation." Personal email correspondence received by Adam Bittner. May 4, 2020.
  16. ↑ Shaner, Lance (Owner of Omega Yeast Labs). "Pellicle formation." Personal email correspondence received by Adam Bittner. April 24, 2020.
  17. ↑ Withee, Jim (President of GigaYeast). "Pellicle formation." Personal email correspondence received by Adam Bittner. April 24, 2020.
  18. ↑ Caudill, Jess (Imperial Yeast representative). "Pellicle formation." Personal email correspondence received by Adam Bittner. April 27, 2020.
  19. ↑ Shayevitz, Avi (Lallemand R&D Associate). "Pellicle formation." Personal email correspondence received by Adam Bittner. April 28, 2020.
  20. ↑ Preiss, Richard (Escarpment Labs R&D associate and one of the founders). "Pellicle formation." Personal email correspondence received by Adam Bittner. April 30, 2020.
  21. ↑ Thread about WLP644 Pellicle Formation. Milk the Funk Facebook group. April 2020.
  22. ↑ "Wiki Kwiki #005 - Lance Shaner of Omega Yeast Labs" (at ~23 minutes) Milk the Funk podcast, December 2019.
  23. ↑ Bumgarner SL, Dowell RD, Grisafi P, Gifford DK, Fink GR. Toggle involving cis-interfering noncoding RNAs controls variegated gene expression in yeast. Proceedings of the National Academy of Sciences. 2009;106(43):18321–18326.
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